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Intake passage time, digesta composition and digestibility in East Pacific green turtles(Chelonia mydas agassizii) at Gorgona National Park, Colombian Pacific

Diego F. Amorocho, Richard D. Reina

School of Biological Sciences. Monash University, Clayton, Victoria 3800, Australia

A B S T R A C TA R T I C L E I N F O

Article history:

Received 6 February 2008

Received in revised form 25 February 2008

Accepted 9 April 2008

Keywords:

Digestibility

Intake passage time

Green turtle

Nutrition

We investigated the food digestibility of East Pacific green turtles, Chelonia mydas agassizii, at tropical coral

reefs of Gorgona National Park in the Colombian Pacific and calculated the intake passage time (IPT) of 3

different diets. We collected 150 faecal samples from turtles (mean straight carapace length 61.34.12 cm

and mean mass 32.3 6.67 kg) to determine digesta composition and for measurement of neutral detergent

fibre (NDF), acid detergent fibre (ADF), sulphuric acid lignin and protein. The mean ( S.D) IPT to recover at

least 73% of external markers (plastic beads) in the faeces was 23.3 6.6 days. The true NDF digestibility and

dry matter digestibility were determined for high protein (fish), plant (fresh leaves of Araceae, Moraceae and

Bombaceae) and mixed (combination of both high protein and plant) diets. NDF values obtained for

digestibility of the protein, plant and mixed diets were 1%, 63% and 49% respectively. There was a large

amount of undigested plant material in the faeces, dominated by fruits of red mangrove ( Rhizophora mangle).

We considered the relationships between the type of food, IPT and apparent digestibility in the context of the

nutritional contribution of an omnivorous diet. Our results suggest that for juvenile green sea turtles in the

Eastern Pacific combined diets are an opportunistic strategy responding to habitat features and supply to

maximise energy acquisition in transitional habitats such as Isla Gorgona.

Crown Copyright 2008 Published by Elsevier B.V. All rights reserved.

1. Introduction

The acquisition of nutrients for all animals depends not only on

feeding but also on mechanisms of digestion and absorption to

process the foods obtained in an interrelated set of needs and

processes (Will et al., 2004). The green sea turtle (Chelonia mydas) is

considered to be primarily a herbivore that feeds throughout most of

its rangeon seagrasspastures, but that will also consume some animal

matter (Seminoff et al., 2006), which it readily accepts in captivity

(Mortimer,1982). Greenturtles canfeed on algae where seagrasses are

not available (Bjorndal,1980; Bjorndal 1985; Forbes,1994; Read, 1991)

or consume both when they are present in the same area (Brand-Gardner et al., 1999; Ferrerira et al., 2006). Other plant material such

as mangrove leavesand fruitshavebeen describedas a substantial and

nutritionally important part of green turtle diets in Australia (Limpus

and Limpus, 2000) and juveniles of the East Pacific green turtle

(Chelonia mydas agassizii) in Colombia (Amorocho and Reina, 2007).

Consumption and effective digestion of a plant diet by sea turtles

presents a significant challenge. Turtles rely on microbial fermenta-

tion in the large intestine to digest plant cell walls, but they can not

chew their food to reduce particle size and facilitate this process

(Bjorndal et al., 1990) because they don't have teeth. Thus, only by

retaining material for longer in the gut with a slow passage rate can

they equal ruminants in the digestion of plant material by fermenta-

tion. Theprimary nutritional endproductsof this fermentationare the

volatile fattyacids (VFA), an important source of energy in herbivorous

sea turtles (Bjorndal, 1997). The quality of forage is a major deter-

minant of the efficiency of digestion and can be assessed by mea-

surement of acid detergent fibre, (ADF, cellulose and lignin) and

neutral detergentfibre (NDF, cellulose, hemicelluloseand lignin) using

the detergent system (Van Soest, 1963; Van Soest and Wine, 1967).

Lignin is the prime factorinfluencing the digestibility of plant cell wall

material and as lignin increases the digestibility, intake and animal

performance usually decrease because the percent ADF and NDFincrease (Van Soest, 1994).

Green sea turtle juveniles over about 40 cm straight carapace

length (SCL) shift their feeding behaviour from carnivorous to

herbivorous when they move from pelagic to neritic developmental

habitats (Bjorndal and Bolten, 1988; Hirth, 1997; Musick and Limpus,

1997). In turtles that feed on seagrass, Thalassia testudinum, in the

Bahamas, nutritional assimilation occurs at a very low intakerate with

high efficiency digestibility of cellulose (89%), hemicellulose (75%) and

organic matter (67%) by cellulolytic microflora present in the gut

(Bjorndal, 1980). The same author suggests that the digestibility

coefficients of NDF and ADF of green turtles foraging selectively on

young blades of the seagrass reflect the specificity of their intestinal

microflora. Bjorndal (1997), proposed that the optimal forage for

green turtles may be that to which its gut microfl

ora are adapted

Journal of Experimental Marine Biology and Ecology 360 (2008) 117-124

Corresponding author. Tel.: + 61 3 99055600; fax: +61 3 99055613.

