Combined Effects of Turbulence and Salinity on Growth, Survival, and Whole‐body Osmolality of Larval Southern Flounder

Abstract

Abstract The southern flounder (Paralichthys lethostigma) is a commercially important marine flatfish from the southeastern Atlantic and Gulf Coasts of the USA and an attractive candidate for aquaculture. Hatchery methods are relatively well developed for southern flounder; however, knowledge of the optimum environmental conditions for culturing the larval stages is needed to make these technologies more cost effective. The objectives of this study were to determine the effects of water turbulence (as controlled by varying rates of diffused aeration) on growth, survival, and whole‐body osmolality of larval southern flounder from hatching through day 16 posthatching (d16ph). Embryos were stocked into black 15‐L cylindrical tanks under four turbulence levels (20, 90, 170, and 250 mL/min of diffused aeration) and two salinities (24 and 35 ppt) in a 4 × 2 factorial design. Larvae were provided with enriched s‐type rotifers from d2ph at a density of 10 individuals/mL. Temperature was 19 C, light intensity was 390 lx, and photoperiod was 18 L:6 D. Significant (P < 0.05) effects of turbulence on growth (notochord length [NL], wet weight, and dry weight) were observed. On d16ph, NL (μm) increased with decreasing turbulence level and was significantly greater at 20 mL/min (64.2) and 90 mL/min (58.2) than at 170 mL/min (56.3) and 250 mL/min (57.2). Survival declined primarily during the prefeeding and first‐feeding stages from d0 to d8ph, then stabilized from d8 to d16ph. In contrast to growth trends, survival (%) on d16ph increased with increasing turbulence levels and was significantly greater at 170 mL/min (57.9) and 250 mL/min (54.0) than at 20 and 90 mL/min (21.4 and 26.2, respectively). Mean rotifer concentrations (individuals/mL) at 24 h postfeeding were significantly higher (P < 0.05) in the low‐turbulence treatments of 20 mL/min (4.48) and 90 mL/min (4.23) than in the high‐turbulence treatments of 170 and 250 mL/min (2.28 and 2.45, respectively). Under both salinities, larval whole‐body osmolality (mOsm/kg) increased with increasing turbulence levels and was significantly higher at 250 mL/min (427) than at 20 mL/min (381), indicating osmoregulatory stress at the higher turbulence levels. On d14ph, larvae in all treatments were positively buoyant in 35 ppt and negatively buoyant in 24 ppt. Results showed that growth of southern flounder larvae in 15‐L tanks was maximized under low turbulence levels of 20 and 90 mL/min, while survival was maximized at high turbulence levels of 170 and 250 mL/min. The data suggested that, in prefeeding‐ and early‐feeding‐stage larvae (which have weak swimming ability), higher turbulence levels improved buoyancy and prevented sinking. In feeding‐stage larvae (which are relatively strong swimmers), higher turbulence levels caused excessive swimming, osmoregulatory stress, and slower growth. Based on these results, we recommend that turbulence levels be maintained relatively high during prefeeding (yolk sac) and first‐feeding stages to maintain buoyancy and survival and then decreased for mid‐ to late‐feeding‐ and premetamorphic stage larvae to optimize prey encounters and feeding efficiency.