Chapter V
CAPTIVE BREEDING AND SEED PRODUCTION
PROTOCOLS OF ETROPLUS SURATENSIS (BLOCH)
5.1. Introduction
Etroplus suratensis is essentially a brackish water fish that has become
naturally acclimatized to freshwaters. Being a high value fish that fetch a high
price, its capture fishery is exposed to high fishing pressures. The species has been
demonstrated to be an ideal candidate for pond culture (Sumitra Vijayaraghavan et
al., 1981) and most adapted for intensive farming in low volume high density
cages (Padmakumar et al., 2004). Although the fish breeds naturally in confined
conditions, production is most erratic; lack of required quantities of fish seed is the
most serious constraint for expansion of its culture. Popularization of commercial
farming depends largely on development of reliable method of seed production
under controlled conditions. Owing to their unique and prolonged parental care and
biparental substrate breeding habit, efforts for inducing them to spawn in captivity
has not been successful.
Propagation of pearlspot is hampered also due to a variety of reasons. In
nature, E. suratensis breeds in shallow, peripheral waters in isolated territories
(Samarakoon, 1985). Such shallow territories are the most seriously exposed to
human interferences. For example, in Vembanad lake, indiscriminate dredging of
sub fossil lime shell deposits for industrial use, by mechanized dredgers, almost
round the clock, bring about cataclysmic damages to the bottom substratum
breeding habitats of pearlspots (Padmakumar, 2003). Increased sedimentation and
siltation of the lake has been yet another factor detrimental to ‘pit caring’ and
‘parental care’, characteristic to this species. The long chain of canals in the
coconut gardens, adjoining the backwater stretches have been the natural breeding
grounds of this species. Unabated reclamation of these shallow wetlands and rapid
landfills during the past two decades in the name of ecotourism and development of
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resorts on the backwater front has been another factor that impeded their natural
recruitment. Further, with the boom in backwater tourism, the demand for
Pearlspot, the brand species of Vembanad, has increased tremendously. Being a
high value fish, selective fishing for this species has been most rampant. Another
persistent threat is from pollution, due to excessive use of chemical pesticides in the
adjoining rice polders for intensive rice farming. Near total disappearance of
E. suratensis in the vast inland water bodies of Kuttanad wetlands, upstream
Vembanad lake has also been attributed to persistent exposure to high concentration
of agricultural pollutants often higher than maximum allowable toxicant
concentration (Sulekha, 2001). Habitat protection and protocols for recruitment
through ranching are two recognised strategies for protection of endemic fish
species affected by habitat loss and hampered natural recruitment. This calls for
efforts to develop protocols for artificial breeding of this species.
5.2. Results
5.2.1. Collection and Maintenance of Brooders
A total of 385 mature fishes and 494 young ones, were collected and
conditioned to maturity in scientifically prepared holding ponds of size 250 sq.m, in
the Regional Agricultural Research Station, Kumarakom. As the fish preferred an
omnivorous diet, as evident from the food and feeding studies, in the holding ponds,
the broodstock was raised on natural algal production facilitated by organic
manuring, supplemented with commercial feed comprising rice bran, ground nut oil
cake and commercial pellet feed. An effective population size (Ne) of minimum
500 (FAO/UNDP, 1981) was maintained and utilized for the captive breeding
trials. E. suratensis stocks, when raised in pond conditions with adequate water
exchange facilities and nutritionally balanced diets was observed, to reach good
gonadal condition during the first year itself. Fishes were also found to breed
profusely in the broodfish ponds when nesting materials were provided to promote
natural breeding.
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5.2.2. Pond Breeding of E. suratensis
Breeding behavior of the fish, closely monitored in an earthen pond
revealed that the breeding activities of E. suratensis, are unique which involved a
series of events such as pairing, nest making and parental care. The stocked fishes
were found to form schools of 15-20 members and among these, males and females
with specific attributes gets ‘attached’ and a spawning pair is formed, over a
period of several days. ‘Spawning families’ were observed to become established
soon after their introduction into the earthen ponds. The most prominent indication
of the pre mating pair formation was the conspicuous intensification and darkening
of color pattern in males. In the case of females, black spots and blotches appear on
the ventral side between pelvic and anal fins, during spawning period. On the other
hand, among males, the breeding tint is more strongly marked, characterized by a
bluish green iridescence and sparkling pearly white spots. Although E. suratensis is
generally stated to be monomorphic, this brilliant breeding coloration among males
may be considered as a sexual dimorphic feature. Soon after the development of
the breeding hue, paired fishes were found to form isolated territories. The paired
fishes form close, stable and long lasting pair bonds and start breeding in regions
devoid of vegetation. The breeding pair starts swimming along the side of the pond
in search of a suitable substratum for nesting. When they find an appropriate
nesting substratum, 11 to 45 cm raised above the ground, the substratum surface is
cleared off attached algae by browsing them over. The fish is observed to browse
the algal growth by rhythmical jaw movements and the collected algae are sucked
deep in to the mouth by regular suction movements at irregular intervals.
Invariably, the fish prefer spawning surfaces available in shallow waters. The
fishes were also found to utilize any type of stationary object such as coconut
leaves, coconut husk, stones, PVC hose pipes, bricks, coconut roots or any such
hard solids available along the shallow periphery of the pond (Plates 8a – 8h). In
the present trial bamboo stumps planted at close intervals were utilized as nest
substrate. The selection of the nesting site, clearance of the site off vegetation and
cleaning of spawning surface are seen completed in 3-5 days. Both male and
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female partners actively engage in nest preparation. After the substrates are cleared
off, attached algae and even the minutest particles adhering to the surface are
cleared, the male fish begin to excite the female by hitting on the vent and nibbling
on the abdomen and the pair swims around the chosen substratum. During
ovulation, the female lay flat on to the spawning site and gently move from end to
end and begin to attach eggs carefully, with the help of its tubular, fleshy and cup
like ovipositor. The female fish then glue their sticky eggs by pressing closely on to
the nest surface, one by one in a single layer, supported by their ventral fins. After
the extrusion of a few eggs, over the selected site in one or two trial runs, the male
fish following close behind, in swift and rapid movements dashes over the freshly
laid eggs, releases a spray of milt and fertilizes them instantly. After the males
have completed their task, the female repeat the process of egg extrusion on to the
nest surface. And this process of egg laying and fertilization is continued several
times and the eggs are placed closely in a chain, not touching each other. The
whole process of spawning is completed in 45 to 60 minutes. The number of eggs
per brood, in the experimental conditions varied from 250 to 1573 with a mean of
854. Nest area per brood was observed to vary from 20 to 49.5cm2. The nest
preferences and the egg yield are given in Table 5.1. Altogether 91 nests are
observed in the pond during the experimental period of one year. Maximum
breeding intensity was observed during September (31 No.) followed by July (15
No.).
Table 5.1. Egg attachment of Etroplus suratensis in wild (pond)
Sl. No.
