BORIS DE LAPEYRE1, ANDREAS MÜLLER-BELECKE1, BERTA LEVAVI-SIVAN2, GABRIELE HÖRSTGEN-SCHWARK1
1Georg-August-University Göttingen, Institute of Animal Husbandry and Genetics, Germany
2The Hebrew University, Department of Animal Sciences, Israel
Unlike other important fish in aquaculture, a suitable tool to induce spawning in the female Nile Tilapia (Oreochromis niloticus) in a given period has not been discovered yet. The benefits of a synchronised spawning behaviour would be a considerable saving in labour as well as greater accuracy in the planning of large batches of fry and reduced water requirements. Hormonal treatments are costly and have not brought about satisfactory results due to the asynchronous and aseasonal nature of O. niloticus with its short spawning interval. The aim of the present study is to evaluate the effect of temperature regimes and stocking density changes on the spawning activity of O. niloticus. In the temperature experiment adult females from the Lake Manzala population, Egypt, were acclimatised in groups of twelve in 300 l glass-aquaria for 1 week at optimum conditions (temperature 27.6C; pH 6.9). The temperature was then decreased to about 22 C for 4 weeks, subsequently the temperature was raised to original level again and the fish were observed for a further 1 week. For the stocking density tests the females were taken from a 1m3 basin (stocking density 29-45 kg m-3) and placed in the glass-aquaria at optimum conditions in groups of twelve, or individually (stocking densities; 13 and 6.5 kg m-3 respectively) and observed for a period of 4 weeks. During all experiments the fish were checked daily for signs of spawning after which they were artificially striped and eggs fertilised. Egg-quality was measured as the percentages of hatchlings after 4 days and swim-up-fry after 9 days. 24% of females spawned within the first 3 days after temperature treatment compared to 7% of the control fish. The long cooling period of the females (n=48) seemed to have a negative effect on the egg-quality with hatching and swim-up-fry rates of 25% and 22% in average while the control values were 53% and 48% respectively. More promising synchronisation came from stocking the females individually without temperature treatment. From 48 experimental breeders, 33% of females spawned within the first 3 days after stocking, and hatching and swim"=up-fry rates were 49% and 41% respectively.
Keywords: Aquaculture, Oreochromis niloticus, spawning, stocking density, synchronisation, temperature