Effect of water quality ununiformity on production of marine medaka

Abstract

The large amount of organic impurities deposited at the bottom of an intensive aquaculture pond often leads to the deterioration of the surrounding environment, creating additional environmental stresses on the farmed fish and affecting its growth. This study investigated the effect of water quality ununiformity on growth, feeding behavior, and stress tolerance of marine medaka (Oryzias melastigma) through differences of residence time in different water quality areas in the same aquaculture system. Through the pre-test, the flow field distribution in the aquarium for cultured experiment was clarified, and the water body in the aquarium was divided into the central area (Ce) and the corner area (Co) according to the water quality difference in different flow fields. Then, seven residence time gradients were set in Co, which were 24 h, 14.4 h, 7.2 h, 4.8 h, 2.4 h, and 0 h, respectively. That was, they accounted for 100% (P100), 60% (P60), 30% (P30), 20% (P20), 10% (P10), and 0% (P0) of a whole day, respectively. The results showed that, all water quality indicators such as dissolved oxygen (DO), pH, nitrite (NO2−-N) and non-ionic ammonia (NH3−N) in Ce and Co were consistent with the requirements of the standard (GB11607–89) during the 28-day cultured experiment, but the water quality in Co was gradually worse than that in Ce, with the prolonging cultured period. In spite of increasing the body length and body weight of medaka in all treatments, their growth rates had significant differences among several treatments (P0 = P10 = P20 > P30 > P60 = P100). And the fish's growth, feeding efficiency, and stress resistance decreased with the increase of residence time in Co (P < 0.05). Through nonlinear regression analysis, it was predicted that the accumulation of residence time of marine medaka stayed in the coner area was >6.667 h, its feeding behavior would be affected, and the corresponding appetite and food intake, immunity to disease and growth rate would all decreased. Attaining uniformity of water quality in aquaculture system may help improve fish welfare and increase the sustainability of aquaculture.