Nesting substrate Nest Area cm2
No. of eggs
No./ cm2
Fertilisation %
Hatching %
1 Coconut-petiole 49.5 990 20 70.71 70 2 Coconut-petiole 36.5 730 20 89.0 2 3 Coconut husk 20.0 960 48 100 4 4 Wooden poles 20.3 1215 60 92.2 13 5 Granite block 22.5 260 12 100 48 6 Coconut leaf 43.7 1573 36 100 10 7 Fire brick 22.9 642 28 98.75 32.4 8 Coconut husk 28.26 1243 44 97.58 38.33 9 Hose tube 48.0 250 5 100 33
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The very next day after egg fixing, prior to hatching, the parent fish start
excavating small cup like pits (‘thadam’) on the pond floor (Plate 9), just below the
selected nesting substrate, where the eggs are laid, by scooping out mud and
cleaning the nearby vegetation by vigorous mouth picking movements. The
bottom pits vary in number from 7 to 15. Nursing pits are excavated on firm and
stable ground in clayey bottom and the fish avoid loose and sandy sediments. The
average size of the shallow ‘pit nurseries’ of pearlspot varies from 3-10cm in
diameter and 2-7 cm in depth. Both the couple participates in the process and males
are more actively involved in this laborious job. On an average 12 pits are observed
per brood. The fish was found to prefer a breeding depth ranging from 15 to
44.5cm. (Table 5.2)
Table 5.2. Details of pit nurseries made by Etroplus suratensis in pond breeding
Water column
(cm) Distance between
pits (cm) Diameter of
pits (cm) Depth of pits
(cm) Sl. No.
No. of pits by a pair Range Av. Range Av. Range Av. Range Av.
1 11 19.0-28 23 2.0 - 9 5.82 7.0-10.0 8.5 3-5.5 4.3 2 9 22-26.5 25 2.0 -9 5.42 5.0-7.0 5.79 4.0-6.0 5.0 3 13 15-26 22 4.0-10 7.5 4.0-9.0 6.21 3.0-5.0 4.0 4 15 17-30 24.4 2.0-8.0 3.91 6.0-9.0 6.0 5.4-7.0 6.0 5 13 29.8-37.2 33.78 3.0-17 6.63 3.5-5.5 4.5 2.0-5.0 3.76 6 7 25-30 27.82 4.0-9.0 6.08 4.0-5.0 4.5 3.0-4.5 4.04 7 15 28-44.5 37.73 2.0-12 5.56 3.0-6.0 3.8 3.5-4.2 3.32 8 10 16.5-22.9 18.92 4.0-19 10.43 5.0-10.0 7.24 4.5-5.5 5.0 9 9 28-36 25.83 7.0-13 10.33 4.0-5.0 4.33 3.0-5.0 3.67
10 14 20-32 26.17 3.5-10 7.10 4.0-6.5 5.0 2.0-7.0 4.0 11 15 25-30 27.6 2.0-8.0 5.13 5.0-6.0 5.63 3.0-6.0 4.8 12 13 29-37 33.2 4.0-7.0 5.44 4.0-7.0 5.7 2.0-6.0 3.7 Av. 12 15-44.5 2.0-17 3 - 10 2.0-7.0
In contrast to the spherical shape of the mature eggs in most teleosts, eggs of
Etroplus are ovoid. The extruded eggs are also demersal, and appear as creamy
yellow elliptical bodies, the major(x) and minor (y) axis measuring 2-2.70 mm and
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1.00 mm respectively. The eggs are seen attached to the nest surface by a stalk in a
single clutch with out any overlap. The whole egg mass appear as patches.
Plate.9. Pits of E. suratensis in earthen ponds
The breeding and courtship behaviour in pearlspots can be categorized into
two phases; pre-courtship and post-courtship behaviour. During the pre-courtship
period, the fish exhibits behavioral activities that largely involve display of
nesting substrate and spawning movements including cleaning of the nest site for
egg deposition. During post-spawning phase, the fish largely display behavioral
activities that help to aerate and guard the eggs and the hatchlings from predators.
After the eggs are laid, the eggs are diligently guarded and aerated by the female,
The male guard the territory, and chases away all intruders. The female with their
rhythmic fanning activity by the pectoral fins continually aerate the eggs. The
guarding female occasionally places its mouth gently against the eggs and sucks
away the adhering particles. This process popularly known as ‘mouthing’ helps to
clean the eggs, a behavior characteristic to substrate guarding cichlids. The eggs
hatches out, generally in 70-72 hours, the newly
hatched hatchlings or ‘wrigglers’ are picked
up by the brooding female in its mouth and are
transferred to the breeding pits on the shallow
pond bottom. As the hatchlings are fully
transferred, the female actively engages in ‘pit
guarding’ and closely care the deposited young
ones in the pits (Plate 10). During this period, Plate 10. Joint parental care
‘pit guarding’
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the parent fish continue fanning of pits with their fins and render oxygenation of the
hatchlings that are sheltered in the pits. During the wriggler stage also, fanning
and mouthing of the brood is continued. Occasionally, a few wrigglers are picked
up by the parents, rolled out in the mouth and returned and some times, the entire
brood of wrigglers are shifted from pit to pit. This process of pit transfer probably
helps to cleanse the sticky larvae by removing the adhering particulate matter.
As the yolk is fully utilized, with in a week, the wrigglers develop their
locomotor abilities and become free swimming and the larvae gradually move out
of the pits, in schools and swim freely in to the open waters, escorted by both the
parents. During this period, the movement of the larvae is largely limited with in
the territorial limits, the young ones when disturbed, are seen returning to the pit
nurseries. Even after the brood reaches free swimming stage, parental protection and
constant vigilance is continued and parent fish continue to attack the potential
predators. Individual fry that stray from the brood are orally retrieved by the parent
and brought back with the others. The parents communicate with the young through
signaling by jolting movements and ‘fin flicking’. The fry respond spontaneously
to this ‘calling behavior’. The parental patronage is continued for about two months
till the young ones scatter out and move freely in search of particulate food
materials.
5.2.3. Captive Breeding of Etroplus Suratensis
As E. suratensis was observed to exhibit such a complex and unique
courtship behaviour, involving pairing, nesting and parental care, captive breeding
of Etroplus suratensis was undertaken, in specially designed artificial raceway
tank provided with simulated natural conditions. The raceway system was devised
as a trapezoidal tank with slopping sides and bottom (Plate 11). The tank
facilitated appropriate and diverse depth situations ranging from 30 to 80 cm. This
system was provided with continuous water exchange that facilitated manipulation
of the breeding environment in terms of temperature regime, turbidity, and a mild
flow. Artificial spawning surfaces, comprising 30-35cm long casuarina poles (25
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nos.) fixed on specially fabricated cement concrete base were placed vertically in
the tanks, approximately 50 cm away from the tank margin at varying depths. The
distance between spawning surfaces were approximately 1 meter, taking in to
consideration the territorial requirements of the species. In the very steep and
sloping margins of the tank, where casuarina poles could not be erected vertically,
bricks and cement concrete sinkers, 8-10 numbers were also placed as nesting
substrate Prefabricated artificial pits measuring 6 cm diameter and 4 cm deep were
also deposited close to the nest substrates to facilitate ‘pit caring’, characteristic to
the species (Plate 12). Selected brood fishes, 70 pairs, of size 15cm (90g) to 23cm
(250g) were placed in the raceway tanks, filled with filtered lake water. In order to
prevent formation of algal blooms and to avoid turbidity, the raceway tank was
provided with adequate shading from above by using 50 percent shade netting.
