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Reviews in Fish Biology and Fisheries 12: 131, 2002. 2003 Kluwer Academic Publishers. Printed in the Netherlands.

1

The potential of fish production based on periphyton

Anne A. van Dam1,3, Malcolm C.M. Beveridge2, M. Ekram Azim1 & Marc C.J. Verdegem11Fish Culture and Fisheries Group, Department of Animal Sciences, Wageningen University, P.O. Box 338, 6700

AH Wageningen, The Netherlands; 2FRS Freshwater Laboratory, Faskally, Pitlochry, Perthshire, Scotland, UK

PH16 5LB; 3Current address: Department of Environmental Resources, IHE-Delft, P.O. Box 3015, 2601 DA Delft,

The Netherlands (E-mail: avd@ihe.nl; Fax: 31-15-2122921; Phone: 31-15-2151712/2151715)

Received 5 December 2001; accepted 17 October 2002

Contents

Abstract page 1Introduction 2

BackgroundObjectives and scope of the review

TerminologyNatural and artificial periphyton-based systems 5

Natural systems with periphytonBrush-park fisheriesTraditional aquaculture systems in southeast AsiaAquaculture experimentsWater treatment with periphyton

Periphyton productivity 10Development of the periphyton assemblage and species compositionBiomass and productivityEffects of environmental factors

Fish 17Morphological and physiological adaptations to herbivoryPeriphyton ingestion by fishPeriphyton as fish feed: proximate composition

Assimilation efficiency and food conversion ratioPotential fish production based on periphyton 21Conclusions and recommendations for further research 24

Role of periphyton in aquaculture systemsAbility of fish to utilize periphytonPotential of periphyton-based fish production

Acknowledgements 26References 26

Key words: fish ponds, herbivory, nutrients, periphyton, phytoplankton

Abstract

Periphyton is composed of attached plant and animal organisms embedded in a mucopolysaccharide matrix. Thisreview summarizes research on periphyton-based fish production and on periphyton productivity and ingestion by

fish, and explores the potential of developing periphyton-based aquaculture. Important systems with periphyton

are brush-parks in lagoon areas and freshwater ponds with maximum extrapolated fish production of 8 t ha1

y1 and 7 t ha1 y1, respectively. Experiments with a variety of substrates and fish species have been done,

sometimes with supplemental feeding. In most experiments, fish production was greater with additional substrates

compared to controls without substrates. Colonization of substrates starts with the deposition of organic substances

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and attraction of bacteria, followed by algae and invertebrates. After initial colonization, biomass density increases

to a maximum when competition for light and nutrients prevents a further increase. Often, more than 50% of the

periphyton ash-free dry matter is of non-algal origin. Highest biomass (dm) in natural systems ranges from 0 to

700 g m2 and in aquaculture experiments was around 100 g m2. Highest productivity was found on bamboo

in brush-parks (7.9 g C m2 d1) and on coral reefs (3 g C m2 d1). Inorganic and organic nutrients stimulate

periphyton production. Grazing is the main factor determining periphyton density, while substrate type also affects

productivity and biomass. Better growth was observed on natural (tree branches and bamboo) than on artificalmaterials (plastic and PVC). Many herbivorous and omnivorous fish can utilize periphyton. Estimates of periphyton

ingestion by fish range from 0.24 to 112 mg dm (g fish)1 d1. Ingestion rates are influenced by temperature, fish

size, fish species and the nutritional quality of the periphyton. Periphyton composition is generally similar to that

of natural feeds in fishponds, with a higher ash content due to the entrapment of sand particles and formation

of carbonates. Protein/Metabolizable Energy (P/ME) ratios of periphyton vary from 10 to 40 kJ g1. Overall

assimilation efficiency of fish growing on periphyton was 2050%. The limited work on feed conversion ratios

resulted in values between 2 and 3. A simple simulation model of periphyton-based fish production estimates fish

production at approximately 2.8 t ha1 y1. Together with other food resources in fishponds, total fish production

with the current technology level is estimated at about 5 t ha1 y1. Because grazing pressure is determined by

fish stocking rates, productivity of periphyton is currently the main factor limiting fish production. We conclude

that periphyton can increase the productivity and efficiency of aquaculture systems, but more research is needed

for optimization. Areas for attention include the implementation and control of periphyton production (nutrient

levels, substate types and conformations), the ratio of fish to periphyton biomass, options for utilizing periphytonin intensive aquaculture systems and with marine fish, and possibilities for periphyton-based shrimp culture.

Introduction

Background

Fish production through aquaculture is realized in

a wide variety of culture systems, from extensive

seasonal ponds to intensive concrete raceways or

floating marine cages. In 1998, 53% of the 30 milliontonnes of finfish, molluscs and shrimp produced in

aquaculture were predominantly cultured in extensive

to semi-intensive pond systems (mainly Chinese carps

like Hypophthalmichthys molitrix, Ctenopharyngodon

idella, and Aristichthys nobilis, all Cyprinidae; com-

mon carp, Cyprinus carpio, Cyprinidae; and Nile

tilapia, Oreochromis niloticus, Cichlidae). Ponds are

also important in terms of production value, account-

ing for some 47% of the total value. Carps and the tiger

shrimp (Penaeus monodon, Penaeidae) are among the

most important commodities (Table 1; FAO, 2001).

All pond species feed low in the food chain, most

being filter feeders, herbivores, or omnivores.Production in extensive pond systems is based

on the natural productivity of the pond and solar

energy. In semi-intensive systems, organic and chem-

ical fertilizers and supplemental feeds are added

whereas intensive systems are based predominantly

on high-quality complete feeds. Only 515% of the

nitrogen added to the ponds as fertilizer is harvested

as fish biomass (Edwards, 1993; Gross et al., 1999).

In feed-driven systems, only 2030% of the nitrogen

in the feed is retained in the fish biomass (Avnimelech

and Lacher, 1979; Boyd, 1985; Jimnez-Montealegre,

2001). The nutrients that are not harvested as fish

biomass either accumulate in the pond sediment,

volatilize, or are discharged into the environment.From economic and environmental points of view,

there is a need to examine options to make aquaculture

systems more nutrient efficient.

Generally, three food pathways can be distin-

guished in aquaculture systems: (1) direct feeding by

the fish on feeds; (2) the autotrophic pathway, in which

solar energy is used by primary producers (mainly

algae) to convert carbon dioxide into organic matter

that can be utilized by fish; and (3) the heterotrophic

pathway, in which heterotrophic organisms (bacteria,

protozoa, and other invertebrates) decompose organic

matter that can be utilized by the fish (Schroeder,

1978). These three pathways are linked throughfluxes of organic and inorganic nutrients. In waste-

fed systems, the heterotrophic pathway can be more

important than the autotrophic pathway, but stable

isotope studies show that a large part of the micro-

bial production in ponds is based on algal detritus

(Schroeder, 1978; Schroeder et al., 1990). Estimates

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Table 1. Importance of pond systems in world aquaculture production of finfish, molluscs, and shrimp in 1998. Data: FAO (2001). CM =coastal marine system; P = pond; R = raceway; C = floating cage

Predominant

Volume Value production

Common name Scientific name 106 MT % 106 US$ % system

Molluscs

Pacific cupped oyster Crassostrea gigas 3.44 11.1 3.27 6.9 CM

Japanese carpet shell Ruditapes philippinarum 1.43 4.6 1.86 4.0 CM

Yesso scallop Pecten yessoensis 0.86 2.8 1.18 2.5 CM

Blue mussel Mytilus edulis 0.50 1.6 0.26 0.6 CM

Blood cockle Anadara granosa 0.25 0.8 0.23 0.5 CM

Mediterranean mussel Mytilus galloprovincialis 0.16 0.5 0.11 0.2 CM

Shrimp

Giant tiger prawn Penaeus monodon 0.58 1.9 3.86 8.2 P

Whiteleg shrimp Penaeus vannamei 0.19 0.6 1.03 2.2 P

Finfish

Silver carp Hypophthalmichthys molitrix 3.31 10.7 3.09 6.6 P

Grass carp Ctenopharyngodon idellus 2.89 9.4 2.66 5.6 P

Common carp Cyprinus carpio 2.47 8.0 2.83 6.0 PBighead carp Aristhichthys nobilis 1.58 5.1 1.45 3.1 P

Crucian carp Carassius carassius 1.04 3.4 0.83 1.8 P

Nile tilapia Oreochromis niloticus 0.79 2.6 0.89 1.9 P

Rohu Labeo rohita 0.75 2.4 1.94 4.1 P

Atlantic salmon Salmo salar 0.69 2.2 2.20 4.7 C

Catla Catla catla 0.63 2.0 0.55 1.2 P

Mrigal Cirrhinus mrigala 0.56 1.8 0.47 1.0 P

White Amur bream Parabramis pekinensis 0.45 1.5 0.54 1.1 P

Rainbow trout Oncorhynchus mykiss 0.44 1.4 1.36 2.9 P/R

Milkfish Chanos chanos 0.37 1.2 0.55 1.2 P

Channel catfish Ictalurus punctatus 0.26 0.8 0.42 0.9 P

Japanese eel Anguilla japonica 0.21 0.7 0.82 1.7 P

Mud carp Cirrhinus molitorella 0.16 0.5 0.16 0.3 P

Total this list 24.01 77.8 32.56 69.2

Total ponds (excluding Oncorhynchus mykiss) 16.24 52.6 22.09 46.9

Total world (finfish, molluscs and shrimp) 30.86 100.0 47.08 100.0

of the proportion of the standing stock of phyto-

plankton that accumulates as sediment in the bottom

range from 20 to 50% per day (Jimnez-Montealegre,

2001). Thus, a large part of the phytoplankton produc-

tion is decomposed on the pond bottom and contrib-

utes to the accumulation of nutrients in the sediment.

