ANDREAS STAMER, ANDREAS MÜLLER-BELECKE, GABRIELE HÖRSTGEN-SCHWARK
Georg-August-Universität Göttingen, Institute of Animal Husbandry and Genetics, Germany
For a profitable Nile Tilapia aquaculture the management of broodstocks by synchronising their spawning activity can be beneficial, especially among intensified conditions, where large amounts of eggs and larvae can be necessary at a given time. Due to the short spawning intervals in Nile Tilapia females (Oreochromis niloticus) the introduction of ovulation by hormonal treatments appears to be difficult and expensive. The aim of the present study was to test the potential of different environmental factors in synchronising the spawning activity of O. niloticus females. Mature females (Lake Manzala population, Egypt) were transferred into 350 l-aquaria in groups of ten to twelve individuals were they were initially kept among optimum water parameters for spawning (temperature: 28°C; acidity: pH6.6; hardness: 9.4°GH; salinity: 0.2ppm). After acclimatisation, treatment groups were adapted to high hardness (30-40°GH) or high salinity (15-22ppm), respectively and kept among these suboptimum conditions. By daily observations for readiness to spawn in treatment groups the potential of the tested environmental influences to suppress spawning activity was determined in relation to adequate untreated control groups. After 21 to 28 days among suboptimum conditions the treatment groups were adapted back to the optimum water parameters and the spawning activity was monitored for another week, to check for initiated synchronous spawns. Furthermore, 18°C-temperature treatments and pH5.0-treatments for 60 hours were similarly tested as short term environmental effects on spawning activity. Treatments and the corresponding controls were carried out in replicates. To determine the influence of the tested environmental conditions on fecundity and egg quality, eggs from spawning females were artificially stripped, inseminated and their developmental rates were observed. The best synchronisation effects were obtained after treatments with 22ppm salinity. In six replicated groups no spawning activity was found during treatments whereas 47% of the females spawned in the week after treatments. In the corresponding control groups 49% of the females spawned but only 11% were achieved during the last week. Although fecundity and hatching rates were negatively effected by high salinity, per group member the output of swimming larvae during the last observation week increased distinctly after treatments (n=285 versus n=101 in controls).
Keywords: Aquaculture, environment, Oreochromis niloticus, reproduction, salinity, spawning, synchronisation