Aspects of the Reproductive Biology of Tilapia Guineensis ( Bleeker , 1862 ) Under Laboratory Conditions

Aspects of the reproductive biology of Tilapia guineensis was carried out in the laboratory. Twelve ripe male and female brooders of T. guineensis (body length, 12.2 12.9cm and weight, 100 – 130g) paired 1≅: in triplicate were made to spawn naturally in glass aquaria (60 x 30 x 30cm 3 ). Results show mean fecundity to be 1,269 eggs per female spawner with mean egg size of 1.48mm. Hatching of eggs started 47-48hours after fertilization at a mean temperature of 27.4 0 C. Percent hatch was 74.8% and length of hatchling as 3.8mm. Time of yolk sac absorption was observed to be 4 days with swim-up fry length, 5.7mm and weight, 1.67mg. Parental care was prominently displayed by both parents.


INTRODUCTION
The brackish water zone of the Nigerian coast which includes creeks, lagoons, rivers and mangrove swamps has tremendous potential for fish farming.It is estimated that the saline swamps in the Niger Delta area alone cover 5,048km 2 and out of which about 3,108km2 could probably be used (Scott, 1966).For the development of brackish water fish farming, several species have been considered as potential candidates.Tilapia guineensis is the only popular and true estuarine species that is commonly cultured in the brackish water region in Nigeria (Horsfall, 2006;Musa et al., 2006).However, experimental reports on its performance in culture are limited (Magnet and Kouassi, 1979).
Tilapia guineensis is a nest builder and has a great capacity to adapt its reproductive behaviour to the environment.In concrete tanks, the eggs are simply laid on the wall to which they adhere (Legendre and Ecoutin, 1989).The development of T. guineensis commences immediately after successful spawning and fertilization of the eggs.Isaac -Harry (1986) reported that T. guineensis is relatively fecund and a multiple female spawner.Fagade (1979) and Eyeson (1983) reported that T. guineensis reproduce all year round in West African regions.Tilapia lives in aquaria spawn throughout the year provided the temperature of the water is maintained at 25-30 o C and other living conditions such as photoperiod and aeration are adequate (Mires, 1977).Data on the reproductive biology of T. guineensis in their natural environment are rather scarce and inadequate (Fagade, 1979;Legendre and Ecoutin, 1989).Therefore the purpose of this study is to provide information on the reproductive characteristics and development of T. guineensis to enhance management and improve the production of seeds for brackish water farming practice.

Experimental Site
The study was conducted between March and April, 1989 at the Department of Fisheries Laboratory of Rivers State University of Science and Technology, Port Harcourt, Nigeria.

Collection of Brood fish
Adult Tilapia guineensis broodfish (body length, 12.2 -12.9 cm and weight, 100 -130g) for experiment were obtained from the University's brackish water fishponds with the aid of cast nets and wire traps.The fish collected were transported alive in plastic containers with water to the Department of Fisheries Laboratory.Individual brooder was identified, hand sexed and separated into separate larger containers.The fish were fed a locally compounded feed of 30% crude protein, twice daily at the rate of 3% of their total weight for 7 days prior to using them for spawning.

Natural Spawning in Aquaria
Three separate glass aquaria measuring 60 x 30 x 30cm 3 were used to spawn T. guineensis in the laboratory.Each aquarium was filled with filtered and aerated saline water to the 40-l mark.Opposite sexes of the broodfish were paired and introduced into each aquarium at 1:1 ratio in triplicate.The pairs were allowed to spawn and fertilize the eggs produced.
The broodfish were not fed during the spawning period but the water was changed daily at 0730 hours and aerated.Fish were observed to spawn after 2 days of pairing.The eggs were observed 12-hourly, counted and a sample of 10 eggs taken and sizes measured.The eggs were allowed to incubate and hatch inside the aquaria through constant mechanical aeration.The male broodfish removed and the sac fry counted by siphoning them into a bowl.Samples of 10 hatchlings were taken and their body lengths and weights measured using a metal rule and a balance.The female broodfish was also removed from the aquaria 4 days later when the fry attained swim-up stage of development.The body lengths and weights of the swim-up fry were also determined similarly as the hatchlings.Water temperatures and salinities were recorded three times daily with a mercury-bulb thermometer (0-100 0 C) and salinity refractometer respectively.Dissolved oxygen was determined by methods described by Boyd (1979).

Spawning Behaviour
In the laboratory, the pair of male and female breeders in the aquarium was observed to flay their tails and nip each other.Both sexes were always found together and aggressive.During spawning, the female laid the eggs at one corner in rows while the male spread milt on the eggs as it swam over them.

Hatching of Eggs
Table 1 gives detail of the spawning data during this experiment.The mean number of eggs produced was 1,269 and these hatched within 47-48hours at a mean water temperature of 27.4 0 C. The mean diameter of an egg was 1.48mm.Guarding and ventilation of eggs was effected by both the male and female parents.Percent hatch of the eggs was calculated as 74.9%.The parents were also observed to occasionally pick the weak hatchlings into their mouths and threw them to a distance.This action probably got the hatchlings detached from their shells.

Water quality measurements
Table 2 shows the summary of the water quality data obtained during this study.Temperature ranged 26.0-28.00 C with a mean value of 27.4 0 C. Mean salinity and dissolved oxygen values were 19.5ppt and 5.5mg/l respectively.

DISCUSSION AND CONCLUSION
The result of the study on the reproductive behaviour of T. guineensis showed that prior to spawning, the breeders cleaned the corners of the aquaria of any dirt.This is a corresponding activity they perform in the wild (Campbell, 1987).The observed egg size for T. guineensis in this study fall into the range observed for Tilapia zilli eggs, 1.1-2.0mm(Jegede and Fawole, 2006).Holden and Reed (1972) stated that these two species may be very similar and only variants.The body length of T. guineensis hatchling was observed to be 3.8mm in this study.Campbell (1987) reported in a review of the biology of T. guineensis that at temperatures greater than 26 0 C, the eggs hatch in 2 days and the yolk sac absorbed 4-5 days later.This information corroborated the results obtained for T. guineensis on corresponding hatching time and temperature.Uka and Sikoki (2011) reported that the salinity range of 0-17 ppt was favorable in the spawning and hatching of fertilized eggs of T. guineensis.The 19.0-20.0ppt salinity range observed in this study was slightly higher indicating that T.guineensis could tolerate more than earlier reported.The dissolved oxygen mean value of 5.5 mg/l equally favoured the performance of T.guineensis during spawning, fertilization of eggs and development into full embryo.
The observations of Hanon (1975) on egg and larvae sizes of 2.7mm and 5.0-5.5mmrespectively were slightly higher than values obtained in this study.This could be due to differences in sizes of fish used for spawning (De Martini, 1991) and the environmental or ecological conditions before and during spawning exercise (Jegede and Fawole, 2006).The report on parental care exhibited by both male and female parents on eggs and hatchlings corroborated the activities of T. guineensis spawners in the wild (Holden and Reed, 1972).This action is vital to the survival of the hatchlings and their early life orientation which is to aggregate together.
The successful breeding of T. guineensis in a glass aquarium other than pond environment pave the way to a practice that would prevent the infiltration of different species into the breeding stock completely.Hence experiments to find clues to the perennial problem of prolific spawning behaviour of Tilapias could be conducted easily.