E-mail address: [email protected] (R.D. Reina).

0022-0981/$ see front matter. Crown Copyright 2008 Published by Elsevier B.V. All rights reserved.

doi:10.1016/j.jembe.2008.04.009

Contents lists available at ScienceDirect

Journal of Experimental Marine Biology and Ecology

j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / j e m b e


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(either seagrass or algae). Thus, it is expected that a less selective diet

or one which is mixed (omnivorous) may change the composition and

function of fermenting bacteria to affect the efficiency of digestion.

However, nutrient gain in the form of VFA may be maximised by the

ability to ingest a greater quantity of food more rapidly ( Bjorndal,

1997).

Large juvenile and adult East Pacific green sea turtles are describedas primarily herbivorous feeding on seagrasses and/or marine algae in

Galapagos and Mexico (Fritts, 1981; Green, 1983; Seminoff et al.,

2002). Nonetheless, along the Coast of Baja California, they occasion-

ally consume red crabs (Lpez-Mendilaharsu et al., 2005) and other

chordates (Casas-Andreu and Gmez-Aguirre, 1980). A significant

amount of animal matter (molluscs, amphipods, sardines and

anchovies) was found in the stomachs of subadult and adult green

turtles captured in the Pacific coastal waters of Peru (Hays and Brown,

1982) and fish eggs, molluscs, polychaetes and jellyfish have been

recovered from the stomachs of turtles caught near the Pacific coast of

Ecuador (Fritts, 1981). These findings suggest a carnivorous trend in

the feeding habits of East Pacific green turtles in the region. This

behaviour has also been observed in individuals consuming a mixed

diet of tunicates (Salpas spp.), terrestrial leaves (Ficus spp.) and algae(Gelidium spp.) at Gorgona National Park, in the Colombian Pacific

(Amorocho and Reina, 2007). The high frequency of tunicates (73.8%)

recovered through oesophageal lavages conducted in 84 juveniles

(N40 cm SCL), showed that Colombian green turtles are omnivores

with a diet biased towards animal matter (Amorocho and Reina,

2007). Sea turtles consistently ingesting a mixed diet would almost

certainly develop a different microbial community than exclusively

herbivorous turtles, in order to degrade the various complex

carbohydrates required to digest each food item efficiently (Bjorndal

1985). So, long intake passage time (IPT) might occur to enable highly

efficient fibre digestibility because the microbial populations in the

green turtle gut may have to adapt continually to changes in the type

and proportions of selected diet components.

Other than stomach content analyses and anatomical descriptions

(Bjorndal, 1997; Green, 1994; Green and Ortiz, 1982; Hays and Brown,

1982), studies on the digestive systemof green turtles in the southeast

Pacific are lacking. Information about the digestibility of a mixed diet

will indicate the real benefit to turtles of eating animal matter to

increase energy gain in a transitional habitat such as Gorgona.

Nutrient availability is limited by the amount of food and by

competition between the rates of digestion and passage. This can be

maximised by ingesting a mixed diet as a way to obtain the essential

aminoacids required for protein synthesis and we may expect that

green turtles at Gorgona will try to obtain the maximum nutritionalbenefit by consuming an omnivorous diet. This might suppose a

digestibility limitation because of the diverse range of food items they

need to eat and the time required for microbial fermentation to

produce VFA. To elucidate these nutritional aspects of the East Pacific

green turtle population foraging at Gorgona National Park in the

Colombian Pacific, we investigated the apparent digestibility of three

types of diet, measured IPT and considered the effect of diet quality on

juveniles' growth. We addressed the following questions: 1. Do turtles

ingesting an omnivorous diet in Gorgona have low digestive

efficiencies? 2. If so, can nutrient gain be maximised by the ability

to ingest a greater quantity of different types of food more rapidly? 3.

If the EastPacific green turtlehas a slow fermenting strategywith high

digestibility offibre and organic matter, is ingesta retained within the

fermentation chamber for long periods? By revealing these aspects ofdiet selection and digestive process, we can begin to understand how

nutrition acts as a regulating mechanism for growth and maturity in

juveniles and the importance of Gorgona as a foraging and develop-

mental habitat for the East Pacific green sea turtle.

