A feeding trial was conducted to investigate the effects of dietary salt supplementation on growth, survival, body composition, and plasma electrolytes of black sea bass Centropristis striata reared under sub-optimal salinity conditions of 10-12.5 g L-1 in a semi-pilot scale RAS. Four test diets were formulated and prepared with different levels of supplemental sea salt (99.86% NaCl, Cargill Salt, Minneapolis, MN): 0% (control), 2.5%, 5%, and 7.5% dry wt. In addition, two control diets were tested for fish raised in full-strength seawater (34 g L-1) in an independent RAS; one was a 0% salt formulated diet and the second was a premium commercial diet (Skretting, Canada). All test diets were isonitrogenous (46% crude protein) and iso-lipidic (12% crude lipid). A 25-m3 automated reservoir system was used to supply brackish water to the low-salinity RAS. Twelve tanks (vol = 2-m3) of the low-salinity RAS were each stocked with juvenile black sea bass (mean wt. = 19.6 g) at a density of 100 fish per tank and at a starting salinity of 34 g L-1. The RAS salinity was gradually decreased at a nominal rate of 1 g L-1 per day until the target salinity of 10 g L-1 was reached in 24 days. Six tanks (2-m3) of the RAS supplied with full-strength seawater were each stocked with 100 fish from the same cohort to serve as a control. After the 8-month feeding trial, fish grown at 10-12 g L-1, were fed their respective diets for an additional 5 weeks under adverse low-salinity conditions in which salinity was further decreased gradually from 10 g L-1 to 4 g L-1 in 30 days.
Percent survival over the initial 8 months remained high (93-100%) among treatments, with no significant differences. The highest weight gain (825%) among treatments was observed for fish fed 2.5% salt in low salinity, whereas the lowest weight gain (765%) was found for fish fed 5% salt. In comparison, fish raised in full seawater showed weight gains of 788-813%. However, no significant differences (P > 0.05) in weight gain were observed among fish under any diet or salinity treatment. Plasma osmolality (mOsm kg-1) for fish raised at salinities of 10-12.5 g L-1 or 35 g L-1 ranged from 336-357 and was not significantly different. No significant differences were observed in plasma electrolytes concentrations among fish fed the various diets, except bicarbonate, which was significantly (P < 0.05) higher for fish fed the commercial diet in full seawater. When challenged with a further reduction in salinity from 10 to 4 g L-1 over a period of 5 weeks, fish a 0% salt diet showed poor survival (29%), not significantly different from fish fed diets with 2.5% (42% survival) and 5% salt (60% survival). However, fish fed diets with 7.5% salt showed significantly higher survival (67%) than fish fed 0% salt (29%). The results suggest that black seabass can be acclimated to and raised at a low salinity of 10 g L-1 with no effects on long-term growth performance, or on fish plasma electrolyte concentrations compared to fish raised in full strength 34 g L-1 seawater. Salt-incorporated diets, however, improved survival under extreme low salinity (~ 4 g L-1) challenge conditions. These findings have potentially important implications for rearing black sea bass in low salinity RAS and for the siting of black sea bass RAS growout operations.