Stocking density effects on growth performance of glass eels (Anguilla bicolor pacifica) in outdoor tank conditions

Optimizing stocking density is necessary to maximize the production efficiency of farmed fish. This study examined the effect of stocking density on the growth performance of the Pacific shortfin eel <i>Anguilla bicolor pacifica</i> reared in outdoor tank conditions. Glass eels (0.72 ± 0.03 g mean body weight [BW]) were stocked at 50 and 100 individuals m⁻² in 6 4-m² outdoor tanks with a flow-through system set up in triplicate. Eels were individually measured for total length and BW, followed by blood sampling at the end of the 286-d trial. Glass eels stocked at a density of 50 m⁻² exhibited significantly higher specific growth rate, feed efficiency, and protein efficiency ratio. However, survival, final BW, percent weight gain, biomass, biometric indices, and Fulton’s condition factor were not significantly different between density groups. Although size class distribution was not significantly affected by stocking density, a higher proportion of larger eels (&gt; 50 g) was evident at low stocking density. The coefficients of variation in both length and weight and in skewness were likewise slightly reduced at low stocking density. In contrast, high stocking density resulted in significantly elevated levels of serum total protein. Partial costs-and-returns analysis showed that 50 m⁻² density resulted in higher net profit and profit index. These results suggest that under outdoor tank conditions, a stocking density of 50 m−2 will result in higher growth rate and feed utilization, less size variation, better physiological condition, and higher net returns for the culture of <i>A. bicolor pacifica</i>.