2. Materials and methods

2.1. Study site

Fieldwork was carried out at Isla Gorgona National Park (2 50' - 3

00'N, 7810' - 7815' W) in Colombia. The island is 9 km long and

2.5 km wide with a total protected area of 617 km2 and a maximum

height of 338 m, located 56 km offshore from the town of Guapi in the

southern Colombian Pacific coast (Fig. 1). The shores of Gorgona are

predominantly steep plunging cliffs, with small sandy and pebbly

beaches supplied on its eastern side by coral reef detritus. The island is

surrounded by near-shore coral reefs where East Pacific green sea

Fig.1. Map of Gorgona National Park in the Colombian Pacific, showing location of the sea turtle in-water enclosure.

118 D.F. Amorocho, R.D. Reina / Journal of Experimental Marine Biology and Ecology 360 (2008) 117-124


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turtles can be found resting or foraging. The IPT, digesta composition

and digestibility experiments were conducted within the constraintsof limited logistic and infrastructure conditions in thefield. Gorgona is

an isolated place without permanent electricity to adequately store

collected samples for some specialised analyses.

2.2. Turtles sampled, confinement and faecal collection

Weconducted IPTand digestibility experiments betweenJuly and

October 2005. Nine green turtles were caught by hand using snorkel

at a depth of up to 7 m in coral reefs of La Azufrada and Playa Blanca.

Straight carapace length (SCL) of all animals was measured as

described by Bolten (1999) and turtles were weighed and double

tagged in the front flippers with Inconel 1005-681S tags (National

Band & Tag Co.), following standard techniques (Balazs, 1999). A

plastic Ziploc bag cut at the bottom was wrapped inside another

plastic bag and covered with nylon cloth to be attached to the

animal's cloaca using a surgicalthread,leavingthe zip side at theend

to enable collection of faeces in a modification of Bjorndal's

technique (1980). Turtles were confined in an enclosure made up of

bamboo, PVC and plastic mesh, in water of approximately 3 m depth

(Fig. 2). Three turtles were held at a time, each in a 2 m2 m2 m

separated compartment of the in-water enclosure for 3 consecutive

sampling periods. Turtles were acclimated to the enclosure for 7 days

and fed with fresh leaves and small fish supplied ad libitum. We

covered the enclosure with 6 mm square plastic mesh to reduce the

possibility of turtles eating food other than that provided in the

laboratory and we checked the enclosure daily to keep it free of

leaves, small fish and tunicates.

2.3. IPT and dietary treatments

After acclimation, we administered external markers to each turtle

before being feeding it with the experimental diet and placing it back

into the enclosure. External markers are synthetic indigestible organic

substances such as plastic beads, that are used in gut passage studies

because they can be counted, are not chewed into smaller pieces and

can easily be recovered from the faeces (Van Soest, 1994). This is

probably the best single measure of food passage through the entire

gut if faeces can be collected (Warner, 1981). We used cylindrical

yellow beads (2.0 - 3.0 mm1 mm packaged into gelatin capsules

containing 20 beads each) and introduced between three and five

capsules into the lower oesophagus by pushing them through a plastic

hose. IPT was calculated from the integrated average of markers

recovered in the faeces following administration.Animals were moved every three days into the laboratory to be fed

with 1 2% body mass of wet food as a maintenance diet (Higgins,

2003). The three experimental diets given to the turtles were: high

protein (fi

sh); plant (a blend of Araceae, Moraceae and Bombacaeaeleaves) andmixed (a combination of animaland plant material). Meals

were prepared in the form of approximately 8 cm3 fish cubes and small

packages of fresh leaves. The mass of food given each time varied with

the willingness of the turtle to ingest it. The diet rations were intro-

duced using 2 plastic hoses to push the food deep into the oesophagus.

The high protein group was composed of turtles with mass ranging

from 27 to 31 kg; the plant group mass was from 31 to 36 kg and the

mixed diet group was from 33 to 48 kg. The three dietary treatments

and mass of food administered to animals are presented in Table 1.

Turtles were taken out of the water every day at 3 p.m. and faeces

collected from each individual's attached bag before returning them to

the enclosure. Meanvalue for water temperature recorded throughout

the three experimental seasons at 3 m below the surface, was 28.3

0.3 C; ranging from a minimum of 27.7 C to a maximum of 29.0 C.

We measured body mass every 7 days and replaced faecal collection

bags as required.

