CONTROL OF REPRODUCTION IN NILE TILAPIA (OREOCHROMIS NILOTICUS) BY MANIPULATION OF ENVIRONMENTAL FACTORS

Abstract

The aim of the study was to determine which holding conditions are most propitious in inducing the greatest number of breeders to spawn within an interval of 5 days. The spawning frequency was defined as the proportion of female breeders that spawned within this given interval of 5 days. To assess fertility and egg quality, the data collected were the number of eggs per gram body mass of the female breeders as well as the percentage of hatchlings and swim-up fry. During the first set of trials, the female breeders were kept at a water temperature of 28°C until they were acclimatised, before the water temperature was reduced to 22°C for a period of 7 days (group I), 14 days (group II) and 28 days (group III). Subsequently the water temperature was returned to the initial 28°C for all three groups of female breeders. There were three corresponding control groups whose water temperature was kept constant at 28°C. The spawning rates in group II and group III (40% and 31%) were significantly higher than those of the corresponding control groups (13%, 11%). However, the percentage of hatchlings in the control (group II: 74%, group III: 58%) was twice as high as in the test groups II and III. During the second set of trials, 2 experiments were tested. In the first experiment, all the females were kept together in an aquarium of volume 1 m3 for one week. They were then moved to individual 0.3 m3 aquaria where they remained for one week. In the process the stocking density was reduced in a ratio of 7:1 (from 48 to 7 kg m-3). In the second experiment, the females were all kept together in a 1m3 aquarium for 3 weeks and 4 weeks with a photoperiod of 6 hours of light and 18 hours of darkness (6L:18D).They were then moved to small individual 0.3 m3 aquaria were they remained for one week at a photoperiod of 12 h light and 12 h darkness (12L:12D). At this point the same reduction of stocking density as in the second experiment occurred (ratio of 4:1). During the last set of trials as in the first experiment of the second set of trials, all the females were kept together in an aquarium of volume 1 m3 for one week. Subsequently, they were then moved to individual 0.3 m3 aquaria where they remained for one week. In the process the stocking density was reduced in a ratio of 4:1. The procedure was then repeated three times (repeat 1, 2 and 3) with the same female breeders It was shown that the spawning rate increased more significantly (second set of trials-experiment 1: 32%) with a greater change in stocking density than was achieved with a smaller change (third set of trials - repeat 1: 25%). Furthermore, it was found that repeated treatment of the same fish (third set of trials) caused an increase in the spawning rate from repeat 1 to repeat 3 (25%, 26%, and 39%). It was the combined effect of a photoperiod change after a 3 week period and a lower change of stocking density (second set of trials-experiment 2) that yielded the highest spawning rate overall (65%). The percentage of resulting hatchlings and swim-up fry was on average higher in all the experiments of the second and third set of trials than it was in the first one. In the present study, the effect of environmental factors such as water temperature , stocking density and photoperiod on spawning in large groups of female O. niloticus breeders was systematically tested (N = 674) for the first time. It was shown clearly that the spawning behaviour of female tilapia breeders can be manipulated in favour of scheduled production of spawn. Thus it is possible to produce a great number of eggs in a predefined period of time with a simple manipulation of stocking density, something relatively simple and fast to realise. If more time is available, a change in stocking density in combination with a 3 week photoperiod of 6L:18D is recommended. What now remains to be verified is whether the trigger that sets off spawning in the female after, for instance, a change in stocking density, is genetically determined. In that case, breeding lines capable of higher spawning rates could possibly be created through selection.