Because many fish species are not able to harvest

phytoplankton directly from the water column, an

extra trophic level is involved in converting phyto-

plankton into fish biomass. With an estimated energy

transfer efficiency of 10% per trophic level (Pauly

and Christensen, 1995), maximum fish yield may be

no more than 1% of the energy fixed by the phyto-

plankton consumed. Fish yields from extensive and

semi-intensive ponds could be up to ten times higher

if primary production could be harvested directly by

herbivorous fish.

Whether phytoplankton can be harvested directly

by fish depends largely on the fish species stocked.

Although species like silver carp and bighead carp

are capable of harvesting microalgae directly, many

species used in aquaculture cannot. Even for Nile

tilapia, generally regarded as a phytoplankton feeder,

it seems questionable whether it can derive enough

energy from exploiting phytoplankton (Dempster et

al., 1993, 1995). Phytoplankton has some other dis-

advantages. Nighttime respiration by phytoplankton

in ponds may lead to oxygen depletion during the

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early morning hours, causing a risk of fish mortality

(Madenjian et al., 1987). Decomposition of sedi-

mented phytoplankon may result in toxic decomposi-

tion products (ammonia, nitrite) and increased oxygen

demand. Phytoplankton blooms are unstable and may

collapse unexpectedly, resulting in a sudden drop in

dissolved oxygen concentrations and fish mortality(Delinc, 1992).

On the other hand, phytoplankton serves a number

of very important functions in pond aquaculture. It

is a net producer of dissolved oxygen, which is

indispensable for fish growth and production (Smith

and Piedrahita, 1988; Teichert-Coddington and Green,

1993). It is also the most important sink of ammonia-

nitrogen, which is excreted by fish and poten-

tially toxic (Hargreaves, 1998; Jimnez-Montealegre,

2001).

Most truly herbivorous fish species feed on larger,

benthic, epilithic or periphytic algae, rather than

on phytoplankton (Horn, 1989). Such algae requiresubstrates for attachment, which are virtually absent in

fish ponds. In response to the high nutrient levels that

are maintained by pond fertilisation and fish excretion,

high-density phytoplankton blooms usually develop.

These reduce the light penetration to the pond bottom,

thus preventing the development of benthic algal mats.

If pond algae could be grown on substrates, more

fish species may be able to harvest them, resulting

in a more efficient utilization of primary production.

Communities of attached algae are generally more

stable than phytoplankton and the risk of collapse is

much lower (Westlake et al., 1980). Some studiessuggest that the production of attached algae per unit

water surface area is higher than of phytoplankton

(Wetzel, 1964). Horne and Goldman (1994) stated

that it is mechanically more efficient to scrape or

graze a two-dimensional layer of periphyton than

to filter algae from a three-dimensional planktonic

environment. Considering all these aspects, it might

be advantageous to develop periphyton-based pond

culture.

Objectives and scope of the review

The main objective of this review is to assess the

potential of periphyton-based fish/shrimp production

in aquaculture pond systems on the basis of the

available literature on periphyton productivity and

on periphyton utilization by fish and shrimp. We

present an overview of data on natural and culture

systems where fish utilize periphyton and describe the

species composition of periphyton and the architec-

ture and functionality of the periphyton assemblage.

The productivity of periphyton in natural systems in

relation to environmental factors, substrate types and

grazing are reviewed and the potential productivity in

culture sytems is estimated. We also review the quality

of periphyton as a fish feed and examine the morpho-logical and physiological adaptations of fish for util-

izing periphyton. Data on periphyton grazing by fish

and the effects of grazing on periphyton productivity

are discussed. Based on this body of information,

we estimate potential periphyton-based fish produc-

tion with a simple simulation model. To conclude, we

indicate knowledge gaps for developing periphyton-

based aquaculture and make recommendations for

further research.

Terminology

Throughout this paper, we will use the term peri-

phyton to indicate the assemblage of attached aquatic

plant and animal organisms on submerged substrates,

including associated non-attached fauna. Several other

terms are used with regard to this assemblage. The

most general terms are aufwuchs (often also written

with a capitalized A, from the original German word

Aufwuchs) and biofilm. Some authors prefer to

talk about attached algae, but this disregards the

many other forms that live in periphyton assemblages.

Aufwuchs includes all the organisms that are attached

to, or move upon, a submerged substrate, but whichdo not penetrate into it, whereas periphyton refers

to the total assemblage of sessile or attached organ-

isms on any substrate (Reid and Wood, 1976; Weitzel,

1979). The difference is in the unattached organisms

that are often found in association with the periphyton

assemblage. Sometimes, the terms euperiphyton

(immobile organisms attached to the substrate by

means of rhizoids, gelatinous stalks, or other mechan-

isms) and pseudoperiphyton or metaphyton (free-

living, mobile forms that creep among or within

the periphyton) are used (Weitzel, 1979). The term

biofilm is preferred in other fields of applica-tion, such as wastewater treatment (Cohen, 2001),

drinking water technology (Momba et al., 2000),

food processing (Joseph et al., 2001) and dentistry

(Rosan and Lamont, 2000) and is used mainly for

attached bacteria and protozoa but not algae (OToole

et al., 2000). Other terms used to indicate periphyton

indicate the substrate on which it grows: epiphyton (on

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plants), epipelon (on sediment), epixylon (on wood),

epilithon (on rocks).

Natural and artificial periphyton-based systems

Natural systems with periphyton

Some of the earlier research on periphyton was carried

out in lakes, and it was shown that periphyton can

have an important share (42.4% for the lake studied)

in the total annual production, especially in shallow

lakes with large littoral zones (Wetzel, 1964). In five

oligotrophic lakes, periphyton contributed 4397% of

the total productivity in the shallow (23 m) zone

(Loeb et al., 1983). In the littoral zone, periphyton can

grow on rocks and sediments but also as epiphyton on

macrophytes.

In natural, unpolluted streams periphyton density

is highest in the mid-waters, where currents are

moderate and erosion and deposition are balanced.

Nutrients, imported from upstream, are absorbed by

periphyton attached in locations with sufficient light

for photosynthesis, such as rocks or the stream bed.

Upstream, the current is stronger, erosional processes

and allochthonousinputs are more important, nutrients

are scarce and shredders dominate the food chain. In

the lower reaches, the currents are slow and deposition

processes are dominant. In this nutrient-rich environ-

ment phytoplankton thrives (Welcomme, 1985). An

example of this trophic gradient was shown in a study

of a grassland stream in New Zealand, where nitrate

concentrations increased along a downstream gradientwhich was reflected in the species composition and

biomass of the periphyton (Biggs et al., 1998a, b). The

key feature of periphyton in running water environ-

ments is its ability to utilize scarce nutrients in a fixed

position favourable for photosynthesis. Once trapped,

nutrients can be recycled within the periphyton

assemblage. A model of periphyton biomass with

nutrient concentration and water velocity as main

driving variables gave good results when compared

to field data from a river in Argentina (Saravia et al.,

1998).

In marine habitats, periphyton is also found inlittoral zones (mangrove forests, estuaries) and on

coral reefs. On coral reefs, the accumulation of

organic material is facilitated by the combination of

a high primary productivity of the attached algae with

nitrogen fixing by cyanophytes, the capture of N from

the surrounding ocean and the recycling of nutri-

ents within the reef. This explains how a relatively

high fish biomass can be sustained in oligotrophic

water. An important part of the primary produc-

tion is transferred to the coral host in the form of

organic exudates. The main limiting factor is the

surface area available for photosynthesis by attached

primary producers (Longhurst and Pauly, 1987). A

range of herbivores, including fish, echinoids, andother invertebrates, graze on coral reef algae. Exclu-

sion experiments with cages have shown that intense

grazing by fish or sea urchins leads to reefs with a

less diverse (in terms of species), lower algal biomass

dominated by the smaller turfs and crustose coral-

lines than ungrazed reefs (Ogden and Lobel, 1978;

Hatcher, 1983; Steneck, 1988). Primary productivity

in coral reefs is very high, but values probably depend

a lot on the part of the reef where the measure-

ment was done (depth, exposure to currents). Algal

productivity is generally believed to increase when

the standing crop is reduced by grazing, because

of reduced self-shading, enhanced nutrient exchangewith the water and maintenance of the plants in the

exponential growth phase (Ogden and Lobel, 1978;

Hatcher, 1983).

Brush-park fisheries

Brush-park fisheries are practiced in a large number

of countries and areas: West Africa, Madagascar,

Sri Lanka, Mexico, Bangladesh, Cambodia, China,

and Ecuador (Kapetsky, 1981). It is a traditional

technology that shares features of both capture

fisheries and aquaculture. Research on brush-parkfisheries in the United States was done as early as the

1930s (Rodeheffer, 1940; in Pardue, 1973). Current

examples are the katha fishery in Bangladesh (also

called jhag, katta or jhata; Wahab and Kibria,

1994), samarahs in Cambodia (Shankar et al., 1998),

and athkotu in Sri Lanka (Senanayake, 1981).

Kathas are constructed from the branches of trees

such as hizol (Barringtonia sp.), jamboline (Eugenia

sp.) or acacia (Streblus sp.). Branches are piled up

between a number of bamboo poles fixed in the bottom

to maintain the structure and delimit the area of the

katha. Kathas are usually built in secondary riversor canals in floodplain lakes. Water hyacinth may be

used to cover the katha. The whole structure can be

69 m long, 26 m wide and approximately 1.25 m

deep. Kathas are usually operated for 57 months

each year, during which period they are fished 3

4 times, principally between September and March

when water levels recede and the water becomes cool.

