The southern flounder (Paralichthys lethostigma) is a commercially important finfish species in the southeastern U.S and a prime candidate for aquaculture. The objective of this study was to study the combined effects of larval stocking densities and rotifer density on growth, survival, and stress resistance in larval southern flounder.

Yolk-sac larvae were stocked into larval densities of 40 and 80 larvae/L under rotifer densities of 10, 20, and 30 rotifers/ml in a 2 x 3 factorial design. Significant (P < 0.05) effects of larval density and rotifer density on growth (notochord length, wet and dry weights) were observed. In general, growth (notochord length, wet and dry weights) was significantly faster at higher rotifer densities. Larval survival (mean = 65.0 %, range = 57.6-74.1%) did not differ significantly (P > 0.05) among treatments, so that treatments stocked at 80 larvae/L produced a higher yield (54.1 larvae/L) than the treatments at 40 larvae/L (21.4 larvae/L). The Survival Activity Index (SAI), a measure of an individual’s ability to resist the stress of an increase in salinity to 40 ppt, was significantly (P < 0.05) influenced by larval density, but not by rotifer density. Under all rotifer densities, larvae stocked at 40 larvae/L showed a higher SAI than those stocked at 80 larvae/L. Although the predator-prey ratio was greater at 80 larvae/L: 30 rotifers/ml (0.375), SAI was higher at the 40 larvae/L: 10 rotifers/ml (14.5 vs. 8.8). This study demonstrated that growth of larval southern flounder was maximized under high rotifer density and low larval stocking densities. While larval survival was not significantly affected by larval or rotifer densities, lower larval density improved stress resistance. Higher larval stocking densities improved yield, but lower stocking densities improved larval quality.