Evaluation of salt-incorporated diets on growth, body composition and plasma electrolytes of black sea bass Centropristis striata reared in a semi-pilot scale low salinity recirculating aquaculture system

To investigate the effects of dietary salt supplementation on growth performance, survival, body composition, and plasma electrolytes of black sea bass Centropristis striata, a feeding trial was conducted in a semi-pilot scale RAS under low salinity (10.8 +/- 0.69 g L-1) rearing conditions. Four isolipidic and isonitrogenous test diets were formulated with graded levels of sea salt (99.86% NaCl): 0%, 2.5%, 5%, and 7.5% dry wt. In addition, two control diets (0% salt test diet and a premium commercial diet) were tested for fish raised in full-strength seawater (34 g L-1). Twelve tanks (vol = 2-m(3)) of the low-salinity RAS were each stocked with 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 and salinity was gradually decreased (0.5 g L-1 per day) to 10.1 g L-1 in 30 days. In addition, six tanks (2-m(3)) of a RAS supplied with full-strength seawater were each stocked with 100 fish from the same cohort, with 3 tanks fed either of the two control diets (0% salt or the commercial diet). After the 8-month feeding trial, fish grown at low salinity were fed their respective diets for an additional 5 weeks under adverse low-salinity conditions in which salinity was further decreased gradually from 10.1 g L-1 to 4 g L-1. Survival over the 8 months remained high (93-100%) among treatments. Growth (% body weight gain) among the treatments in low salinity ranged from 766 to 825%, comparable to fish raised in full seawater (788-813%). Plasma osmolality (mOsm kg(-1)) for fish ranged from 336 to 357 among all treatments. However, no significant relationships between weight gain, feed conversion ratio, survival, plasma osmolality, and electrolyte concentrations and dietary salt level were observed among fish raised in low salinity. When salinity was further reduced from 10.1 g L-1 to 4 g L-1, fish fed 0% salt showed poor survival (29%), whereas fish fed 7.5% salt showed highest survival (67%), and a significant (P < 0.05) linear relationship between dietary salt and survival was observed. The results suggest that black sea bass juveniles can be raised at a low salinity of 10.1-12.3 g L-1 with no negative effects on long-term growth performance. Salt-incorporated diets, however, improved survival under extreme low salinity (similar to 4 g L-1) challenge conditions. These findings have important implications for rearing black sea bass in low salinity RAS and for the siting of black sea bass RAS grow-out operations.