2.4. Digesta composition and digestibility analyses

Collected faecal material was initially sorted into categories (fruits,

leaves, mixture, coral andplastic).Wet mass wasmeasured for thefirst3

categories and then samples were preserved in 70% ethanol in plastic

bags and stored at 4 C in a portable fridge for organic matter (OM), acid

detergentfibre (ADF), neutraldetergent fibre (NDF) and lignin analyses.

At the conclusion of the experiment, attached cloacal bags were

carefully removed from turtles and iodine (10%) applied around the

cloaca before releasing the animals at the approximate site of capture.

Faecal samples were dried to constant mass at 60 C for 48 to 60 h

and stored in desiccators (Wood and Wood,1981). Following return tothe mainland, samples were dried again at 105 C for 12 hours,

Table 1

Total mass (g) of protein and plant feeding rations supplied every 3 days to 9 turtles

confined in a near shore captive enclosure at Gorgona National Park, Colombia

Turtle Type of diet Plant ration (g) Protein ration (g) Total intake (g)

T1 Mixed 175 320 495

T2 Mixed 225 100 325

T3 Mixed 125 85 210

T4 Mixed 223 210 433

T5 Mixed 500 500 1000

T6 Plant 292 - 292

T7 Plant 300 - 300T8 Protein - 534 534

T9 Protein - 500 500

Fig. 2. In-water enclosure to keep turtles confined near shore El Poblado beach. The enclosure was sunk to the water level after being placed in position.

119D.F. Amorocho, R.D. Reina / Journal of Experimental Marine Biology and Ecology 360 (2008) 117-124


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weighed and ground in a Wiley Mill. Coral and plastic debris were

excluded because they were not digested and were probably

consumed incidentally by turtles while feeding on other food items.

Faecal samples were aggregated for the whole collection period of

each animal, then all faecal and food samples were analysed fororganic matter by heating in a muffle furnace at 550 C for 12 h. Total

nitrogen was measured by the micro-Kjeldhal method and crude

protein including both true protein and non-protein nitrogen was

estimated by multiplying total nitrogen content by 6.25, while ADF,

NDF and sulphuric acid lignin analyses were performed following

standard procedures (Van Soest, 1963; Van Soest and Wine, 1967).

Food samples were also analysed for organic matter, crude protein,

ADF, NDF and lignin. Hemicellulose content was calculated by

subtracting ash-free ADF from ash-free NDF, and cellulose was

calculated by subtracting lignin from ash-free ADF following proce-

dures applied for green turtles previously (Bjorndal, 1980). Duplicate

analyses were performed when the quantity of material permitted.

3. Results

All results are reported as the meanS.D.

3.1. Animal size and gut length

The mean SCL of the 9 studied turtles was 58.23.6 cm. (ranging

from 52.2 to 62.2 cm) and mean mass was 32.36.7 kg (ranging from

26 to 48 kg). One of the turtles (T7) died 12 days after capture and a

necropsy revealed a 5 cm long-line J shaped hook # 7 (Mustard Co.)

embedded in the oesophagus, that was presumably accidentally

ingested by the turtle from some fisheries activity prior to capture.

Large pieces of plastic debris were distributed all along the gut and

may have also contributed to the cause of death. The entire digestive

tract of this turtle (63.7 cm SCL, 36.0 kg mass) from the beak to the

cloaca was 6.53 m in length. The necropsy showed that plant matter

consumed by the turtle had travelled along the gut in the form of

ingesta packages (bolus), with fresh leaves observed between the

oesophagus and the stomach (0 - 70 cm), boluses of partially digested

plant material located in the midgut (70 - 318 cm) and large amountsof mangrove fruits were distributed from mid to hindgut (318 -

650 cm). All of the external markers given to the animal on day one of

the experiment were contained within boluses distributed along the

midgut (371 - 411 cm) at the time of death 12 days later.


No markers were recovered from Turtle T1 after 38 days and only

one marker was recovered from turtle T2 in 32 days, so these two

turtles were released to avoid health problems that could result from

extended captivity. Excluding T1, T7 and T8 due to zero or low mark

recovery, the mean IPT for the 6 remaining turtles was 23.36.6 days

(559 hr) ranging from 22.0 6.9 days (528 hr) for thefirst recovery day

and 24.76.6 days (593 hr) for the last recovery day of markers(Table 2). We ended theexperimentwhen we recovered at least 73% of

beads from faeces (Fig. 3).