5.2.3.1. Maturation Assessment and Selection of Brooders
In order to enable natural pairing, captive stocks of male and female
broodfish raised in the holding ponds were stocked in the breeding raceways at 1:1
ratio by number. Paired fishes in the broodstock pond were also picked up and
transferred. Fishes were selected based on their external secondary sexual
characters, which were prominent during the breeding season. Ripe and gravid pre
spawning females were identified by its enlarged, reddish and swollen genital
papillae, which is modified in to a fleshy ovipositor. Since milt was not expressible
from the males, they were identified by their thin and pointed genital papillae and
peculiar coloration. Color differences among males were marked during mating
time. Males appear brilliantly colored, and pigmentation of the males begins to
intensify as nest construction commences. Apparently female mate choice appears
to be correlated with this male coloration. In sexually motivated males, as a
common pattern during courting, black occipital stripes were found to appear
between the eye and opercular spot. The swollen cup like ovipositor/ genital
papillae in females and intense display of pigmentation among males during
courting period can be considered as the ‘on heat’ signs of male and female
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fishes (Plate 13). Such paired fishes were seined out from the holding ponds,
assessed of their maturity and stocked in the raceway system for breeding.
Shortly after release, the fishes were found to form 2-3 groups as observed in
the pond breeding trials. Breeding pairs were formed invariably from members with
in the groups and after pairing, the pair gets separated from rest of the group and the
pair was found to search for the nesting substratum at shallow water depths.
Presence of adequate number of spawning surfaces was observed to enhance pair
formation.
5.2.3.2. Nest building and care of young
Pair bonding among the mates was almost stable and not momentary.
Apparently, mate selection was guided by some specific attributes characteristic
among members within a group. In order to enhance pair formation among the left
over members and find choice pairs of appropriate attributes, it was felt necessary to
increase the size of the groups. For this additional numbers of fishes in readiness to
breed were collected from the holding ponds and added to the breeding tanks at
periodic intervals. The paired mates released were observed to clear off algal growth
from the nesting substrate, by browsing with their truncated cone like snout. The
modified mouth also helped in suction feeding. The female fish was found to probe
the artificial nesting substrates with the genital papillae and spontaneously spawn
with one or two trial runs, using the modified cup like ovipositor. At this point of
time, the male and female positions parallelly one behind the other, lay flat to the
spawning surface and then began to attach their adhesive eggs on to the substrate.
After the female has completed attaching one or two rows of eggs, the male
following close behind spontaneously hover over the egg mass, deposited by the
female and fertilizes the eggs by sprinkling milt over the egg patches. Duration of
sperm motility was found to vary from 3 to 4 min. Eggs are never laid on top of
other eggs but are concentrated in a single layer so that one egg just touched the
other (Plate 14). Video graphic recordings of the courtship behaviour and spawning
sequences (Plates 15a – 15h) indicated that the orientation of the spawning partners
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one to the other was characteristic. During spawning, the partners assumed a
ritualistic side-by-side position. Body contact between male and female fish
occurred only infrequently and incidentally as it did not appear to serve any
particular function. The water temperature during courting and breeding varied from
25-270C .
In all, 50 nests were made by the stocked fishes, within the raceway system
during the experimental period of 12 months, during 2004-05. Over 75 percent of
the nest were confined to shallow clear areas at depths between 12 and 45cm. The
maximum depth selected for nest formation with in the raceway was 53.5cm. The
breeding performance of E. suratensis in the artificial raceway system is
summarized in Table 5.3. In the tank breeding system, the percentage success of
breeding was as high as 71 percent, much higher than earthen pond system.
Evidently, the fish was found to exhibit a perceptible preference to cement concrete
substrates as compared to other materials. The mean number of eggs within a nest
was observed to be 32 per cm2. The patch size of eggs per nest was found to vary
from 18 to 56cm2 (Plates 16a – 16h). The number of eggs released per spawning
ranged from 382 to 1966 with a mean of 830. The number of eggs varied according
to the size of the fish and a female fish of average size, 100g was observed to lay
around 250 eggs, where as a female fish of size 200-300g produced more than
1000 eggs. The breeding intensity was however, higher under earthen pond
situations where increased mate choices were available as compared to artificial
tank conditions. The fertilization rate in the confined raceway system fluctuated
between 82.3 to 100 percent.
Egg incubation and hatching could be achieved in two ways, (1) with in the
breeding tank ensuring care of the breeding pairs and (2) in separate larval rearing
tanks with out parental care. For egg incubation, the ‘nest’ along with the eggs were
‘robbed’ from the females, and immediately transferred to indoor incubation tanks,
each of 1.1 t capacity provided with continuous aeration. When the parent fishes
were allowed to care the eggs, the fish was observed to places its mouth gently
against the eggs and was seen to suck away loose particles. The fanning parent
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holds position close to the clutch and facilitated water movement in high amplitude
by their ritualistic pectoral fin beats.
Table 5.3. Egg attachment of Etroplus suratensis in raceway systems
Sl.No. Nesting substrate
Nest Area cm2
No. of eggs
No./ cm2 Fertilisation %
Hatching %
1 Earthen pots 56 951 17 75.7 90* 2 CC. substrate** 32 851 27 90 # 3 CC. substrate 52.5 1371 26 90 # 4 CC. substrate 32 420 13 100 63* 5 Wooden poles 18 792 44 82.32 20* 6 Wooden poles 37.8 1966 52 98 29* 7 Hose tube 19 382 20 100 99 8 Fire brick 18.3 439 24 100 # 9 CC. substrate 18 633 35 95 63
10 CC. substrate 54.4 390 7 98 90 11 CC. substrate 27 1350 50 98.7 34* 12 CC. substrate 28 477 17 95.8 65
* incubation inside the laboratory # eggs are removed by the parents
** Cement concrete substrate
When eggs were incubated separately, without parental patronage, hatching
of eggs occurred in 70-72 hrs under a temperature regime of 25-27oC. The
hatching was protracted and the whole brood was observed to hatch out in a long
interval of 24 to 26 hrs after the first hatching. Embryonic development, hatching
and larval development stages, captured in a Magnus Imaging System connected to
a computer monitor is given in Plate 17 and 18 a-c. The hatched out larvae, or the
‘wrigglers’, were heavily yolked and observed to sink to the bottom. They were
found to instinctively congregate on the tank floor. In the tank incubation,
involving parent fishes, fanning and mouthing of the brood were continued. During
parental care, the fish responded even to the slightest disturbance and was found to
remove all the eggs away from the substratum when bothered. Eggs were also
removed at times when the water became more turbid. Parents continued to
maintain a constant current of water over the pits by fanning with their fins and was
found to clear off any adhering foreign particles on to the larval body.