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For fishing, the katha is encircled with a net and all

branches are removed. Fishing the whole katha may

take several days and usually involves 45 persons

using scoop nets. Harvests range from 100 to 1000

kg, depending on the size of the katha. Similar fish-

eries are the kua fishery where branches are placed

in natural or excavated depressions at the beginningof the rainy season (Wahab and Kibria, 1994) and the

juk fishery in Kaptai Lake (Ahmed and Hambrey,

1999).

Much better studied are the acadjas in West

African coastal lagoons. An acadja is an artificial

reef made of tree branches (mangrove poles or some-

times palm trunks) and installed in water of about

11.5 m deep. It attracts fish and is colonized by

periphyton which serves as food for the fish. Most

important species in the brush-park fisheries of Benin

are the blackchin tilapia (Sarotherodon melanotheron,

Cichlidae) and bagrid catfish (Chrysichthys nigro-

digitatus, Bagridae), but many other species arereported from other countries. After encircling the

acadja with a net and removing the branches, the fish

are removed, if necessary using traps or baskets. In

some areas, fishing is done by hook and line. Apart

from attracting fish from outside, fish also reproduce

inside the acadja system. Production figures reported

are high, from 420 t ha1 y1 (Welcomme, 1972;

Hem and Avit, 1994).

Because of their profitability, acadjas proliferated

in West-Africa which led to resource use conflicts:

competition with navigation for space in the lagoon,

competition with capture fisheries for wild fish stock(although it is also claimed that brush-parks may be

beneficial to capture fisheries because fish disperse

to adjacent open waters; Welcomme, 1972). There

are also negative environmental impacts, such as

increased silting in the lagoons due to accelerated

sedimentation around the brush-parks, organic pollu-

tion caused by the decaying branches in the water,

increased erosion as a result of deforested catchment

areas and a net export of nutrients in areas of intense

acadja harvesting (Durand and Hem, 1996; Weinzierl

and Vennemann, 2001).

An experiment with the so-called acadja-enclos

was reported by Hem and Avit (1994). They compared

three 625 m2 enclosures in the Ebri lagoon in Ivory

Coast (salinity 09): one empty, one with a 100 m2

acadja of Echinochloa pyramidalis (a floating macro-

phyte), and one with a 100 m2 traditional acadja

made of the usual tree branches. Fish recruited to

these systems naturally by swimming through the 14-

mm mesh surrounding nets. After 12 months, total

biomasses of 11.7, 18.2, and 80.5 kg, respectively

were harvested from the three enclosures. Blackchin

tilapia was the dominant fish species in the enclosure

with tree branches. Subsequent trials with different

sizes of acadja-enclos (2002500 m2) yielded on

average 1.8 t ha1

. Because of the high requirementsfor wood, additional trials with bamboo sticks (10

sticks m2, appoximately 6 cm diameter) were done,

leading to average yields of 8.3 t ha1. The authors

expect even higher yields if a scheme of successive

selective harvesting would be employed.

Similar experiments in Sri Lanka using different

mangrove and non-mangrove tree species to construct

brush-parks of 4-m diameter resulted in comparable

yields (extrapolated: 2.312.9 t ha1 y1, assuming

10 productive months per year and depending

on substrate species) of mainly green chromide

(Etroplus suratensis, Cyprinidae), streaked spinefoot

(Siganus javus, Siganidae), dory snapper (Lutjanusfulviflammus; Lutjanidae), prawns (Penaeus spp.,

Metapenaeus dobsoni and Macrobrachium spp.) and

ornamental fish (Costa and Wijeyaratne, 1994).

Traditional aquaculture systems in southeast Asia

In the Philippines, Indonesia, and Taiwan, the tradi-

tional culture system for milkfish (Chanos chanos,

Chanidae) in coastal ponds was based on mats

of benthic algae, protozoa, and detritus (in the

Philippines called lablab) stimulated by organic

fertilization. In Indonesia, mangrove leaves (notablyAvicennia sp.) and twigs were used, whereas in

the Philippines green manures or copra slime were

applied. Inorganic fertilization was rare. Supple-

mentary feeding with rice straw, rice bran, oil

cake, wheat starch, water hyacinth, or other macro-

phytes was sometimes applied. In the shallow ponds

(0.30.7 m water depth), a thick mat of algae

developed, consisting of cyanobacteria (e.g., Oscilla-

toria, Lyngbya, Phormidium, Spirulina, Micro-

coleus, Chroococcus, Gomphosphaeria) and diatoms

(e.g., Navicula, Pleurosigma, Nastogloia, Stauroneis,

Amphiora, Nitzschia and Gyrosigma). Other benthicflora and fauna as well as filamentous green algae were

also ingested. Apart from the target species, about

20% of the harvest in Java could consist of prawns

(Huet, 1986). Benitez (1984) distinguished between

floating lablab that contained 15% protein (ash-free

dry matter basis), and benthic lablab with only 6%

protein, and reports an observation by fish farmers

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that milkfish consuming filamentous green algae had

a slower growth rate than fish eating lablab consisting

of unicellular algae and diatoms. Nowadays, many

of these traditional systems have been replaced by

deeper milkfish ponds with higher stocking density

and supplemental feeding.

Another traditional culture system is rice-fishculture. Rice fields harbour lots of phytoplankton and

filamentous higher algae, but studies on rice field

ecology do not reveal much about periphyton (Roger,

1996). Not much is known about the natural feeds

consumed by fish in these systems. Periphytic detrital

aggregate was the most important item in the food

of Nile tilapia and common carp in rice fields in

northeast Thailand (Chapman and Fernando, 1994).

Fish feeding on periphyton on rice plants could be

so vigorous that the rice plants were observed to be

shaking (Chapman, 1991). Similar observations are

reported from rice-fish studies in Vietnam (Rothuis et

al., 1999) and in Bangladesh (Gupta et al., 1998).

Aquaculture experiments

Pardue (1973) reports two experiments from Alabama

with bluegill (Lepomis macrochirus, Centrarchidae)

grown in plastic circular tanks (stocking density 2

m2, 3 m diameter, water volume 5,400 l) equipped

with yellow pine boards with surface areas equivalent

to 20, 40, 60, 80 and 100% of the tank surface. Fish

production increased linearly with increasing surface

area, and the highest mean yield with 100% surface

area added was 384 kg ha1

of bluegill in 180 days(y = 243.34 + 1.408x, in which y = fish yield [kg]

and x = added substrate [%]). In a subsequent exper-

iment with 40 and 100% additional surface area and

three levels of fertilization, there was no difference

between the two levels of added surface but a strong

effect of fertilization with the highest yields (mean:

430 kg ha1) achieved under complete fertilization

(8-8-2 NPK at 112 kg ha1, applied 6 times during

the 180 days culture period). The increase in bluegill

production was linked to the increase in macroinverte-

brates on the substrates, notably Diptera, Hemiptera,

Odonata, and Plecoptera.

Cohen et al. (1983) added substrates to 200-m2

ponds for freshwater prawn (Macrobrachium rosen-

bergii) in Israel in an attempt to mimic the natural

conditions for prawn growth by increasing the avail-

able surface area. The substrates consisted of two

horizontal layers of plastic 2-cm mesh nets with

corrugated plastic pipes attached to them (8-cm

diameter, 1520 cm long). Each pond was stocked

with 2,000 juvenile prawns (2 g) and 15 common

carp (100 g). The ponds were fertilized with chicken

manure and a 25% pellet feed was given during the

night. Selective harvesting of large prawns was done

to prevent suppression of growth caused by territorial

behaviour. The total marketable yield from ponds withsubstrates was 2,850 kg ha1 in six months, against

2,500 in ponds without substrates. The average final

weight of the prawns was also higher with substrates

(40.3 g versus 35.8 g). There was no difference in

survival.

Another set of experiments with Macrobrachium

rosenbergii was done at Kentucky State University. In

a first experiment, substrates consisting of 3 horizontal

levels of plastic mesh sheet, suspended 30 cm apart in

a PVC pipe frame were placed in 400 m2 ponds. The

substrates increased available surface area by about

20%. The substrates increased the yield of prawns

stocked at 0.33 g individual weight and 59,280 indi-viduals ha1 from 1,060 kg ha1 (without substrates)

to 1,268 kg ha1 in 106 days. Mean size at harvest

was also bigger (37 g against 30 g without substrates)

and the number of mature females increased with

substrates (Tidwell et al., 1998). In a similar experi-

ment with two stocking densities and substrates that

added 80% of available surface area to the ponds, the

substrates produced an increase in yield from 1,243

to 1,469 kg ha1 in 95 days (averages for stocking

densities 60,000 and 120,000 ha1, size at stocking =

0.24 g) and a decrease in food conversion ratio from

2.9 to 2.4 (Tidwell et al., 1999). In a third experi-ment, the density of the substrates was varied at a

fixed prawn density of 74,000 ha1 with a stocking

weight of 0.24 g. Substrates consisted of 120 cm wide,

7.03.5 cm mesh polyethylenepanels suspended hori-

zontally across the ponds, 30 cm above the bottom

with 30 cm between the layers when multiple layers

were installed. Three treatments were created, corre-

sponding to surface area increases of 0, 40 and 80%.

There was a positive linear response of yield on

substrate density: Y (kg/ha) = 1466.3 + 4.4604 X

(% increase in surface). Feed conversion ratio was

inversely related to increase in surface area: Y (FCR)

= 2.784 0.0052 X (% increase in surface). There

was no significant difference in individual weight at

harvest (Tidwell, 2000).

Bender et al. (1989) produced a microbial mat by

applying32 g dm m2 of native (Dominican Republic)

grass clippings to small (3014 cm), shallow (4.5 cm)

laboratory ponds and inoculating with Oscillatoria sp.