3.3. Digesta composition and digestibility analyses

Faecal sample percentage wet mass of mangrove fruits, leaves,

remains composed of fragments of leaves and metabolic excretions

termed mixture, coral and plastic debris are shown in Fig. 4. Entire or

Table 2

Recovery of administered markers from 6 turtles. Mean Ingesta Passage Time (IPT)=

23.36.6 days

Turtle SCL

(cm)

Number of

administered

markers

Percentage

of markers

recovered

First recovery

day

Last recovery

day

T2 59.0 100 96% 16 17

T3 60.5 95 86% 14 19

T4 60.8 60 100% 19 21

T5 54.8 60 82% 23 27

T6 62.2 60 100% 30 32

T9 52.0 64 73% 30 32

MeanSD 58.2 3.6 7 3.2 17.3 89.4 10.0% 2 2.0 6.3 24.7 6.0

T1 was excluded because no markers were recovered by day 38 and animal was

released. T7 died after 12 days of treatment due to a hook ingested prior to experiment.

T8 was released after day 31 without releasing markers.

Fig. 3. Percentage of markers recovered and time elapsed for over 73% of markers to be

recovered from faecal samples of 6 green turtles under 3 dietary treatments. P=plant,

M=mixed, F=fish (high protein).

Fig. 4. Percentageof different categoriesof ingested items collected fromfaecal samples

of 9 green sea turtles at Gorgona National Park.

Table 3

Percentages of organic matter (OM), neutral detergent fibre (NDF), acid detergent fibre

(ADF),ligninand protein of diet components and3 main faecalcategories (leaves,fruits,

mixture) collected from 9 green turtles at Gorgona National Park, Colombia

Diet Sample OM(%) NDF(%) ADF(%) Lignin(%) Protein(%)

Fish 87 1 0.1 0 98

Leaves 73 63 53 38 8

Faecal Sample OM(%) NDF(%) ADF(%) Lignin(%) Protein(%)

MIXTURE

Mean SD 68 3 74 19 58 6 28 5 16 2

Min 63 0 47 20 12

Max 74 87 68 36 19

LEAVES

Mean SD 52 33 45 42 45 27 22 13 12 8

Min 3 0 6 5 2

Max 74 87 68 36 19

FRUITS

Mean SD 58 24 61 30 49 20 25 11 14 6Min 3 0 6 5 2

Max 74 87 68 36 19



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chopped pieces of mangrove fruit were present in faeces throughout

the experiment in all 9 assessed turtles. Percentages of OM, NDF, ADF,

lignin and protein from fish and plant dietary treatments, as well as

the 3 main faecalcomponents are presented in Table 3. Through direct

observation of the faecal material it appears that animal food (i.e.fish)

was entirely digested, but some plant material (particularly mangrove

fruits) showed little visible sign of digestion (Fig. 5). True NDF andapparent dry matter digestibility (ADM) of the assessed turtles are

presented in Table 4.

4. Discussion

4.1. Turtle size class

East Pacific green turtles using coral reefs and surrounding waters

of Gorgona National Park were small and large juveniles (52.0 62.2

SCL), confirming recruitment of young turtles at the size of 50.0 -

70 cm SCL to neritic habitats of this protected area. The mean size of

58.23.6 cm at Gorgona is similar to that measured from juveniles at

Baha Magdalena and Estero Banderitas in Baja California Sur, Mexico

(Lpez-Mendilaharsu et al., 2005) and is typical of juveniles on the

Pacific coast of the Baja California Peninsula (Koch et al., 2007) in

protected reefs or mangrove habitats where they can find food and

shelter. However, juveniles in Gorgona, consume a mixed diet biased

towards animal matter (Amorocho and Reina, 2007) while those in

Baha Magdalena feed mostly on algae and seagrasses (Lpez-

Mendilaharsu et al., 2003; Lpez-Mendilaharsu et al., 2005; Seminoff

et al., 2002). At Gorgona, it is advantageous for juvenile green turtles

to consume a high protein diet with good nutrient assimilation to

contribute energy for growth while juveniles migrate between

foraging and developmental habitats. In comparison, stable isotopes

in scutes ofN36 cm young green turtle recruited to neritic habitats off

Great Inagua (Bahamas), reveal rapid ontogenetic shift from carnivor-

ous to herbivorous lifestyle (Reich et al., 2007). Green sea turtle

juveniles at a size class of 50 - 70 cm continue feeding mainly on

animal matter or at least consuming an omnivorous diet in neritic

near shore habitats of Gorgona (Amorocho and Reina, 2007) in an

ontogenetic pattern different to that observed in green turtles of the

Caribbean. These different dietary patterns are probably indicative of

the transitional nature of the Gorgona habitat as a stop-over point in

turtle migration.


