EFFECTS OF SALINITY AND DENSITY ON GROWTH AND SURVIVAL OF SPOTTED SEATROUT Cynoscion nebulosus REARED IN RECIRCULATING AQUACULTURE SYSTEMS

Eric Gigli*, Reginald Blaylock, and Eric Saillant
Thad Cochran Marine Aquaculture Center, University of Southern Mississippi
Ocean Springs, Mississippi 39564
Eric.Gigli@usm.edu

The spotted seatrout (Cynoscion nebulosus) is the most recreationally targeted fish in the northern Gulf of Mexico. Aquaculture of this species is being developed both for stock enhancement and for commercial production intending to supply local and regional markets. Aquaculture in recirculating aquaculture systems (RAS) presents several potential advantages over traditional pond-culture including the possible use of higher stocking density and reduced environmental impacts. Currently, culture of seatrout in RAS for stock enhancement is performed at a salinity of 25 psu and density of 20kg/m3 or less. Culture at higher density would contribute to improve the cost effectiveness of the production process. In addition, culture at low salinity would be beneficial in inland areas where saltwater is not easily accessible. This study aimed to document the effects of rearing density and salinity on spotted seatrout zootechnical performance during grow-out in RAS in order to assist in the evaluation of protocols involving high density and/or low salinity for this species.

The first experiment examined the effect of density on growth and survival of juveniles. Twenty-five day old fish were stocked into two systems featuring two 10 m3 tanks each. One tank in each system was stocked at 200 fish/m3 and the other tank was stocked at 300 fish/m3.  Each system was equipped with a propeller-washed bead filter, a moving bed bioreactor, a protein skimmer, an oxygen cone, and an auto-controlled heater/chiller unit. Salinity, temperature, dissolved oxygen, pH, alkalinity, ammonia, and nitrites were monitored daily. pH and alkalinity were adjusted as necessary using sodium bicarbonate. After 180 days, 200-300 fish from each tank were harvested, counted, weighed to the nearest g, and measured (total length mm). Survival was similar among tanks and averaged 69.8% during the experimental period. Density at the end of the experiment ranged between 8-12kg/m 3. Growth differed between systems but not between the two tested densities. Differences in growth rate between the two systems may have been due to periodic degradation of the water quality recorded in one of the two systems.

The second experiment examined the effect of salinity on growth and survival. Twenty-five day old and 180 day old fish were stocked into four systems featuring four 1 m3 tanks each. Each of the systems was run at a different salinity (10, 15, 20, and 25ppt). Two tanks in each system were stocked with twenty-five day old fish at a density of 280 fish/m3 based on the results of the previous study. The other two tanks in each system were stocked with 180 day old fish at a slightly lower density (210 fish/m3) because higher survival was expected in that group. Fish were fed 1.5-5% body weight per day and samples were collected from the entire population every 75 days. After the first 75 days, the mean weight of the 15psu group (20.98g and 105.13g) was higher than that in the three other treatments but corresponding differences in length were minimal (.1 - 4.5%). The 15psu fish were 4% heavier than the 10psu fish, 6.5% heavier than the 20psu fish, and 14.5% heavier than the 25psu fish. This trial will continue until fish in each group reach approximately 365 days old at which point tanks will be harvested to compile data regarding product quality and yield.