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Both parents were found to take turns at guarding and fanning them. When
eggs were incubated in spawning tank under parental care i.e., when the brood
fishes were left undisturbed along with eggs , hatching rate varied from 63 to 99
percent, while in artificial incubation without parental care, ie., eggs robbed from
the parents and incubated artificially, the hatching rate was found to range from 20
to 90 percent.
A comparison of the fry yield under artificial tank breeding and natural
breeding in earthern pond condition revealed that fry yield under tank breeding is
perceptibly high,(mean yield 20 to 99 percent), as compared to earthern pond
system (2-70 percent). Under the tank breeding system, a total of up to 2375
hatchlings were obtained from a single brood and this was apparently very close to
the fecundity of the species.
5.2.3.3. Egg Incubation in Larval Rearing Tanks
In situations when eggs were removed from the parents and separately
incubated in Larval Rearing Tanks (LRT), the eggs were found to become
increasingly infected with fungal elements. However, when parents were also
transferred to the larval rearing tanks, in the assumption that they will take care of
the transferred brood, curiously, the fish was found to prey on their eggs. Such a
behaviour was noticed even in the raceway breeding tanks, when the brood was
disturbed, the parent fish was observed to devour the developing eggs. On the
contrary, when the eggs were retained undisturbed for hatching ensuring care by
parents, the parent fishes were found to continually pick up the eggs in their
mouth, destroying the dead and infected eggs especially those that are attacked by
fungus.
Brooding males were more aggressive and were found to push and strike
the intruders using their body or caudal fin, through rapid jerking movements. The
guarding males defended the nest from other fishes and at times, strike the intruders
with its head. And during such ‘head butts’ males strike its intruder on the flanks,
most often dislodging the scales from the opponent(Plate 19). Some times, two
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males may also engage in fight, head to head. At this time, their coloration was
found to intensify markedly.
5.2.4. Embryonic Development
The eggs of E. suratensis, is elliptical in shape, with an average length of
2.2mm and width of 1mm. Like other substrate brooding cichlids, the eggs remain
cemented to the nesting object by a stalk. Eggs of Etroplus are pale yellow in
colour and after fertilization the color changes slowly and they become brownish
just prior to hatching. Cell division in E. suratensis eggs were found to be very
slow, probably due to the presence of large amount of yolk (Plate 17). After 1hr of
fertilization, a blastodisc readily become recognizable on the anterior side of the
yolk. The cleavage is restricted to the animal pole region. The first cleavage is
initiated at 1.30hrs resulting in the formation of two blastomeres of equal size. The
second cleavage results in four equal blastomeres at 2hrs, and the third division
results in 8 blastomeres of equal size followed by the fourth division leading to
sixteen celled stage, at 4 hrs. At this stage, the cells appear irregular in size. Further
division results in thirty two celled stage at 4.30 hrs and from this time onwards the
cell division is no more equal. Cell boundaries are clear, while the cells appear
more or less crowded and occupy the whole space in two layers.
The sixth cleavage results in the formation of 64 celled stage and at 7.30hrs
a multilayered blastula is formed. By 12 hrs, the blastomeres appear as a mass of
cells above the yolk mass. Gradually, the cells spread over the yolk, moving
towards the vegetal pole. As epiboly progresses, the blastoderm at the animal pole
thins out and extend on to either side of the yolk. The yolk mass at this stage
bulges out into the animal pole and the blastoderm appear as a transparent
crescentic mass which is the first sign of formation of embryonic membrane. The
gastrulation commences at about 18 hrs. The germinal ring become differentiated at
this point of time and by 21 hrs, the germ ring invade and migrate towards the
equator of the yolk. The extension of the germinal ring is gradual and slow owing
to the enormous quantity of yolk present.
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By 24.30 hrs, almost 50 percent of the yolk becomes covered by the
developing cells. At this stage, a thin layer of cells were found to envelop the yolk.
By 30 hrs, the anterior region of the embryo expands and the neural plate, appear as
a layer dorsally at the axial end. A thickened median ridge also grows downwards
along one side enveloping the yolk. By 33.30 hrs, head fold appears and the embryo
extends from the anterior end as an elongated tube. The cephalic end of the embryo
gradually gets lifted up and optic buds become visible. At this point of time, somites
gradually become recognizable. Rudiment of heart appears, at about 38-40 hrs and
rhythmic movement of the heart become visible. At 41 hrs, the cerebral region
become differentiated and at the cephalic end otic capsules also become
conspicuous. At 42 hrs, pumping of the two lobed transparent heart, ventral to the
cephalic end become conspicuous and the beats could be counted as 86 per minute.
At this stage 22 pairs of somites are clearly visible. Head and tail is more distinct,
projecting distinctly and the embryo covers almost 50 percent of the yolk at this
point of time. By 43 hrs, cerebral vesicles become clear and heart beat become
more rapid. Blood streaming to the tail region is clearly visible at 49 hrs and the
embryo begins twitching at irregular intervals. By 52 hrs, the tail becomes free
and by 54 hrs, the eye becomes enlarged and prominent. Embryo at this time
encircles almost 60 percent of the yolk and become ‘C’ shaped. The embryo also
exhibit wriggling movements at times within the egg case. By 60 hrs, the embryo
covers almost 80 percent of the yolk and the to and fro streaming of blood become
conspicuous. By 64th hr embryo appear to encircle over 90 percent of the yolk.
Heart beats become rapid and regular, at beat count, 128 per minute. The embryo
completely encircles the yolk by 66 hrs. Prior to hatching, tilting and wriggling
movement of the embryo become more frequent. Gradually the embryo wriggles
violently inside the egg case. The egg case become thinner and it ruptures dorsally
at the cephalic end. Hatching occurs at 73 hrs and the tail emerges out first. The
embryo remain quite for sometime, within the broken egg shell and with the
lashing and wriggling of embryonic movements, the opening get widened. During
further development, the yolk become transparent and stellate chromatophores and
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the oil globules become visible. At 78 hrs, the heart beat become more rhythmic
and rapid, and beat at 138 per minute. Slowly, the membrane near the head also
gets separated and the hatchling becomes free from the shell membrane by 75-80
hrs. This process of hatching is protracted i.e. time interval between the rupture of
egg membrane and emergence of hatchling is quite long. Hatching of eggs in one
brood is also protracted and is completed only in 24-26 hrs. All the developmental
stages of the egg are summarized in Table.4
Although natural breeding of Etroplus has been indicated to be linked to
lunar cycle, such a correlation was not evident in controlled raceway system.