There is very little information about the use of external markers(beads) to estimate time elapsed between ingestion and egestion in

sea turtles. The percentage of bead recovery in faeces of 6 turtles was

not always over the 95% typically applied in ruminant passage studies

(Van Soest, 1994). However, our mean of 89% (from the 73% to 100%

range) of markers recovered was similar to the 88% - 100% collected in

3 juvenilesof similar size class (50.3 55.2 cmSCL)in Australia (Brand

et al., 1999). It is possible that some markers were lost through small

holes between the collection bags and the skin around the cloaca. It is

unclear why no markers were recovered after at least 32 days from 2

turtles. We suspect that they may have been regurgitated following

administration and escaped through the mesh of the enclosure but

cannot exclude the possibility that their movement through the gut

was retarded for some unknown reason.

Mean IPT for captive turtles was long with a mean time offirst andlast marker recovery 528to 593h compared to digesta retentiontimes

of 156 to 325 h estimated in wild immature green turtles at MoretonFig. 5. Pieces of a) the mangrove fruit Rhizophora mangle and b) leaves of Ficus spp.

collected in the faeces of 9 assessed turtles.

Table 4

Number of days that turtles were confined, total food intake, mean Intake Passage Time

(IPT), total mass of faeces, percent neutral detergent fibre (NDF) and percent apparent

drymatterdigestibility (ADM) of 9 green seaturtles at GorgonaNational Park, Colombia

Turtle Days

confined

Number

of feeds

Total

intake (g)

IPT

(days)

Wet mass

of collected

faeces (g)

NDF

(%)

ADM

(%)

T1 38 13 3519 - 725 77 89

T2 25 8 5850 16 866 82 67

T3 19 6 1853 16 321 77 75

T4 25 9 4138 25 841 75 78

T5 31 10 10000 - 3377 60 66

T6 19 7 2051 20 687 62 67

T7 13 4 1200 - 1649 - 61 - 37

T8 31 11 5674 31 535 19 91

T9 31 10 5000 31 746 22 85

MeanSD 26 8 9 3 4365 270 0 23.3 7.2 1083 933 59 25 77 10

Mean time elapsed between first and last recovery day of markers . NDF and ADM

values of T 7 are negative because intake was less than the excreta produced after

12daysof captivitypriorto death from unrelatedhook ingestion.These values were not

included for mean and SD estimation.



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Bay, Australia (Brand et al., 1999). This difference is a consequence of

the experimental design and different techniques used in both studies.

In the Moreton Bay study, turtles were freed into the wild to resume

feeding after the markers were administered, then recaptured and

sacrificed after 5 9 days. This was before they excreted any beads, so

IPT was estimated from beads counted at some point along the gut

following necropsy. We think that our measurements of IPT are moreprecise because the passage of beads was measured all the way

through from ingestion to egestion, with excreted beads collected

daily in attached cloacal bags. Additionally, the necropsy of turtle T7

that died from the unrelated hook ingestion, showed that markers

were only half to two thirds the way along the gut after nearly 300 h

(12 days). The point of the gut that the markers reached was similar to

that of Brand et al. (1999) in a comparable time, but our results

indicate that it took about another 11 days for markers to reach the

end of the gut and be excreted.

4.3. Digesta composition

The presence of large amounts of undigested mangrove fruits in

faeces of all sampled turtles refl

ects the abundance of this food itemin Gorgona waters. Mangrove fruits are found drifting in surface

currents carrying organic matter from the mainland rivers of la Tola,

Sanquianga and Patia in South Western Colombian Pacific. The

abundance and availability of Rhizophora mangle fruits in superficial

waters of Gorgona varies between seasons and may influence die-

tary changes in foraging green turtles from plant to animal matter

(tunicates). The presence of undigested mangrove fruits in the faeces

of all turtles can be associated with seasonal abundance prior to the

experiment. Thelow percentage of NDF(63%) in fresh mangrove fruits

might be responsible for the high intake of this food item before our

study was conducted. The consequence of consuming Rhizophora

mangle needs to be considered in further studies, since mangroves

play a key role in the nutritional ecology of immature green turtle

populations in the Pacific (Limpus and Limpus, 2000).

4.4. Digestibility

From direct observation of digesta composition in the entire gut, it is

evident that green turtles at Gorgona are opportunistic consumers.

Distinct packages of food separated in boluses containing mangrove

stalks, small fish and leaves reflects the omnivorous behaviour of

juveniles in the wild. This also indicates howoften they switch between

animal and plant food items. A similar pattern of shifting diet was

observed in Australia, with separate boluses representing different

feeding bouts divided along the digestive tract (Brand et al., 1999).