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The experiment was repeated with larger (135 m)

concrete pondsand a water depth of 20 cm. Apart from

the microbial layer growing on the grass substrate, a

detrital gelatinous deposit developed on the pond sedi-

ment. Weight gain of 0.53.5 g Nile tilapia in a 2-week

growth trial was higher with the silage-microbial mat

than with a commercial catfish feed applied at 3% BWday1. For the microbial mat to induce fish growth, it

had to be offered in the same pond where it was grown.

Microbial mats or detrital material grown in one pond

and offered to fish in another pond (either with or

without sediment) did not induce growth in the fish. It

was concluded that the integration of the detrital mate-

rials with the biomass is crucial for the productivity of

such a system. Phillips et al. (1994) obtained signifi-

cant differences in final individual weight of Nile

tilapia between ponds with and without similar micro-

bial mats, but no data on mortality and total yield were

given.

At the Asian Institute of Technology in Bangkok,Shrestha and Knud-Hansen (1994) carried out two

experiments in concrete tanks (2.521.1 m) using

vertically suspended corrugated plastic sheets (7.7

m2 extra surface area per tank) as substrates with

inorganic fertilization (2.1 g N m2 week1). Sex-

reversed all-male Nile tilapia were stocked (20 g

fish1, 3 fish m2) for 56 days. Microscopic examina-

tion of gut contents and periphyton, as well as obser-

vation of feeding behaviour showed that the fish were

feeding on the periphyton. In the first experiment, net

fish yields were not significantly different between

tanks with and without substrates (1.05 and 0.88 gm2 d1, respectively). Although there was a differ-

ence in mean periphyton density between tanks with

and without fish (0.78 and 0.93 mg dm cm2, respec-

tively), the difference was not statistically significant.

The second experiment compared the plastic substrate

with a similar surface area of bamboo poles. Fish yield

was higher with bamboo poles than with plastic (3.43

and 2.51 g m2 d1, respectively) and on this occa-

sion, there was significantly less periphyton in tanks

with fish (0.80 against 1.71 mg dm cm2 on plastic

sheets; bamboo substrates were only tested with fish).

From the data on dry matter and ash-free dry matter,

it can be seen that the ash content of the periphyton

(appr. 50%) was consistently much higher than in the

suspended solids (consisting mainly of phytoplankton;

ash appr. 20%), with the exception of periphyton on

bamboo (appr. 20%).

Shankar and co-workers (Shankar et al., 1998;

Ramesh et al., 1999; Umesh et al., 1999) used

bio-degradable substrates to stimulate fish produc-

tion. In two preliminary experiments, production

of common carp, Mozambique tilapia (Oreochromis

mossambicus, Cichlidae) and rohu (Labeo rohita,

Cyprinidae) in concrete tanks was 4550% higher

with sugarcane bagasse as a substrate, compared to

control tanks without substrates. A subsequent biggertrial compared dried sugarcane bagasse, paddy straw,

and dried water hyacinth (Eichhornia sp.) leaves,

suspended as 6090 cm bundles in 25 m2 concrete

tanks with soil bottoms. Substrate density was 12.5

kg tank1 (as the authors stressed the biodegradability

of the substrates, substrate density was expressed as

mass per surface area) and the tanks were fertilized

with cow manure and urea. A mixture of common carp

(2.1 g) and rohu (1.5 g) were stocked at 13 and 12 per

tank, respectively. The experiment lasted 133 days and

on day 70, 7.5 kg of fresh substrates were suspended.

Both rohu and common carp grew best in the treat-

ment with sugarcane bagasse, yielding 3,088 g tank1,against 2,873, 2,403 and 1,865 g with paddy straw,

Eichhornia and in tanks without substrate, respec-

tively. In all substrate treatments, fish survival on

average was higher than in the control (85.793.7%,

versus 81.3%). The increased fish production could, in

the absence of major differences in dissolved oxygen

and ammonia concentrations, be attributed to the

periphyton on the substrates, as shown by total plate

counts of bacteria in the water and on the substrates,

and by phytoplankton and zooplankton enumeration.

The superior fish production with bagasse was attrib-

uted to its higher fibre content and surface area,favouring better bacterial growth and subsequent fish

production than the other two substrates. Zooplankton

density in the water of the bagasse treatment was

higher than in the other treatments. The authors

concluded that biodegradable substrates led to better

results than less degradable substrates like bamboo or

non-biodegradable substrates like PVC and plastic.

Bratvold and Browdy (2001) studied changes in

water quality and microbial community activity due

to AquaMatsTM substrates (3.4 m2 per m3 tank)

added to polyethylene tanks stocked with Litopen-

aeus vannamei postlarvae. Shrimps were fed with

a commercial feed. Compared to tanks with only

sand sediment or tanks without sediment, tanks

with substrates and sand sediment had a higher pH

and higher total photosynthesis, lower abundance of

pelagic bacteria and phytoplankton, lower turbidity,

lower ammonia and orthophosphate and higher nitrifi-

cation. Shrimp production was significantly higher

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(1.69 vs. 0.981.07 kg m2 in 100 days) and

food conversion ratio (feed given/shrimp produced)

was significantly lower (1.5 vs. 1.92.1) in the

tanks with substrates compared to the tanks without

substrates.

Keshavanath et al. (2001b) reported an experi-

ment with different types of substrates for enhancingthe production of mahseer (Tor khudree, Cyprinidae).

Bamboo poles, PVC pipes and sugarcane bagasse

substrates were placed in 25 m2 concrete tanks with

mud bottoms and fingerlings of about 3 g were stocked

at densities of 1, 1.5, and 2 fish per m2. After 90 days,

the highest net production with bamboo substrate

was 447 kg ha1 at the highest fish density, against

399 kg ha1 with the PVC pipes. In the bagasse

treatment, all fish died due to low oxygen concen-

trations. In subsequent experiments (Keshavanath et

al., 2002) using the same tank systems, the effects

of periphyton, supplemental feeding and their combi-

nation were investigated using mahseer (3.5 g) orthe fringed lipped peninsula carp (Labeo fimbriatus,

Cyprinidae) (0.73 g), both stocked at 25 per tank.

Two densities of bamboo poles (98 or 196 poles per

25 m2) were compared with tanks without substrates.

Periphyton alone and feeding alone led to compa-

rable fish yields that were significantly higher (by

3075%) than the yields obtained without periphyton

or feed. There was a significant effect of substrate

density on fish survival in both species. The combi-

nation of periphyton and feeding resulted in even

higher yield increases (5487%). The higher substrate

density improved yield only slightly without feedingand not at all with feeding, suggesting that at the

fish stocking density used the carrying capacity of

the periphyton was never exceeded. A similar exper-

iment with red tilapia (Oreochromis mossambicus

O. niloticus hybrid) gave similar results, with even

more pronounced differences between tanks with and

without substrates. Highest tilapia yields were 1,834

g per 25 m2 without, and 2,142 g per 25 m2 with

feeding in 75 days (Keshavanath, unpublished results).

Sugarcane bagasse was again used as a substrate in

a farm trial. At densities of 156 bagasse bundles

(about 28 kg) per 100 m2, total fish yield of catla

(Catla catla, Cyprinidae), rohu, and common carp was

13,104 and 14,842 g per 100 m2 in 180 days without

and with feed, respectively, compared to 8,076 g in the

control without feed or periphyton (Keshavanath et al.,

2001a).

At the Bangladesh Agricultural University in

Mymensingh, Bangladesh, research on periphyton

started with a series of monoculture experiments in

which the performance of several fish species with

periphyton was assessed. An experiment in six 75

m2 ponds compared the production of 2.1 g orange-

fin labeo (Morulius calbasu, Cyprinidae) stocked at

1 m2 with and without substrates made of kanchi

(bamboo trimmings). While no differences in waterquality were observed, survival (8790% versus 72

77%) and fish growth were higher with substrates

than without, resulting in a net yield of 713 kg ha1

versus 399 kg ha1 in 120 days. These relatively

low yields could be explained by the sub-optimal

water temperatures that prevailed during the experi-

ment (23.632.7 C) (Wahab et al., 1999). Rohu (10 g

at 1 m2) and kuria labeo (Labeo gonius, Cyprinidae;

4 g at 1 m2) were then grown with bamboo substrates

at 9 poles m2 (but leaving the pond perimeter free;

total substrate area was about 75 m2). Net rohu yield

with substrate was significantly greater (1,901 kg ha1

in 120 days) than without substrate (1,073 kg ha1).For kuria labeo (separate experiment), yields were not

significantly different (794 and 788 kg ha1, respec-

tively, in 120 days). The kuria labeo were never

observed to feed actively on the periphyton whereas

rohu could be clearly seen eating the periphyton(Azim

et al., 2001a). It was concluded that rohu and orange-

fin labeo are more suitable candidates for periphyton-

based aquaculture than kuria labeo.

To utilize the other food sources in the pond

(plankton, detritus) as well, polyculture of rohu and

catla was investigated. With different stocking ratios

and bamboo substrate, the growth and total yields fromany combination of rohu and catla were higher by 3

40% (individual weight) and 50300% (total yield),

than those from rohu or catla in monoculture. The

combination of 60% rohu and 40% catla was optimal,

resulting in a net yield of 586 kg ha1 70 d1 (Azim

et al., 2002a). Using this stocking ratio, another trial

with and without bamboo substrates was carried out to

verify the effect of substrates with this species combi-

nation. Survival, growth rate, individual weight at

harvest as well as net yield of both rohu and catla,

were significantly higher in the ponds with bamboo

substrates. In ponds with substrate, not only the

net yield of periphyton-feeding rohu was higher (by

160%), but also that of surface-feeding catla (220%).