However, spawning frequency was highest during quarters close to new moon phase
from dwadashi to chathurdasi in the tank conditions. This lunar periodicity was
however more perceptible in earthen pond system.
Table 5.4. Embryonic development in Etroplus suratensis
Time after fertilisation
Developmental stages of eggs
1.00 Blastodisc appears 1.30 Two blastomeres 2.00 Four blastomeres 4.00 Blastula appears
10.00 Transparent crescent shaped blastula 18.00 Germ ring is conspicuous 38.00 Heart beating starts 42.00 Head and tail region become more conspicuous 46.00 Cerebral vesicles are clear 48.00 Twitching movement starts 52.00 Tail become free 54.00 Eye auditory vesicles more conspicuous 64.00 Embryo encircles 90% of the yolk;
Head region more prominent 73.00 Egg membrane breaks 80.00 Hatching
As the yolk gets absorbed and the pectoral fins become active in 5-6 days,
the young become free-swimming and the parents lead them out to the open waters.
The family unit moves about slowly guarded by parents and the fry begin to feed on
food particles on or near the substrate. The turbulations on the tank bottom created
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by the parent fish by their fin movement appeared to help in making available
suspended particles as food to the young ones. The high fry yield under protracted
parental care is apparently linked to increased availability of particulate feed and for
the young.
5.2.5. Larval And Post Larval Development
5.2.5.1. Newly hatched larva
Newly hatched E. suratensis larvae is approximately 4.5 mm in size. The
yolk is voluminous with large oil globules and the larvae being heavy sinks to the
bottom. Yolk sac is ovoid and is enormous; broader at the proximal end and narrow
towards the distal end and the yolk extends almost upto the anal opening. The
hatchling is characterized by large pigmented eyes and olfactory pits. Mouth and
jaws are not fully developed at this stage. Anal opening is seen located 4-5 somites
below the level of yolk sac. Fin fold appear continuous from 3rd somite dorsally,
seen attached upto the yolksac on the ventral side. Heart is prominent and is seen
pulsating, located between the head and yolksac, ventral to the eyes. Caudal
circulation through dorsal and ventral blood vessels and interconnecting canals is
conspicuous, and is visible through the transparent body. Notochord and myotomes
are clear, with little ossification in the skeleton. A full complement of fins is also
absent. The newly hatched larvae congregate in pits and bottom corners of the
larval rearing tank, head down and tail up with lashing movements and apparently
exhibit positive geotaxis and a negative phototaxis.
5.2.5.2. Hatchling – 2 day old
In about 24 hrs after hatching, the larva attains an average length of 5.0 mm
(Plate 18a). Yolk sac that remain attached to the head region of the larvae become
separated and the head appear free from the yolk mass. The yolk sac becomes
gradually reduced and large eyes appear deeply pigmented. Otolith is clearly visible
within the transparent body. The two chambered heart located ventrally anterior to
the globular yolk is observed to pulsate vigorously at 99 beats per minute. Mouth
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cleft appears during this period, although not with any marked mobility. At this
stage, blood circulation is rapid and continuous, discernable through the transparent
body. Dorsal fin fold also appears continuous through the caudal end.
5.2.5.3. Hatchling – 3 day old
The larvae measures 6.0mm in length on the 3rd day(Plate 18b). Head region
is well developed with the formation of a characteristic spout like mouth. Lower
jaw begin to exhibit rapid movements. Yolk sac become reduced to half its size and
heart appear spherical with rapid pulsation. The transparent body becomes
pigmented gradually.
5.2.5.4. Hatchling – 4 day old
The larvae measures 6.0mm in length on day 4, starts gliding on the bottom
and begins to swim up with yolk sac down (Plate 18d). Pigmentation become
remarkable on the head and trunk region. Alimentary canal becomes visible through
the body, marked by intermittent pulsative movements. Gradually yolk mass
decreases in size. Upper and lower jaw becomes well developed with characteristic
jaw movements. At this stage, the young ones are found to congregate inside the
artificial pits. Since the fins are not functional, larvae are seen to creep around the
tank bottom to form gregarious patches.
5.2.5.5. Hatchling – 5 day old
The larva attains a length of 6.5mm on fifth day (Plate 18c). The yolksac is
greatly reduced at this time. Fin rays in the caudal region become conspicuous and
the fin fold appear continuous.
5.2.5.6. Hatchling – 6 day old
The larva measures 6.5mm and the yolk sac is almost resorbed by the 6th
day. Pectoral fins also begin to appear at this period. Fin rays become conspicuous
throughout the length of the fin fold. Operculum also becomes conspicuous and the
gills become functional. When kept in glass tank, inside the laboratory, the young
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ones are seen to congregate near the aeration points and are found to move in
swarms.
The fry become free swimming on the 6th day and move in shoals guided by
the parents, swimming mostly underneath the parents. The yolk gets fully resorbed
by this time and the hatchlings attain an average size of 7-8 mm.
5.2.5.7. Nursing of hatchlings
The larvae, after the resorption of yolk, begin to feed on particulate food
matter. They feed on the food particles kept in suspension probably by the
bioturbulation or fin digging activity of the parent fish, in natural pond bottom. The
characteristic vertical bands on the hatchling appear on the body when the
hatchlings attain a size of approximately 1.5cm. In the fry nursing tanks, the young
ones attains a size of about 20mm in one month and freely feed on zooplankton and
powdered Higashi-Fresh (Protein 20 percent). The larvae attain a juvenile size of
3- 3.5cm, in two months of nursery rearing (Plate 20). The juveniles during this
stage is characterized by a black spot on the dorsal fin which gradually disappears as
they attain fingerling size of 4.5- 5cm .
The fry accepts supplementary feeds comprising ground nut oil cake, rice
bran and commercial feed pellets. The young ones now begin to feed on
filamentous algae on the submerged bamboo poles fixed at close intervals in the
nursery system.
5.3. Discussion
E. suratensis, a cichlid endemic to the region, is one of the most potential
candidate species for aquaculture with immense commercial possibilities.
Popularization of farming of this species calls for standardized methods for seed
production. With its opportunistic ability to feed on a variety of alternative food
resources, cichlids are considered extensively adapted to trophic specializations.
However, as observed in this study, it is most conservative with regard to breeding
behavior. As the fish display complex breeding habits and parental behavior,
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traditional methods of breeding inducement by hormonal manipulation has not been
effective.
5.3.1. Pond breeding of E. suratensis
Placement of brood stock in open ponds and pond spawning has been the
only method of seed production of E. suratensis, followed for long.
Investigations on captive breeding of this species under controlled conditions by
manipulation of the breeding environment revealed that in this species, breeding is
preceded by a very complex pair bonding. Fecundity of this species being low, the
actual yield of hatchlings per brood also has been very poor. The realized
fecundity has not been more than half of the potential fecundity. The sperm motility
is higher i.e. 3-4 minute in E. suratensis. Nikolskii (1963) observed that the
duration of sperm motility is higher in slow flowing waters, than fast flowing
situations (i.e. 10-15 sec.).