The NDF digestibility of six turtles from which we recovered

markers varied, with apparent dry matter digestibility (ADM) highest

in the protein (fish) diet group, lowest in the plant diet group and

intermediate in the mixed diet group, although sample size was smallin the first two groups. The protein diet (NDF 21 2%) seems to be well

digested, with our measured ADM of 85 91% very similar to that for

green turtles (82 90%) fed commercially prepared high protein diets

(Hadjichristophorou and Grove, 1983; Wood and Wood, 1981). As a

result, juveniles in Gorgona might grow more rapidly than those

feeding on sea grass or algae in the Caribbean (Bjorndal, 1985) and

Australia (Brand et al., 1999), considering protein as the limiting

nutrient responsible for growth and sexual maturity. Thus, we concur

with Bjorndal (1985) that growth rates in wild green turtles are under

nutritional rather than genetic control. We propose that the need for

rapid growth is indicated by the diet biased toward animal matter of

juvenile turtles at Gorgona (Amorocho and Reina, 2007), in contrast to

the assumption that juveniles must exclusively consume plant matter

as they approach adulthood.For animals fed with plant diet, 63% of organic matter consisted of

NDF and ADM was 67%, in agreement with the ADM of 66% measured

from green juveniles consuming Thalassia testudinum blades from

grazed plots in Union Creek, Bahamas (Bjorndal, 1980). NDF digest-

ibility for turtles varies depending on its composition, because

hemicellulose can be partially digested by sea turtles but lignin can

not. In addition, ligninand cutin appearto reduce digestibility of other

NDFcomponents. NDFdigestibility also depends on the species andits

digestive anatomy, size and strategy and so ruminants digest moreNDF than hind-gut fermenters such as sea turtles.

4.5. Comparisons with ruminants

In general there are two options available for a herbivore: to either

maximise digestive efficiency or to maximise intake (White et al.,

2007). These are competing process because digestion is a time-

dependent process, so that maximum digestion requires a long time. If

each unit of forage must be held in the digestive tract a long time

during microbial fermentation, then a species maximising digestive

efficiency cannot eat a large amount of food because the capacity of

the gut is limiting. Conversely, a species maximising intake will

sacrifice digestive efficiency because rate of passage must be increased

to make room for the additional forage and therefore potentiallydigestible material will pass through the digestive tract (Mertens and

Ely, 1979). Ruminants are more efficient at digesting the low and

medium quality forage normally consumed by these animals, usually

in 24 - 48 h, but ruminants are constrained by rumen fill and rate of

passage on how much they can eat in a day (Leng, 1990). Hind-gut

fermenters like green sea turtles are not similarly constrained and

thus can compensate for lower digestive efficiency by simply eating

more. When food is not limiting, hind-gut fermenters can attain an

equal or higher rate of energy (or digestible dry matter) intake.

Ruminants may be at an advantage when food quantity is limited, but

hind-gut fermenters such as green sea turtles usually have the option

of consuming more of a higher quality food, which is generally

abundant in Gorgona. Analysis of plant and animal characteristics that

influence digestion and intake suggests that maximum intake of

digestible drymatter is influencedmore by theproportion offiberthat

is indigestible and the rate of passage than by the rate of fiber

digestion. Thus, with a slow passage rate and increased digestibility,

the green turtles at Gorgona maximise nutrient gain in the form of

VFA through an omnivorous diet.

4.6. Temperature implications

Temperature can be a major variable limiting digestive efficiencies

in poikilothermic animals. For this reason, the intake rate and

digestive efficiency of turtles will be affected by geographical and

seasonal variations in water temperature. Body temperature of green

turtles in Gorgona are not more than one or two degrees C different

to the water temperature at 3 m depth, which is not sufficient to

significantly affect IPT or decrease digestibility. Thus, the microbialincubation temperature of these turtles was considerably cooler than

that of endothermic ruminants, further contributing to relatively

lengthy gut passage times. Reptiles can attain digestive efficiencies

roughly comparable to those of ruminants by subjecting feed to

microbial fermentation processes for longer periods of time (Mackie,

2002). This strategy is not feasible for endothermic herbivores with a

rapid metabolic rate and limited time available to extract energy and

nutrients (Farlow, 1987). Thus the main advantage of mastication in

ruminants is to reduce the time needed to attain a digestibility that

reptiles can achieve simply through a longer IPT. Consumption of a

mixed diet and a regulated temperature are the key aspects to main-

tainthe efficiency of the microbial fermentation process driving intake

rate, passage time and digestibility of green turtles in developmental

and feeding grounds of Gorgona. Water temperatures of 21.3 - 25.5 Crecorded in feeding grounds of Moreton Bay, Australia (Brand et al.,

1999) and of 19 28 C in Baja California, Mexico (Lluch-Belda et al.,



8/9


2000), are similar to those measured in Gorgona and are typical of

tropical waters. Fluctuation in temperature over the course of the year

will have some effect on the fermentation process, digestive efficiency

and IPT for a poikilotherm animal. This might be the case for turtles

feeding on seagrasses in the Bahamas during atypical El Nio years,

when normal temperature ranging from 24 29C rises toabove 34C

(Bjorndal, 1980), increasing the rate of digestion and food intake.