The combined net production of the two species in

ponds with substrates was 180% higher (1,652 kg

ha1 90 d1 on average) than that of control ponds

(577 kg ha1 90 d1). In the same experiment, it was

found that the addition of 15% orange-fin labeo to the

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10

optimum mix of catla and rohu further increased total

production by 40% (2,306 kg ha1 90 d1) (Azim et

al., 2001b).

Water treatment with periphyton

Some investigators have attempted to use fish andperiphyton for removing nutrients from wastewater.

The nutrients acccumulated in the periphyton are

removed by harvesting the periphyton. This can be

done manually or mechanically, but fish can also be

used. Drenner et al. (1997) used Mozambique tilapia

and central stoneroller (Campostoma anomalum,

Cyprinidae) for harvesting periphyton grown in

circular tanks fed with mock wastewater treatment

plant effluent. Maximum removal rates were 48.4

mg total phosphorous (removed in sediments and fish

biomass) per m2 water surface per day.

Periphyton productivity

Development of the periphyton assemblage and

species composition

Development of a periphyton layer on a clean surface

generally starts with the deposition by electrostatic

forces of a coating of dissolved organic substances

(mainly mucopolysaccharides), to which bacteria are

attracted by hydrophobic reactions (Hoagland et al.,

1982; Cowling et al., 2000). The presence of free-

floating organic micro-particles in eutrophic watersstimulates this process. Bacteria actively attach using

mucilaginous strands. This can take a week, but in

some studies this was observed within days and even

within a matter of hours. It is not clear whether

bacterial colonization is a prerequisite for subsequent

attachment of other organisms, or what the exact

role of the bacteria is in this process (Hoagland

et al., 1982). In the days that follow, algae start

to grow. Low-profile diatoms appear first, followed

by small pennate diatoms, short-stalked and longer

stalked species and then by diatoms with rosettes

and mucilage pads. In the final stages of develop-

ment, species of green algae with upright filaments

or long strands can grow (Hoagland et al., 1982;

Horne and Goldman, 1994). On plants, the periphyton

community is attached on gelatinous stalks of algal

and bacterial mucus interspersed with deposits of

calcium carbonate (Wetzel, 1975).

In their study with grass silage, Bender et

al. (1989), using colony counts of nitrogen-fixing

bacteria, microscopic identification of species and

characterization of chemotactic response on agar

plates, showed that a chemotactic response to the lactic

and acetic acid in the silage from bacteria in the sedi-

ments was the first step in the colonization of the grasssubstrates. The bacteria bloomed in the water and

produced slimy exudates that annealed the mirobes

to the silage after which cyanobacteria invaded the

substrates and caused a further increase in biomass.

Using microprobes, it was shown that within the mat,

different heterogeneous micro-environments existed

with oxic and anoxic zones.

Periphyton organisms have various ways of

attaching to the substrate: stalks with sticky ends

(e.g. ciliates), sticky capsules (bacteria and bluegreen

algae), cushions of filaments (seaweeds, algae and

aquatic mosses), muscular suction pads (snails),

glue (barnacles) or simply clinging to the substrate(e.g., insect larvae). Attachment to sediment can

be achieved by rooting (especially higher plants),

rhizoids (seaweeds on corals), and with a muscular

foot (clams) (Reid and Wood, 1976).

During the development of the periphyton layer,

conditions for growth of the various algal species

change drastically. As the density of organisms

increases, there is more competition for substrate

surface area and this affects the composition of the

periphyton community. The organisms also compete

for carbon dioxide, nutrients and light. This explains

the development of the periphyton layer away from thesubstrate, resulting in something that can be compared

to the canopy of a terrestrial forest (Hoagland et

al., 1982; Figure 1). Another strategy for ensuring

optimal nutrient and light conditions is shown by

pennate diatom and cyanobacterial cells that can move

around the substrate. They glide by excreting a

polysaccharide mucilage that sticks to the substrate

(diatoms) or by using contractile fibrils in their cell

walls (cyanobacteria). In this way, they can move

away from areas where light or nutrients have become

limiting (Horne and Goldman, 1994). It is prob-

ably also a way to escape being covered by sedi-

ment deposits (Hutchinson, 1975). Some diatoms

remain at the base of the periphyton assemblage

throughout its development, withstanding extremely

low light conditions, while other species move around

the periphyton layer looking for the best conditions

available (Johnson et al., 1997).

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11

Figure 1. Range of vertical structure in the periphyton community. Drawn to scale, 400. From: Hoagland et al., 1982. Reproduced withpermission, Botanical Society of America.

Coral reef algae can be classified as algal

turf (110 mm), macroalgae or fleshy algal pave-

ment (larger than 10 mm) and crustose algae

(encrusting noncalcified algae or calcareous, crustose

corallines) (Steneck, 1988). The epilithic algal

community (EAC) of coral reefs generally consists

of a mixture of turf and crustose algae (Klumppand McKinnon, 1992). Species diversity within the

periphyton assemblage can be high. Planas et al.

(1989) found on average 41 different species in

periphyton on ceramic tiles. Wahab et al. (1999)

encountered 12 genera of Bacillariophyceae, 25 of

Chlorophycaea, 10 of Cyanophyceae, 4 of Eugleno-

phyceae, 1 of Rhodophyceae and 5 of zooplankton in

periphyton, as well as a variety of macrobenthic organ-

isms, notably chironomid larvae on scrap bamboo

substrates in fishponds in Bangladesh. Lam and Lei

(1999) found 81 algal species in periphyton on glass

slides in the Lam Tsuen River, Hong Kong. Konan-

Brou and Guiral (1994) found 24 species of algae inthe periphyton community on bamboo substrates in

acadjas in Cote dIvoire.

Often, one or a few species of algae dominate the

assemblage. The EAC of coral reefs is often domi-

nated by filamentous Chlorophytes and Rhodophytes

(Klumpp and McKinnon, 1992). On plastic substrates

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in tilapia cages in Bangladesh, filamentous Chloro-

phyceae and Myxophyceae dominated the periphyton

before fish stocking, whereas after stocking of the

fish diatoms became more important. Diatoms could

be attached directly to the substrate but were also

found as epiphytes on larger filamentous algae. In

addition, freshwater oligochaetes, Protozoa, Rotifera,and coelenterate Hydrozoa were observed (Huchette

et al., 2000). Dominant species in the periphyton of

the acadjas were the filamentous algae Rhizoclonium

riparium (Chlorophyceae) and Lyngbia (Cyano-

bacteria) in the surface layers, and Audouinella

daviesii (Rhodophyceae) in the deeper layers. Diatoms

of the genera Nitzschia and Melosira grew on the

filamentous algae (Guiral et al., 1993; Konan-Brou

and Guiral, 1994). In eutrophic Lake Valencia,

Venezuela, the basis of the periphytic detrital

aggregate (PDA) growing on macrophytes (Potamo-

geton sp.) consisted of filamentous cyanophytes,

which were also the dominant phytoplankton in thelake. Diatoms, bacteria, and amorphous detritus were

the main components of the periphyton attached to the

cyanophyte matrix (Bowen, 1979).

The composition, biomass, and productivity of the

periphyton community vary with season, year, loca-

tion, and grazing pressure. On coral reefs, the structure

of the community is often different depending on

whether it is located on the reef flats, the inner or

outer shelf, windward or leeward side, and shallow

or deep parts and slopes (Klumpp and McKinnon,

1992). Within a single water body, there can be

a considerable overlap in species between phyto-plankton and peripyton (Havens et al., 1996). In fish-

ponds in Bangladesh, 23 of the 39 genera found in the

periphyton were common to the phytoplankton (Azim

et al., 2001a).

The algal contribution to the dry matter can be

estimated from the ratio of ash free dry matter to

pigment, called the autotrophic index (AI, mg ash free

dry matter/mg chlorophyll a on a given surface area;

APHA, 1998). Huchette et al. (2000) reported that AIs

were between 150 and 300 in ungrazed conditions and

remained stable around 300 when grazed. Azim et al.

(2002b) reported AI values ranging from 189 to 346 in

freshwater fertilized ponds without fish, depending on

substrate. The values decreased with time, indicating

that ash-free dry matter of non-algal origin dominated

in young periphyton. In general, 1 mg chlorophyll a is

equivalent to 6585 mg algae (Dempster et al., 1993;

APHA, 1998), so more than 50% of the periphyton

ash-free dry matter is not of algal origin.

Biomass and productivity

Table 2 gives an overview of the standing crop of

periphyton found on different substrates in various

kinds of natural and culture systems. Because of

the wide variation in methods used, environments,

and species composition it is difficult to comparethe biomass figures directly. The biomasses found

in natural systems (streams, lakes, and coral reefs)

can be greater than those found in brush-parks and

culture systems (tanks and ponds). In a comparison of

periphyton biomassin a wide range of natural systems,

biomasses of up to 2350 mg m2 chlorophyll a were

reported (Westlake et al., 1980). The main reasons for

this are probably the higher fish densities and asso-

ciated grazing intensity in culture systems, although

biomasses in culture systems without fish were still

lower than those in, for example, coral reefs. Likely,

there is a strong effect of the algal species composi-

tion, with especially high biomasses attained by thefleshy macroalgae that make up the algal assemblage

on lightly grazed coral reefs. Furthermore, substrate

type has a strong effect on the density of periphyton,as

shown by the differences between different substrate

types in experiments in India and Bangladesh (Azim

et al., 2002b; Keshavanath et al., 2001b).

Table 3 shows data on periphyton productivity.