Although, the breeding biology of the fish with regard to incidence of
gravid females and gonad maturation indicated that the major spawning season of
E. suratensis in Vembanad Lake is from February-April and June-October, the fish
could be made to breed round the year under captive conditions. Nevertheless, in
controlled breeding system also, maximum spawning success is achieved during
June-September. In the artificial raceway system also, apparently, the fish was
found to breed profusely in conditions of low turbidity, facilitated through periodic
water replacement.
5.3.1.1. Decreased turbidity favour visual displays
Visual contact between the parents and the offsprings appear to be a critical
requirement for spawning of E. suratensis (Breder and Rosen, 1966; Keenleyside,
1979; Blumer, 1982; Gross and Sargent, 1985). On the basis of the intensity of nest
building, Ward and Samarakoon (1981) observed that E. suratensis in coastal
lagoons of Sri Lanka breeds during two periods in a year, when the salinity is high
and turbidity is minimal. These authors therefore inferred that fast flowing murky
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waters are inimical to survival of E. suratensis. It appears that the silt laden murky
waters and heavy river discharges during monsoon apparently restrict visual
signaling and threatens the survival of young. In Kerala backwaters, E. suratensis
is reported to exhibit intense spawning during November-March with a peak during
December-January (Jayaprakas,1980; Krishnan and Diwan,1990). This should be
linked to high transparency of water with the cessation of monsoon and incursion of
saline waters. It may be inferred that pearlspots prefer water conditions and
situations that ensured visual contact with the young for breeding. Jalabert and
Zohar (1982) also observed that visual stimuli hastens ovulation but with little
influence on oogenesis. Cole and Ward (1969) also observed that visual displays are
important in the parental behavior of chromides. In terms of selection of nest sites
or selection of mates also, visual displays might be apparently important and it is
presumed that clean water favour this behaviour.
Ward and Samarakoon (1981) observed that E. suratensis reproduced twice
during the year when water conditions were favorable for nest construction and
maintaining visual contact with offspring. These authors have also indicated
decreased water turbidity and increasing salinity as two important factors that favor
spawning. Samarakoon (1983) observed that readiness of the fish to breed is
dependent not only on conditions of turbidity but also on current speed. De Silva et
al.(1984) also presumed that E. suratensis is a visual feeder and so they preferred
clear water for breeding, probably because this ensured feed availability for the
young ones.
5.3.1.2. Shallow water conditions
In Chilka lake, Kowtal (1976) observed that E. suratensis breeds round the
year with peaks in summer and winter, when favorable condition becomes available.
Jhingran (1991) reported that in shallow confined waters, the fish breed almost
round the year, while in backwaters, the fish breeds twice a year. Apparently, the
response of fishes in confined condition is at variance from natural systems and
Fryer and Iles (1972) cited a number of examples to signify that fishes in confined
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conditions might breed more frequently and at a smaller size than natural
conditions.
5.3.2. Artificial raceways and simulated situations
In the raceway system devised, under simulated environment,
E. suratensis could be stimulated to spawn round the year. Rapid removal of eggs
and young ones from the breeding tanks and rearing them in separate systems,
possibly have reduced parental care period. Lee (1979) observed that parental care
suppresses expression of full reproductive potential and fry removal shortens the
time interval between spawning. The present observations on round the year
spawning under controlled raceway systems support the hypothesis that removal of
brood from the caring parent can help to reduce the spawning interval. Peters
(1983) found that the average time period between spawning in tilapias can be
shortened by removal of freshly spawned hatchlings. Verdegem and McGinty
(1987) also indicated that frequent removal of incubating eggs in O.niloticus is one
method that increase spawning frequency and fry production.
In the present study, under controlled conditions in sloping raceway system
E. suratensis was found to prefer breeding in shallow depths. In nature, although
the fish is reported to be eurytopic, and occupy a variety of different habitats;
nursing of juveniles is accomplished in shallow fringe regions, while the maturing
ones move progressively to deeper waters. Ward and Samarakoon (1981) also
observed that E. suratensis migrate to shallow water for breeding. Ward & Wyman
(1975) made a similar observation that sub littoral regions, close to the marginal
vegetation is the habitat of the young ones. Ward and Samarakoon (1981) observed
that unlike E. maculatus that select regions rich in vegetation, E. suratensis select
more specific regions in open locations devoid of vegetation, as nest site. While
E. maculatus breed in several patches in vegetation rich areas, E. suratensis select
bare areas fringed with vegetation. Although parents and nests are exposed in such
open locations exposing them to predation, detection of predator appears to be better
facilitated in such situations. The observation on the abundance of young ones and
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the breeding ones in the shallow littoral regions and fish of larger size in the deeper
regions (Winn, 1956), indicates that conservation management of pearlspot calls for
more co-ordinated approaches concentrated in shallow waters, the natural breeding
habitats of the species.
5.3.3. Artificial spawning substrates
The higher rate of spawning in the artificial raceway system is invariably
linked to increased availability of artificial nests substrates provided with in the
tanks. In fact, an upsurge in spawning frequency was observed when spawning
surfaces were supplied. However, the artificial, pre fabricated breeding pits
provided were seldom utilized by the parent fish. The fish rather preferred to utilize
the cleaned floor of the tank for hatchling care. Occurrence of large schools of fry
containing many more individuals than a typical single brood, is indicative of the
communal care behaviour in E. suratensis as observed also by Ward and Wyman
(1975, 1977). E. suratensis has been reported to prefer a variety of nest substrates
in natural pond situations viz., stones, coconut shells, coconut petioles, tiles, bamboo
and wooden pieces. Samarakoon (1985) observed that E. suratensis spawned rapidly
in ponds supplied with spawning surfaces as compared to situations without
appropriate breeding substrates, when similar environments were provided. Sultana
et al., (1995) also stated that in nature, the fish spawn on a variety of hard objects
that facilitate attachment of eggs and this include stones, pieces of wood, coconut
husks, water logged coconuts, mid ribs of coconut and palm leaves tiles, bamboo
pieces, asbestos sheets, and any other submerged objects situated at a depth not
more than 100 cm. A sticky material produced at the time of ovulation by the
granulosa, around the special threads emerging from the zona radiata has been
indicated to glues the eggs into the substrates (Nicholls and Maple, 1972).
Incidentally, the spawning fecundity was quite high for fishes bred in tank
breeding trials as compared natural system. This is apparently due to the better
nutrition of the broodfish raised under captive conditions, as the number of eggs
released per spawning naturally increases with body weight and physical well being
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of the fish. Food availability and feeding intensity has been reported to influence
rapid maturation of gonads ( Sobhana and Nair,1980)
5.3.4. Spawning rhythm and lunar periodicity
In most teleosts, spawning periods appear to be adjusted to environmental
factors such as photoperiod, temperature, salinity, rainfall etc so that they are
suitable for rearing offspring. Avault (2006) has described key factors that
stimulate, inhibit or exert regulation on various stages of breeding cycle. In
temperate zones, where photoperiod and temperature are of greater magnitude,
spawning occur during limited period in a year. In tropical regions, temperature is
rarely the limiting factor, the effect of seasonal rainfall is most apparent.