4.7. Conservation and management issues

The death of turtle T7 due to a hook embedded in its oesophagus

raises the issue of high sea turtle mortality in artisanal and industrialfisheries. Incidental capture in long-line fishing gear has been

documented as the major threat for green sea turtle populations in

the Pacific Ocean (Seminoff, 2004). Technological changes in current

fishing practices and the use of de-hookers need to be rapidly

introduced in order to reduce greensea turtle by-catchin Colombia. In

addition, the amount of plastic (1.6%) that we recovered from the

faeces of turtles indicates the magnitude of marine pollution affecting

thefeeding behaviourand healthof green sea turtles. It is importantto

continue documenting the occurrence and impact of plastic debris indiets consumed by sea turtles for the design and implementation of a

regional management strategy in the Pacific. Results from our study

show how the transitional feeding grounds of Gorgona National Park

provide nutritional opportunities that benefit turtles' digestive

efficiency that can be translated into growth andfitness. For managers

to better protect them, it would be important to address the

abundanceof debrisand quality of food items ingested by omnivorous

juveniles in the protected habitats and to ensure that Gorgona

remains available as a transitional habitat.

4.8. Conclusions

It seems that for juvenile green sea turtles in the Eastern Pacific

combined diets are an opportunistic strategy responding more to

habitat features and supply rather than to the species' herbivorous

preference. Perhaps foraging patterns in juveniles are driven more

by the rookery geographical distribution, type of habitat and food

availability than by ontogenetic changes experienced by animals

when they enter neritic shallow waters. This relationship may be the

reason for IPT differences observed in assessed turtles, suggesting that

the cellulolytic gut microflora is capable of degrading plant cell walls

and protein with acceptable digestibility efficiency under a mixed diet.

Thus, juveniles recruited to Gorgona's habitats seem to be consuming

a better quality and more degradable mixed forage that increases

growth rates for more rapid achievement of sexual maturity. In regard

to our experimental questions posed earlier, we can say the following:

1) turtles in Gorgona consuming an omnivorous diet exhibited high

fibre digestibilities (82 88%), similar to the 72 91% reported for

green turtles in the Caribbean (Bjorndal, 1980); 2) NDF of 77% for themixed (omnivorous) diet suggests that intake does not necessarily

occur rapidly. However, the animal might be compensated with

nutrients for growth supplied by the high protein component of the

diet and efficiency of thefibre content digestibility; 3) An IPT between

14 and 32 days seems to be a reasonable time for an ectothermic

animal to efficiently digest meals composed of different types of food.

Further research combining microbiological techniques, molecular

tags, stable isotopes and satellite tracking, will contribute to further

understanding the foraging ecology and importance of Gorgona

National Park as a transitional habitat for growth, performance and

productivity of East Pacific green turtles.

Acknowledgements

We thank the US National Fish and Wildlife Foundation (NFWF),

The Rufford Small Grants Foundation (UK), the Colombian National

Parks Administrative Unit (UAESPNN), Centre for Research and

Environmental Development (CIMAD, Colombia), the International

Centre for Tropical Agriculture (CIAT), the Zoo of Cali (Colombia), the

R.E. Train Education for Nature (EFN), the University of West Indies

(Barbados), and the Wider Caribbean Sea Turtle Network (WIDECAST),

for their financial, logistic and human contributions to this project.

Our special gratitude to A. Pavia, M.J. Restrepo, J. Hoyos, A. Ruiz, J.Puertas, J. Rodriguez, L. Merizalde and other volunteers for their

energy and enthusiasm proved while catching and processing turtles

during the fieldwork. We also thank Dr. Fernando Gast from the

Colombian Biodiversity Research Institute Alexander von Humboldt

and Dr. Fernando Castro from the Department of Zoology of the Valle

University for their support during this investigation. This study was

conducted under research permit DTSO 0029 from the UAESPNN

(Colombia) and ethics approval BSCI /2003/04 from Monash Uni-

versity (Australia). [SS]

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in

the online version, at doi:10.1016/j.jembe.2008.04.009.

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