Again, comparisons are difficult, but the productivity

of the acadja systems was the highest (7.9 g C

m2 d1). In this study, not only area-specific but

also chlorophyll-specific productivity was higher in

periphyton than in phytoplankton (highest contrastmeasured on one day was 22.5 and 5.9 mg C (mg

chla)1 h1 for periphyton and phytoplankton, respec-

tively) (Guiral et al., 1993). Coral reefs are also

very productive, with net productivity of up to 3

g m2 d1 (see Table 3). Productivity measure-

ments were much lower in temperate lakes. An inter-

mediate estimate (1.7 g C m2 d1) was made in

a pond in Bangladesh (Azim et al., 2002b), based

on the periphyton biomass after the first two weeks

of colonization on clean bamboo substrates. The

other measurements were made using UV-transparent

respiration chambers.

Effects of environmental factors

Nutrients

Inorganic nutrients can have a strong effect on

periphyton biomass, as shown by numerous enrich-

ment studies in both natural and artificial systems

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Table2.Periphytonbio

mass(standingcrop)invariousnaturalandar

tificialsystems.Biomassisexpressedasgash

-freedrymatter(AFDM),orasmgchlorophylla(Chl-a)perm2of

substratearea

Systemtype

Su

bstratetype

AFDM

Chl-a

Remarks

Reference

(gm2)

(mgm

2)

Stream/river

Na

turalstones

048.4

Glacialstream,Antarctica

IzaguirreandPizarro,1998

Ce

llulose-acetate

0.820

7.14

6

Amazonriver,dependingonzone

Putz,1997

Wood

733

1342

Billabong(lenticsystem),onEucalyptu

scamaldulensis

ScholzandB

oon,1993

Lake/reservoir

Sediment

76.83

97

Nutrient-poorvs.enrichedsites,NorthernOzarks,Missouri(USA)

Lohmanetal.,1992

X-

rayplates

0.43

3.1

Oligotrophiclakewithtroutfarm,Patagonia,Argentina.Higher

BafficoandPedrozo,1996

valuesnearfishfarm

Na

turalstones

350

2100

10lakesofvaryingtrophy,Quebec,Canada

Cattaneo,19

87

De

adreedstems

550

Oligo-mesotrophiclake,Netherlands.L

owbiomassinsummer,

MeulemansandHeinis,1983

highinwinter

Coralreef

Co

ralrock

132683

Dependingonalgaltype,Bonaire(NetherlandsAntilles)

Bruggeman,

1995

Co

ralrock

400700

6001200

Macroalgae,GreatBarrierReef

Hatcherand

Larkum,1983

Co

ralrock

160240b

Mainlycrustoseandturfalgae,GreatB

arrierReef.

Klumppand

McKinnon,1992

Brush-park

Ba

mboo

0.238

2.51

54

DependingondepthandlocationEbrie

Lagoon,Cote

Konan-Brou

andGuiral,1994

dIvoire

Ba

mboo

7.225

4.11

67a

Dependingondepthandsalinity/season

,EbrieLagoon,CotedIvoire

Arfietal.,19

97

Culturesystem

Plasticsheets

3.39.5

5m2concretetanks,AsianInstituteof

Technology,Bangkok

Shresthaand

Knud-Hansen,1994

Ba

mboo,kanchi,hizol

19113

125228

75m2fishpondsMymensingh,Bangladesh

Azimetal.,2002b

Ba

gasse,bamboo,PVC

718

60300a

25m2concretetanks,Mangalore,India.Highervalueswithoutfish

Keshavanath

etal.,2001b,2002

Watertreatment

Glass,sanddisk,

70100

Biofilmonbiologicalreactorswithlayerofsubstrate,value

Apilanezetal.,1998

system

Ac

tivecarbon

dependingonsubstrate

aTotalpigment(chlorophylla+pheophyton).bConvertedfromgcarbonbyassuming48%Cinafdm.

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Table 3. Net productivity of periphyton in various natural and culture systems. Productivity is expressed as g C m2 d1 and was convertedto these units assuming 50% C in dm and 12 hours active feeding per day

System type Substrate type Net productivity Remarks Reference

(g C m2 d1)

River Cellulose-acetate 0.0821.04 Amazon river, depending on zone Putz, 1997

L ake 0.020.20 Littoral zone, 5 oligotrophic lakes, California/Nevada Loeb et al. , 1983USA

Reed stems 0.140.72 Littoral zone, oligo-mesotrophic lake, Netherlands Meulemans and Heinis, 1983

Debris and plants 0.73 Whole-lake estimate, large shallow lake Wetzel, 1964

Coral reef Coral rock 0.61.1 Great Barrier Reef. Lower on slopes, higher on flats Klumpp and McKinnon, 1992

Coral rock 1.792.15 Great Barrier Reef. Damselfish territories Klumpp and Polunin, 1989

Coral rock 1.982.00 Papua New Guinea, turf algae Polunin, 1988

Coral rock 1.83.1 Virgin Islands Carpenter, 1986

Coral rock 2.164.80 Several Pacific benthic reef communities Marsh, 1976

Coral rock 0.642.00 Fringing coral reef, Bonaire Van Rooij et al., 1998

Brush-park Bamboo 7.9 Guiral et al., 1993

Fishpond Bamboo 1.7 Estimated from increase in total biomass on clean Azim et al., 2002b

substratesFishpond Grass silage 7.5 Bender et al., 1989

(e.g., Aizaki and Sakamoto, 1988; Lohman et al.,

1992; Ghosh and Gaur, 1994). Periphyton biomass

and productivity can thus be used as indicators of

eutrophication in natural waters (e.g., Mattila and

Raeisaenen, 1998). In a fertilization experiment in

ponds, a quadratic relationship between periphyton

biomass and fertilization level was established, with

a maximum periphyton biomass (mean biomass over6 weeks) of 3.3 mg cm2 dry matter realized with

fertilization rates of 4,500, 150, and 150 kg ha1

of cow manure, urea, and TSP, respectively (equiva-

lent to 1.5 times the standard rate for fishponds

in Bangladesh). Phytoplankton biomass increased

linearly with increasing fertilization rate up to 2 times

the standard rate (Azim et al., 2001c).

Investigative studies of nutrient limitation show

mixed results. In most freshwater studies, phos-

phorous was identified as the limiting nutrient (e.g.,

Ghosh and Gaur, 1994; Vymazal et al., 1994), but

nitrogen (Barnese and Schelske, 1994), carbon

(Sherman and Fairchild, 1989) and silica can also be

limiting, depending on the algal species and on other

environmental factors such as hardness and acidity.

High Si:P and N:P ratios favoured diatoms, and low

N:P and Si:P ratios favoured cyanophytes in a reser-

voir in Patagonia (Baffico and Pedroso, 1996). Simi-

larly, high Si:N or Si:P ratios favoured diatoms, low

N:P ratios favoured cyanophytes and high N:P ratios

favoured chlorophytes in periphyton of the Baltic Sea

(Sommer, 1996).

Whether or not nutrient enrichment stimulates

periphyton productivity also depends on the type of

substrate. Benthic periphyton has an advantage over

phytoplankton because it is closer to the nutrient-rich

sediment and the interstitial pore water, or in the caseof epiphytes to macrophyte nutrients. In a series of

whole-lake experiments, it was shown that periphyton

on sediments utilized the nutrients in the sediment

pore water and therefore, responded much less to

enrichment than periphyton growing on wood in the

same lake (Blumenshine et al., 1997). Epilithic algae

are more likely to become nutrient-limited because

they have to absorb nutrients from the water (Sand-

Jensen and Borum, 1991).

Lower nutrient concentrations do not necessarily

mean lower biomass and productivity. In an experi-

ment with an artificially created upstream-downstream

gradient, there were large differences in nutrient

concentrations between upstream and downstream

parts of the stream, but differences in periphyton

biomass were poorly related to gradient. More nutrient

recycling took place downstream (Mulholland et al.,

1995). Similarly, in a modelling study of nutrient

enrichment in seven New Zealand streams, biomass

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levels in the area studied were lower than predicted

using the calibrated model. This was ascribed to

differences in other factors such as increased grazing,

shading, or differences in substrate characteristics

(Welch et al., 1992).

Apart from the impact of enrichment on

periphyton, periphyton has an effect on the nutrientconcentration in the overlying water. Periphyton

lowered the phosphorous of the overlying water

(Hansson, 1989; Bratvold and Browdy, 2001) and

sediment (Hansson, 1989). By lowering the nutrient

concentration of the water, periphyton can affect

the growth of the phytoplankton, as was shown

in a study in Swedish lakes (Hansson, 1990). In

aquaculture experiments with periphyton, ammonia

concentrations in tanks with periphyton were lower

than in control tanks, indicating a stimulating effect

of the periphyton on nitrification (Langis et al.,

1988; Ramesh et al., 1999; Bratvold and Browdy,

2001).Organic nutrients are also important for the hetero-

trophic components of the periphyton. The activity

of ectoenzymes in a Mediterranean river was higher

during periods of high dissolved organic matter

concentrations (Roman and Sabater, 2000). Such

enzymes are retained in the periphyton layer by

the extracellular polysaccharide matrix (Thompson

and Sinsabaugh, 2000). Similarly, the organic load

caused by decomposing salmon carcasses led to

increased stream periphyton growth (Fisher-Wold

and Hershey, 1999). Pulses of highly available

carbon created a change in the composition ofthe biofilm from chemoautotrophic to heterotrophic

organisms, and biofilms adapted their metabolism to

the prevailing environmental conditions (Battin et al.,

1999; Butturini et al., 2000). The algae from the

periphytonare important suppliers of organic matter to

the heterotrophs. In river biofilms, maximum enzyme

activity was seen with an algal biomas that was

two to three times as high as the bacterial biomass.

