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

Ž . The productivity of Nile tilapia Oreochromis niloticus L. broodfish, spawned in large hapas Ž 2. 120 m 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 Ž . spawning hapa throughout the experiment 0 day or exchanged for females maintained in separate Ž . Ž . single sex groups at high density for a period of 7 days at each 3.5 days or alternate 7 days Ž 2. harvests. Exchanged females were conditioned in separate hapas 20 m in the same pond. Ž . Production of eggs per unit area were significantly higher P-0.05 in females exchanged Ž y2 y1. Ž y2 y1. weekly 7 days, 64 eggs m day or at each harvest 3.5 days; 55 eggs m day than for Ž y2 y1. fish remaining in the spawning hapas throughout the trial 0 day, 33 eggs m day . A greater Ž . proportion of conditioned females 3.5 days and 7 days was spawned than in the control group Ž . 18% females compared to 11% per harvest . Conditioning of female broodfish had no effect on Ž . Ž y1 y1. the egg clutch size P)0.05 . Female productivity eggs kg day was significantly lower for Ž . fish exchanged after every harvest 3.5 days than those remaining in spawning hapas for alternate Ž . Ž . Ž harvest 7 days or throughout the experiment 0 day . Gonadal steroid testosterone, T and . oestradiol, 17b-E and calcium levels sampled at the end of the experiment reflected spawning 2 performance; serum T and E were highest in exchanged females. Non-spawned fish had elevated 2 Ž steroid levels compared to females incubating seed at harvest e.g., 19.5–91.4 and 7.2–21.5 ng y1 . ml T for non-spawned and spawned fish, respectively . Stocking of the Indian Major carp ) Corresponding author. University of Stirling, Institute of Aquaculture, Stirling, Scotland FK9 4LA, UK. 0044-8486r00r$ see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S0044-8486 99 00368-3 ( ) D.C. Little et al.rAquaculture 186 2000 77–88 78 Labeo rohita in spawning hapas to control fouling did not improve water quality and led to a Ž . 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. q 2000 Elsevier Science B.V. All rights reserved.