Effect of broodfish exchange strategy on the spawning performance and sex steroid hormone levels of Oreochromis niloticus broodfish in hapas

The productivity of Nile tilapia (Oreochromis niloticus L.) broodfish, spaw ned in large hapas (120 m(2)) in ponds under different exchange and managem ent strategies, was monitored over a 70-day period. Female fish were checke d for eggs twice weekly and returned to the same spawning hapa throughout t he experiment (0 day) or exchanged for females maintained in separate singl e sex groups at high density for a period of 7 days at each (3.5 days) or a lternate (7 days) harvests. Exchanged females were conditioned in separate hapas (20 m(2)) in the same pond. Production of eggs per unit area were sig nificantly higher (P < 0.05) in females exchanged weekly (7 days, 64 eggs m (-2) day(-1)) or at each harvest (3.5 days; 55 eggs m(-2) day(-1)) than for fish remaining in the spawning hapas throughout the trial (0 day, 33 eggs m(-2) day(-1)). A greater proportion of conditioned females (3.5 days and 7 days) was spawned than in the control group (18% females compared to 11% p er harvest). Conditioning of female broodfish had no effect on the egg clut ch size(P > 0.05). Female productivity (eggs kg(-1) day(-1)) was significan tly lower for fish exchanged after every harvest (3.5 days) than those rema ining in spawning hapas for alternate harvest (7 days) or throughout the ex periment (0 day), Gonadal steroid (testosterone, T and oestradiol, 17 beta- E-2) and calcium levels sampled at the end of the experiment reflected spaw ning performance; serum T and E-2 were highest in exchanged females. Non-sp awned fish had elevated steroid levels compared to females incubating seed at harvest(e.g., 19.5-91.4 and 7.2-21.5 ng ml(-1) T for non-spawned and spa wned fish, respectively). Stocking of the Indian Major carp Labeo rohita in spawning hapas to control fouling did not improve water quality and led to a significant depression of seed production (P < 0.05). Regular exchange o f female broodfish from spawning hapas with conditioned fish can improve pr oductivity of hapa-in-pond systems. (C) 2000 Elsevier Science B.V. All righ ts reserved.