Bacteria are likely to utilize the algal exudates and

lysis products, as well as photosynthetically produced

oxygen, whereas algae utilize the inorganic carbon

produced by the heterotrophs (Kuehl et al., 1996;

Roman and Sabater, 2000). Organic matter quality

affects the rate at which it is processed, as shown by

differences in turnover times between two rivers with

different sources of organic matter (Roman, 2000).

Dissolved organic matter may play a role in deter-

mining the structure of the periphyton. Periphyton

communities treated experimentally with dissolved

organic carbon contained less mucilage than untreated

controls (Wetzel et al., 1997).

Grazing

Grazing is the most important determinant of

periphyton biomass. On coral reefs, algal communities

can be grazed down completely by fish or echinoids(e.g. Hixon and Brostoff, 1981; Hay, 1981). Exclusion

and removal experiments on coral reefs showed that

algal standing crop could increase 1.5 to 15-fold when

grazers were excluded (Hatcher and Larkum, 1983).

Not all components of the periphyton assemblage are

equally susceptible to grazing. Diatoms belonging to

the overstory of the periphyton layer were removed

by grazing snails while more prostrate basal cells

(e.g., Stigeoclonium sp.) were unaffected by grazing

(McCormick and Stevenson, 1991; Hill et al., 1992;

Steinman et al., 1992). Generally, periphyton algal

diversity is lower when grazed (Jacoby, 1987; Horn,

1989; Swamikannu and Hoagland, 1989).Most studies of grazing have been done with

invertebrate grazers such as snails and insect larvae

while studies with fish are much less common.

Huchette et al. (2000) compared the species composi-

tion of grazed (four weeks) and ungrazed periphyton

communities on plastic substrates in tilapia cages in a

Bangladesh river. Four weeks after stocking with fish,

the filamentous algae were reduced to short colony

lengths and other species such as Ankistrodesmus

became more important in the periphyton. Grazing

also resulted in a size reduction of epiphytic diatoms.

Grazing tilapia preferred the larger-sized diatoms(Melosira spp., Cycotella spp.) as shown by a larger

proportion of these species in the stomachs. The fish

were not grazing on the periphyton only, as shown

by a higher diversity of diatom species in the fish

stomachs than in the periphyton and the presence of

nanoplankton in the fish stomachs. Grazing resulted in

a much lower standing biomass of periphyton than in

the ungrazed treatment. Similarly, periphyton biomass

was 60% lower on nets in cages stocked with Kariba

tilapia (Oreochromis mortimeri, Cichlidae), redbreast

tilapia (Tilapia rendalli, Cichlidae) and Nile tilapia

than in unstocked cages (Norberg, 1999). However,

in a study with redear sunfish (Lepomis microlophus,

Centrarchidae) and snails, it was shown that the

fish had a positive effect on periphyton biomass by

reducing the grazing on the periphyton by snails.

Nutrient concentrations in the water were also higher

with fish, but this did not affect the periphyton much

(McCollum et al., 1998). In streams in the Ozark

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Mountains (Missouri, USA), stones were covered

by cyanobacteria (Calothrix sp.) when exposed to

grazing by fish and invertebrates. Once protected from

grazing, diatoms would overgrow the cyanobacteria

within four to ten days. Grazing minnows could strip

the diatom layer in a matter of minutes, after which

regeneration of the cyanobacteria happened in 11 days(Power et al., 1988). Predators of grazing fish can also

affect the density of periphyton layers, as shown by

a study in streams in Panama where bands of high

periphyton biomass occurred in the top water layer

(

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branches, jute sticks, or bundles of sugarcane bagasse

are likely to become more important in pond aquacul-

ture. In periphyton research, a variety of other mate-

rials is used including ceramic tiles, glass fibre filters,

and glass slides. For sampling microbial periphyton

communities, and especially protozoa, polyurethane

foam has been recommended (Cairns et al., 1979).Substrate type has a distinct effect on periphyton

growth. A comparison of periphyton growing on

natural (leaves) and artificial (glass slides) substrates

in a Malaysian river, totals of 37 and 35 species,

respectively, were found of which 25 were common

to both substrate types (Nather Khan et al., 1987).

In a study in Swedish lakes, glass tubes supported

greater numbers and a more diverse periphyton

community than wood substrates, whereas plastic

substrates supported only a layer of bacteria (Danilov

and Ekelund, 2001). In studies in India and

Bangladesh, bamboo resulted in greater maximum

densities of periphyton than PVC pipes or sugarcanebagasse bundles (Keshavanath et al., 2001b; Azim

et al., 2002b). The reasons for these differences are

unknown, but they may be attributable to leaching of

nutrients or toxic substances from the substrates, or

differencesin surface roughness. Similarly, the density

of biofilms of Salmonella sp. on plastic, cement,

and steel surfaces was found to differ by an order

of magnitude, probably due to differences in hydro-

phobicity of the materials (Joseph et al., 2001). Most

microorganisms are hydrophilic and probably adsorb

more strongly to hydrophobic surfaces (Cowling et

al. 2000). Generally, plastic and PVC also performedworse in terms of fish production than bamboo or

tree branches (Shrestha and Knud-Hansen, 1994;

Keshavanath et al., 2001b).

Interactions

Grazing has an overriding effect on biomass while

the effect of nutrients is not discernible or much

less apparent due to the ability of the periphyton

to recycle nutrients and to utilize nutrients from the

substrate (Steinman et al., 1992; Hill et al., 1992;

Pan and Lowe, 1994). Greater nutrient levels lead to

greater grazer biomass, indicating that the nutrients

are effectively passed on to higher trophic levels. Low

nutrient concentrations limit periphyton biomass but

when nutrient supply increases, competition for light,

both within the periphyton and with other primary

producers, becomes more important. In lakes, phyto-

plankton can contribute to shading at high nutrient

concentrations (Hansson, 1992).

Fish

Morphological and physiological adaptations to

herbivory

Although animal components constitute an important

part of periphyton layers, periphyton is best util-ized by fishes that are adapted to herbivorous diets.

These adaptations reflect their evolution from carni-

vores to herbivores. Specialization towards a plant

diet required adaptation of the teeth (from grasping

to cropping teeth), a grinding apparatus (a pharyngeal

mill or a gizzard-like stomach), a less acidic stomach

(because plant cell walls are destroyed mechanically

and acidity possibly interferes with the ingestion of

carbonates from coral substrates or sand), and a

longer gut length to increase the residence time, thus

ensuring better digestion (Horn, 1989). Fishes gener-

ally possess the enzymes to digest the contents of plant

cells, but lack the enzymes capable of disrupting cellwalls by digesting the beta-linked polymers, such as

cellulase (Lobel, 1981). Cellulase activity in fish guts

can probably be ascribed to microorganisms coloniz-

ing the fish gut contents (Stickney and Shumway,

1974). Many important pond aquaculture species

possess some of these adaptations to herbivory and are

probably able to utilize periphyton. However, some

other species that seem to be adapted to a herbivorous

diet do not utilize periphyton well (e.g., catla; Azim et

al., 2002a). The reason for this is not clear, but it seems

likely that they lack the ability to crop the attached

vegetative part of the periphyton. Detritivorous tilapiasare exceptional in the sense that they use low stomach

pH (as low as 1.4) to digest cell walls (Beveridge and

Baird, 2000).

Periphyton ingestion by fish

Most of the quantitative data on periphyton grazing

is from coral reef studies and deals with species that

are not of direct interest for aquaculture. For white-

spotted devil (Plectroglyphidodon lacrymatus, Poma-

centridae) of 62.7 mm mean standard length, a grazing

rate of 304.3 g C (g fish)1 y1 was measured on

shallow fringing reefs in Papua New Guinea (Polunin

and Brothers, 1989). For the same species in a coastal

lagoon in Papua New Guinea, mean consumption was

estimated at 1.57 g dm d1 for a 14 g fish (Polunin,

1988). For another damselfish species, the Australian

gregory (Stegastes apicalis, Pomacentridae) on the

Great Barrier Reef, ingestion rates of 7731433 mg

C fish1 d1 were measured (mean body weight 63

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g; Klumpp and Polunin, 1989). For herbivorous reef

fish, Van Rooij et al. (1998) estimated an allometric

relationship between carbon intake (g d1) and body

weight (g) from several literature sources: C intake =

0.0342W0.816 (r2 = 0.946, n = 13, W = 10100 g).

In a study in a West-African lagoon, Pauly (1976)

found that a 20 g blackchin tilapia consumed 1.5 gdetritus dm day1. Dempster et al. (1993) measured

the ingestion of algae by Nile tilapia fingerlings (47

mm mean standard length), both of planktonic Micro-

cystis aeruginosa and from a periphytic assemblage

consisting mainly of Oscillatoria sp. Mean ingestion

rate when only periphyton was offered was about 1.8

mg dm (g fish)1 h1 (measured during a 4-hour

period). In a second experiment with periphyton densi-

ties ranging from 10 to 40 g m2, ingestion rate was

independent of periphyton density (mean: 3.68 mg

dm g1 h1). In a study with indigenous carps in

Bangladesh, periphyton grazing rates of 0.51.2 (kuria

labeo), 0.20.79 (rohu) and 0.21.1 mg dm fish1

h1 (orange-fin labeo) were measured (Rahmatullah

et al., 2001). Azim et al. (2003) measured periphyton

consumption in three size groups (7, 25 and 150 g)

of Nile tilapia in the laboratory and found ingestion

rates of 0.90, 0.18 and 0.02 mg dry matter g1 h1,

respectively. Ingestion rate of Nile tilapia increased

when periphyton density on the glass plates increased.