Breeding of E. suratensis appeared to be linked to the lunar cycle and over
30 percent of the breeding occurred during dwadashi to chathurdashi. Probably,
this is a physiological adaptation. Being an estuarine fish though adapted to thrive
in freshwaters, the tidal amplitude facilitated by the lunar phase provided stable
conditions for ovulation as well as egg care. Lam (1983) indicated that in many of
the tropical fishes, peak spawning activity is often associated with lunar cycle,
rainfall or floods. Mathew et al. (2002) observed that in case of Groupers,
Epinephalus tauvina spontaneous spawning occurred 3-4 days before or after the
new moon phase. Sundararaj and Krishnamurthy (1975) reported that abundance of
zooplankton favored seasonal reproduction of E. suratensis and in Vellar estuary,
South India, such a situation of higher food availability occurring at salinity ranging
from 10-31 ppt coincided with peak breeding.
5.3.5. Coloration and reproductive readiness
As observed in the present study, several authors have indicated that male
coloration intensifies during courtship (Greenwood, 1974; Fryer, 1977; Fryer and
Iles, 1972). Colour patterns are a very important channel for communication in
cichlids (Nelissen, 1991) and this is attributed as an important component of the
specific mate recognition system in cichlids (Ribbink, 1990). Anatomical and
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physiological evidence also strongly suggests that cichlids have strong color vision
(Muntz, 1976). Similar to pearlspots, Rothbard and Pruginin (1975) observed that
during period of courtship, tilapia displays highly intensified body pigmentation
much stronger and more typical among males. McElroy and Kornfield (1990) stated
that the breeding coloration among males is an adaptive mechanism to advertise its
presence and reproductive readiness. Endler (1995) suggested that physical
attributes of habitat, light condition, water clarity etc affect considerably the
expression of male secondary sexual traits and the female enticement and
preferences for these traits apparently influence pairing process. This obviously
elucidates the importance of physical environment in triggering breeding.
Ward and Wyman(1977) and Yamaoka(1991) observed that the nesting of
green chromides is influenced not only by the availability of spawning sites but also
lack of incidence of orange chromides, their major predators that raid and prey on
their eggs. In nature, the obvious predators or cannibals of pearl spots have been
identified to be orange chromides (Ward and Wyman, 1977) as schools of adult
orange chromides raid the nesting locations of pearlspot. In the tank breeding, the
total protection from such predators apparently ensured higher survival.
5.3.6.Territoriality and spawning
In the raceway system devised, the fish was observed to utilize artificial
substrate with in specific territories and at specific distance. Samarakoon (1981)
also observed that E. suratensis breed in isolated territories of approximately 2m in
diameter. And hence, it is reasonable to suggest that spawning surfaces should be
placed at interval of 2-4m to ensure the required degree of isolation and encourage
nesting. Despite closer spacing of the substrates in the raceway system, nesting
occurred only at a wider space interval. This apparently highlights the territoriality
of the species.
The percentage success of breeding was quite high at 71 percent in the
raceway breeding system. It appears that the probability of bringing together male
and female fish with attributes necessary for successful pairing is enhanced in
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controlled tank breeding, where the group size of broodstock is considerably
increased. Samarakoon (1985) observed that when E. suratensis is stocked in a sex
ratio @ 1:1 under earthen pond system, less than one third of the fishes were found
to breed. It was also observed that pair formation and nest site selection occurs only
within groups, and among members of a groups less than 50 percent succeed in
pairing. Legendre and Trebaol (1996) observed that in case of mouth breeding
cichlids S. melanotheron, spawning interval is considerably increased when sex
ratio is in favor of females which implies that higher sex ratio does not significantly
increase individual spawning frequency.
5.3.7. Tank breeding with Parental patronage
One of the problems encountered in the traditional pond breeding of
pearlspot by environmental manipulation has been the lower yield of seeds in such
systems. And the most serious practical difficulty encountered in this method is
capturing of the juveniles. As the juveniles sink to the bottom, pond draining
becomes inevitable. Such exhaustive collections also bring about unwanted
disturbances to other pair bonded brooders. This difficulty is overcome in
controlled raceway systems where seed recovery is facilitated with less effort.
In tank incubation, with out parental patronage, egg masses were exposed
to fungal menace more often. Some times, heavy infestation with fungal mat
resulted in large scale mortality of eggs. Egg masses cared by the parents were
infected with aquatic fungi less frequently. Takahashi et al. (2004) observed that
under conditions of low parental care, infections are characteristic in large broods.
Bergmann (1968) observed that egg guarding fishes facilitated cleaning of
guarding eggs by two unique cleaning process viz., ‘mouthing’ and ‘snapping’. In
mouthing, the parent fish places its mouth gently against the eggs and sucks away
loose particles. Dead or fungus ridden eggs are removed by a more vigorous mouth
contact, often called ‘snapping’. However, when the brood was disturbed, the
parent fish was found to devour the developing eggs. Fryer (1959), Fryer and Iles
(1972), Holzberg (1978), Ribbink et al., (1980,1981,1983) have postulated that all
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cichlids regardless of their trophic specializations, even most specialized herbivores,
opportunistically prey upon their eggs and embryos if given a chance to do so. Fox
(1975) while reviewing cannibalism stated that increasing population density
coupled with food scarcity are factors that encourage cannibalism.
In the present study also, a more synchronized hatching was apparent when
egg hatching was facilitated by parental patronage. Obviously, parental guarding
accelerated and synchronised hatching and this was well facilitated by mouthing and
nibbling of the eggs by the parents. Zoran and Ward (1983) also observed that in
E.maculatus the embryos kept without parents hatch later, and over a more extended
interval, than those attended by parents.
5.3.7.1. Substrate brooding to mouth brooding among cichlids
Pearlspots were found to spend 68-77 percent of the time budget for
guarding nest and 6-10 percent of their time in interaction with intruders. In
E. maculatus, where the male and female parent alternately cared the young this
time allocation is only half (Ward and Samarakoon, 1981). Cichlids are unique in
their mating behavior and the prolonged period of active parental care of the young
is a characteristic feature. They also guard their offspring for several weeks even
after hatching. In E. suratensis, parental care starts from fertilization onwards and
both parents are actively involved in this process. The parent fish, mostly female,
brood over the fertilized eggs and facilitate an environment suitable for their normal
growth and development. Such a prolonged and close association between parents
and offspring also acts as a two-way communication between generations (Breder
and Rosen, 1966; Keenleyside, 1979; Blumer, 1982; Gross and Sargent, 1985).
