Effects of light intensity and salinity on growth, survival and whole-body osmolality of larval southern Flounder paralichthys lethostigma

Abstract

The southern flounder Paralichthys lethostigma is a high-valued flatfish found in estuarine and shelf waters of the south Atlantic and Gulf coasts of the United States. Wide temperature and salinity tolerances exhibited by juveniles and adults make it a versatile new candidate for commercial culture, and studies are underway in the southeastern U.S. to develop hatchery methods for this species. The objectives of this study were to establish illumination and salinity conditions that optimize growth and survival of larval southern flounder reared through the yolk-sac and first feeding stages to 15-d post-hatching (15 dph). Early embryos were stocked into black 15-L tanks under light intensities of 5, 50, 100, and 1,000 lx and at salinities of 24 and 34 ppt in a 4 × 2 factorial design. Significant (P < 0.05) effects of both light intensity and salinity on growth and survival were obtained, with no interaction between these effects. On 11 dph and 15 dph, growth was generally maximized at the intermediate light intensities (50 and 100 lx) and minimized at the extremes (5 and 1,000 lx). By 15 dph, growth was higher at 34 ppt than at 24 ppt. Survival to 15 dph showed trends similar to those of growth. Survival was higher at 100 lx (avg. = 46%, range = 41-54%) than at 5 lx (avg. = 11%, range = 6-17%) and higher at 34 ppt (avg. = 43%, range = 31-55%) than at 24 ppt (avg. = 17%, range = 8-38%). Whole-body osmolality (mOsmol/kg) was significantly lower in larvae reared at 24 ppt (avg. = 304, range = 285-325) through 11 dph than in larvae reared at 34 ppt (avg. = 343, range = 296-405). Larvae reared under the extreme light intensity treatments (5 and 1,000 lx) at 34 ppt appeared to exhibit osmoregulatory stress, particularly on 11 dph, when a marked increase in whole-body osmolality was observed. The mid-intensity treatments (50 and 100 lx) at 34 ppt optimized growth and survival of larval southern flounder in this study; and elicited the most stable osmotic response. These conditions appear to be consistent with those that southern flounder larvae encounter in nature during this early developmental period.