A comparison of these observations after conver-

sion to the same unit (mg dm [g fish]1 d1; Table

4) shows that ingestion rates estimated for coral reef

fish are generally higher than those measured in the

laboratory. Information on grazing by fishes can beobtained from observations of ingestion (number of

bites during a defined time period) in relation to gut

fullness, through exclusion experiments in the natural

habitat (using cages to exclude fish from certain areas,

after which grazing rates follow from biomass differ-

ences between areas with and without cages) or by

measuring biomass differences on artificial substrates

in the laboratory before and after grazing. Advantage

of the observation and exclusion methods is that

the measurements are relatively unaffected by hand-

ling stress, while laboratory studies often involve

short study periods that are hard to extrapolate to

longer periods of time. Laboratory conditions (solitary

fish, sometimes unialgal diets, various sources of

stress) strongly affect the natural feeding behaviour

of fish, which probably leads to underestimations

of periphyton consumption with these experiments

(Horn, 1989; Azim, unpublished results).

Apart from fish weight, temperature is important.

A linear relationship between the number of bites

per hour and temperature was observed in damselfish

(whitespotted devil), with the mean number of bites

doubling when water temperature increased from 26

to 32 C (Polunin and Brothers, 1989). There are

also differences between species. For some species,periphyton is simply not a preferred food. Yields of

the indigenous kuria labeo in ponds with periphyton

in Bangladesh were not different from control ponds

without substrates, suggesting that this labeo did

not eat the periphyton (Azim et al., 2001a). Simi-

larly, catla were not observed to feed on periphyton

growing on bamboo substrates in ponds (Azim et al.,

2002a). Common carp were observed to lose weight

in tanks with periphyton substrates while Nile tilapia

gained weight by utilizing the periphyton (Van Dam,

unpublished).

Periphyton nutritional quality also plays a role in

determining ingestion rates. Fish feeding on algae,macrophytes or detritus ingest larger quantities to

compensate for the low energycontent. To compensate

for low protein content, they selectively feed on

the components with the highest protein content,

thus maximizing the dietary protein to metaboliz-

able energy (P/ME) ratio (Horn, 1989; Bowen et

al., 1995). Stoplight parrotfishes (Sparisoma viride,

Scaridae) on Carribean reefs spend 7090% of the day

foraging, ingesting huge amounts (sometimes more

than 60% of their own body mass per day) of low-

quality food (P/ME ratios ranging from 58 mg kJ1)

(Bruggemann, 1995).

Periphyton as fish feed: proximate composition

The composition of natural food for pond fishes has

been reviewed by Hepher (1988). Except for macro-

vegetation and some algae, most natural food types

are rich in protein. Fluctuations in composition are

mainly caused by differences in ash content. The

composition of detritus is highly variable and depends

a lot on the source material. Generally, periphyton

composition is similar to other natural food types. One

difference is the high organic matter content of the

periphyton assemblage due to the mucopolysaccharide

matrix. Nielsen et al. (1997) determined the composi-

tion of biofilms and found that the extracellular poly-

meric substances accounted for 5080% of the total

organic matter. Protein was the largest fraction of the

extracellular substance, despite the emphasis that is

ususally put on polysaccharides. Periphyton can also

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Table 4. Comparison of periphyton ingestion rates from different experiments. Data were converted to mg dm (g fish)1 d1

Species Location Fish size Grazing rate Source

(g fw) (mg dm g1 d1)

Sarotherodon melanothron Lagoon, Ghana 20 75 Pauly, 1976

Plectroglyphidodon lacrymatus Shallow fringing reef, Papua New Guinea 16.9c 98.8a,b Polunin and Brothers, 1989

Stegastes apicalis Great Barrier Reef, Australia 63 24.545.5b Klumpp and Polunin, 1989

Plectroglyphidodon lacrymatus Coastal lagoon, Papua New Guinea 14 112.1 Polunin, 1988

Oreochromis niloticus Laboratory, periphyton on glass plates 3.7d 21.644.2e Dempster et al., 1993

Oreochromis niloticus Laboratory, periphyton on glass plates 7 10.8e Azim et al., 2003

Oreochromis niloticus L aboratory, periphyton on glass plates 25 2.16 Azim et al., 2003

Oreochromis niloticus L aboratory, periphyton on glass plates 150 0.24 Azim et al., 2003

Labeo gonius Laboratory, periphyton on glass plates 10100 614.4e Rahmatullah et al., 2001

Labeo calbasu Laboratory, periphyton on glass plates 10100 2.413.2e Rahmatullah et al., 2001

Labeo rohita Laboratory, periphyton on glass plates 10100 2.49.5e Rahmatullah et al., 2001

Assumptions: aAssuming 365 days in one year. bAssuming 50% C in dm. cCalculated with L-W relationship from same publication.dCalculated with L-W relationship for O. niloticus from Fishbase (Froese and Pauly 2001). eAssuming diurnal feeding (12 h day1).

trap exogenous organic matter from the water column,which may account for a large part of the bio-mass

(Sansone et al., 1998). Organic compounds adsorb

to particulate and colloidal calcium carbonate that is

formed during photosynthesis (Wetzel, 1975).

Table 5 compares the composition of a number

of natural pond foods with that of periphyton.

Characteristics of natural foods are the low dry

matter content, and variable protein, energy and ash

contents. Periphyton grazed from coral reefs, mixed

with sediment material, had the lowest protein and

energy content, whereas bacteria, zooplankton and

insects had the highest nutritional quality. Generally,periphyton has a high ash content compared to other

natural feeds. Partly, this is due to the high content

of carbonates that are formed (Wetzel, 1975) and

the entrapment of inorganic particles. We estimated

the P/ME-ratios of the various food types, assuming

that metabolizable energy is 60% of gross energy

(see Table 5). P/ME-ratios ranged from about 6 in

periphyton ingested by parrotfish and in calcareous red

algae to more than 60 in bacteria, with the majority

of values between 10 and 40 kJ g1. Values esti-

mated for periphyton grown on different substrate

types in aquaculture ponds ranged from 15.6 to 25.5

kJ g1. There seemed to be an effect of substrate

type on the nutritional quality of the periphyton, with

bamboo substrates giving better quality periphyton

than jutestick and branches of the hizol tree (Azim

et al., 2002b). P/ME ratios for natural feeds range

typically range from 1 to 30 mg kJ1 (Bowen et al.,

1995).

There is little information on other indicators ofperiphyton nutritional quality. Phillips et al. (1994)

analyzed the amino acid profile of proteins from a

microbial mat grown on grass silage. A comparison

with the essential amino acid requirements of several

fish species shows that the microbial mat was only

deficient in valine.

Assimilation efficiency and food conversion ratio

Montgomery and Gerking (1980) measured assimi-

lation efficiency in cortez damselfish (Eupomacentrus

rectifraenum, Pomacentridae) and giant damselfish(Microspathodon dorsalis, Pomacentridae) from Baja

California, Mexico feeding on various species of

marine algae. There was a large difference between the

proximate composition of the periphyton mat (81.6%

ash, 1.7% protein in dm) and the stomach contents of

cortez damselfish (50.2% ash, 26.1% protein), which

was attributed to selective feeding. The giant damsel-

fish fed non-selectively on algal turf consisting solely

of the red alga Polysiphonia. The assimilation effici-

ency on a biomass basis was 2024% and for protein

5767%. A digestibility study with periphyton mats

grown on grass silage showed a dry matter digesti-

bility of 62% and 60%, and protein digestibility of

81% and 75%, for Nile tilapia and silver carp, respec-

tively. Drying significantly lowered the digestibility of

the material to less than 50% on a dry matter basis

(Ekpo and Bender, 1989; Bender et al., 1989). Table

6 summarizes data on assimilation efficiency (AE, or

digestibility) of fish feeding on periphyton and some

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Table5.Comparison

ofperiphytonnutritionalqualitywithothertyp

esofnaturalfoods.P/ME-ratioswereestimatedbyusassumingthatmetabolizableenergyis60%ofgrossenergy

forallfoodtypes

Foodtype

Subtratetype

Drymatter

Protein

Lipids

Carbohydrates

Ash

Energy

P/ME-ratioa

Source

(%)

(%

dm)

(%dm)

(%dm)

(%dm)

(kJg1)

(mgkJ1)

Bacteria(Pseudomo

nassp.)

80.4

1.7

16.9

20.5

65.3

MattyandSm

ith,1978

Bacteria

5.4

19.7

45.7

Hepher,1988

Algae(Spirulinasp.)

53.8

2.5

11.1

19.6

MattyandSm

ith,1978

Algae

14.121.7

17.631.3

3.79.9

26.946.7

9.315.8

32.7

Hepher,1988

Macrophytes

15.8

14.6

4.5

13.9

16.3

14.9

Hepher,1988

Zooplankton

7.335.0

41.564.3

19.026.4

9.228.2

5.119.6

20.123.8

40.1

Hepher,1988

Insects

14.826.0

34.768.8

4.918.6

20.122.5

3.711.8

20.523.6

39.1

Hepher,1988

Molluscs

32.2

39.5

7.8

7.5

32.9

16.3

40.5

Hepher,1988

Detritus

91.5

12.4

19.7

Hepher,1988

Averagenaturalfood

14.2

52.1

7.7

27.3

7.7

Hepher,1988

Greenalgae

Braniticboulders

10.2

4.5

59.9

25.4

13.8

12.4

MontgomeryandGerking,1980

Brownalgae

Graniticboulders

8.3

4.8

55.7

31.3

11.7

11.7

MontgomeryandGerking,1980

Redalgae

Graniticboulders

7.7

2.1

51.9

38.3

10.4

12.4

MontgomeryandGerking,1980

Calcareousredalgae

Graniticboulders

1.2

1.7

15.5

81.6

3.3

6.1

MontgomeryandGerking,1980

Algalmat

Graniticboulders

1.7

2.1

14.6

81.6

1.71

6.1

MontgomeryandGerking,1980

Periphyton+sediment