Overwintering performance of Nile tilapia Oreochromis niloticus (L.) broodfish and seed at ambient temperatures in northern Vietnam

Two experiments (E1 and E2) to assess the performance of tilapia broodstock and tilapia sex‐reversed fry in overwintering were conducted at the Research Institute for Aquaculture No.1 (RIA‐1) in the cold seasons of 1995–96 and 1996–97. Nile tilapia Oreochromis niloticus (L.) broodstock of the Thai, GIFT, Egypt and Viet strains were overwintered in deep and shallow ponds, as well as in deep and shallow hapas suspended in a single deep pond for evaluation of the influence of overwintering systems on the survival and growth of fish. Large (> 1 g) and small (< 1 g) tilapia seed were overwintered in deep hapas‐in‐ponds for comparison of their performance. In 1995–96, the coldest pond water temperature was 10–11 °C, and survival of tilapia broodfish overwintered in deep and shallow hapas‐in‐ponds was 99.6–100%. This was significantly (P < 0.05) higher than fish stocked in deep and shallow ponds (74.4–90%). The survival rate of larger monosex tilapia fry was 54%, which was significantly (P < 0.05) higher than that of smaller fry (33.4%). In 1996–97, the lowest pond water temperature was 15.8 °C, and fry showed similarly high survival rates in all treatments (97–100%). There was no significant difference between fry in the two size classes. The results of this study clearly indicate that hapas‐in‐ponds are useful for reducing the risk and improving the survival of tilapia broodstock and fry in the cold season. Differences in the decline in ambient temperatures year on year mean that the need for special overwintering conditions varies. Hapas‐in‐ponds are a low‐cost overwintering method that can be one of the appropriate strategies for tilapia seed production under the variable, cool temperature regimes in northern Vietnam.