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, spawned in large hapas (120 m(2)) in ponds under different exchange and management strategies, was monitored over a 70-day period. Female fish were checked for eggs twice weekly and returned to the same spawning hapa throughout the experiment (0 day) or exchanged for females maintained in separate single sex groups at high density for a period of 7 days at each (3.5 days) or alternate (7 days) harvests. Exchanged females were conditioned in separate hapas (20 m(2)) in the same pond. Production of eggs per unit area were significantly 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% per harvest). Conditioning of female broodfish had no effect on the egg clutch size(P > 0.05). Female productivity (eggs kg(-1) day(-1)) was significantly lower for fish exchanged after every harvest (3.5 days) than those remaining in spawning hapas for alternate harvest (7 days) or throughout the experiment (0 day), Gonadal steroid (testosterone, T and oestradiol, 17 beta-E-2) and calcium levels sampled at the end of the experiment reflected spawning performance; serum T and E-2 were highest in exchanged females. Non-spawned 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 spawned 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 of female broodfish from spawning hapas with conditioned fish can improve productivity of hapa-in-pond systems. (C) 2000 Elsevier Science B.V. All rights reserved.