Nevertheless, in E. suratensis, both the parents were actively involved in parental
care and nest guarding. Biparental guarding is considered a primitive pattern of
parental care as compared to mouth breeding form of parental care seen among
Tilapias. Oppenheimer (1970) elucidated that mouth breeding syndrome is a highly
derived form of parental care. This behaviour is derived from an ancestral substrate
brooding form, in which parental acts such as mouthing of eggs and transferring of
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the embryos were incorporated. Perrone and Zaret (1979) concluded that biparental
care is a serious evolutionary problem, because in terms of investment, both parents
are not required to raise a brood. However, according to Perrone and Zaret (1979),
the biparental care is an adaptation necessary for survival as this would permit
parents to go foraging alternatively and this would help to keep them in good
physical health for future guard duty. Baylis (1981), while tracing the evolution of
parental care in fishes observed that the major factors that favour parental
behaviour are 1)defense of nest site 2)facility for multiple spawning 3) preservation
of optimal quality of the nesting site and 4) pre-spawning care of hatching eggs.
Noakes and Balon (1982) characterized fish species in to two categories, ‘r
selected’ strategists with a life history pattern that include short growth interval,
early maturation, high fecundity, reduced parental care and short life span and ‘K
selected’ strategists when these species have a prolonged growth interval, deferred
maturation, reduced fecundity, increased parental care and extended life span.
Noakes (1981) observed that ‘K selected’ condition similar to that of Pearlspots is
a more mature and competitive condition, evolved through ecological succession.
Growns (2004) on the other hand classified fishes with a breeding strategy that
involve parental care that ensures protection of young and therefore lower
mortality rates among young ones falling under Guild C. Apparently E. suratensis
with characteristic parental care, nest building and protection of young can be
categorized to include in this category.
5.3.7.2. Parental care behaviour
Fanning is the most common parental care behavior carried out by the
substrate brooders (Keenleyside,1991). The fanning parent holds position close to
the clutch and moves the water with large amplitude beats of the pectoral fins.
Fanning is reported to not only prevent dirt settling but also increase the oxygen
level of water near the eggs. The current of water created over the eggs helps to
remove metabolic wastes. According to Zoran and Ward (1983), both active and
passive fanning are observed in most substrate brooders such as E. maculatus,
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Herotilapia multispinosa etc. (Baylis, 1974; Smith-Grayton and Keenleyside,
1978).
5.3.7.3. Pit caring
Despite deposition of artificial pits, in the raceway system the parent fish
was observed to seldom use this facility for pit caring. However, in the larval
rearing tanks, where the eggs were incubated artificially, the wrigglers sought
protection in these structures. The average diameter of the fry pits are reported to be
5.2cm and 4.2cm in deep (Jayaprakas, 1980). Similar to E. suratensis, Ribbink et
al (1981) observed that the nesting pits of T. rendalli, a substrate spawning guarder
is usually not more than 6cm deep and a single nest has been found to have 5-24
pits of varying depths . The wrigglers remain in the pits attached by mucous
threads from three pairs of head glands, where as in mouth brooders these glands are
vestigial or absent (Keenleyside, 1991).
5.3.7.4. Parental care and larval nutrition
Poor fry yield in larval rearing tank when eggs are incubated without the
support of the parent indicate that parents helps in providing the required nutrients
by their bioturbulation activity. Fin digging and micro nipping are two parental
behavior in cichlids which makes food more readily available for the fry
(Quertermus and Ward,1969; Williams,1972; Noakes,1979). During fin digging,
the adults settles on the substrate and with vigorous, rapid beats of the pectoral fins,
stirs up loose materials for feeding the fry. Where as, in ‘micro nipping’ the fry
ingest mucous from the body of the parent and the parent’s epidermal mucous
production is increased during the fry-brooding period, and the young fish
regularly bite at the adult’s body and swallow the mucous (Hildemann,1959).
Both these behavior are regular and common in E. suratensis and E.maculatus even
in the presence of other foods (Ward and Wymann, 1977). Similarly, Seth (2001)
reports that the nest guarding males of giant river catfish, Aorichthys seenghala,
release a white creamy secretion locally known as ‘Chara’ and the young ones are
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nursed and fed over this. Since the major component of parental mucous is
protein, apparently this assist in nutritional management of broods and
larviculture (Khen and Chien, 2006). This would imply that for better fry survival
and fry yield in tank breeding and hatchling care without parental patronage, there
is a dire need to develop alternate sources of nutrition so as to ensure high
hatchling survival.
5.3.8. Larval Development and Fry Nursing
E. suratensis exhibits a prolonged parental care and adults are found with
young ones till they attained size up to 40mm. This would explain the poor growth
rate of E. suratensis in pond culture compared to open water cage culture, where
breeding is totally restricted. However, the parent-offspring association in
E. suratensis is comparatively short as compared to E. maculatus where it has
been reported to be very much extended and last up to 6 months (Ward and Wyman,
1975,1977). Since gonad development is inhibited during parental care, which is
precisely the period when prolactin is supposed to be acting, the spawning interval
is considerably prolonged in such species.
Balon (1975) identified three developmental period within the embryonic
period: the cleavage phase, embryonic phase and the eleutheroembryonic phase.
The process occurs within 12h in Danio rerio, while it takes more than 100 days in
some salmonids. Panikkar (1920) reported that the incubation period of eggs in
E. suratensis vary from 82 to 100 hours. Experiments in relation to amount of yolk
and developmental rates shows that the hatchlings of guarder species retain much
smaller amount of yolk become free swimming in a much earlier state of
development (Noakes, 1991). The embryonic period begins with fertilization of the
egg and ends with the transition from endogenous to exogenous feeding.
The larval period begins with transition to exogenous feeding and terminated
with metamorphosis in to adult organs such as differentiation of the median fin fold
and ossification of the vertebral centra (Noakes and Balon,1982). The larval
development period includes the yolk sac larvae period and the post yolk sac
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larvae period (Liang et al., 2003). In case of E. suratensis, the yolk sac to larval
period commences when the embryo become free of the egg membranes (hatching)
and ends when the yolk of larvae is completely absorbed. The latter stage begins
with complete absorption of yolk occurring on the 6th day and ending with the
emergence of scales. During this period, the body length has been observed to
increase from 4.5 to 6.5 mm on the average and the young ones begin to feed on
attached algae on the substrates. The larvae moves in schools along with the
parents and it reduces the probability of any one individual being preyed by a
predator.
In the present study, seed production has been facilitated in artificial
raceway system by selective stocking of paired brood stock, and by providing
simulated environmental conditions such as transparency of water conducive for
spawning. Feasibility of egg incubation in artificial larval rearing system with and
with out parental patronage was also facilitated. In the context that seed
production of E. suratensis under pond condition has been most unpredictable, and
natural spawning has been dependent almost entirely on a variety of factors the
present study assumes immense significance. The technology of captive breeding
devised and demonstrated with least dependence on finer protocols of hormonal
manipulation is extremely important as it opens up a simpler technology for mass
production of seeds of E. suratensis, a high value species preferred for culture in
fresh and brackish waters.