THE EFFECTS OF STOCKING DENSITY ON GROWTH, SURVIVAL, AND RESISTANCE TO ACUTE CROWDING OF POST-METAMORPHIC STAGE BLACK SEA BASS Centropristis striata IN A PILOT COMMERCIAL SCALE NURSERY
The black sea bass Centropristis striata is a high value, high demand marine finfish with promising commercial aquaculture potential. A bottleneck to commercial culture is a reliable and cost-effective source of fingerlings to support growout operations. Maximizing safe stocking density of metamorphic stage juvenile fish in nursery tanks (NTs) while maintaining high fish health and growth rates is critical to efficient use of hatchery and nursery resources and, hence, to maximizing fingerling quality, output and affordability. This study investigated the effects of NT stocking density of post-metamorphic stage black sea bass on survival, growth and resistance to acute crowding to the transport-ready juvenile stage 60 days post-hatching (60 dph). Juveniles (47 dph, mean wt. = 0.54 g) were stocked into six 300-L fiberglass NTs supported by a recirculating seawater (34 g/L) system. Two NT stocking densities were compared in triplicate; a low density (4.5 fish/L, 1,350 fish/tank, 2.43 kg/m3) and a high density (6.5 fish/L, 1,950 fish/tank, 3.51 kg/m3). Fish were fed a UNCW-formulated diet by hand every morning and at 2-h intervals by automatic belt feeder over the ensuing 12 hours to satiation. Temp was 23.5 oC and photoperiod was 18 L: 6 D. Survival of fingerlings to 60 dph under both NT densities was high (mean = 87.7 + 0.03%) and not significantly different as final biomass densities under the low and high NT densities reached 6.66 and 8.98 kg/m3, respectively. Fish in the low density NTs (mean = 1.68 + 0.026 g) were significantly (P < 0.05) larger than those in the high density NTs (mean =1.58 + 0.004 g). FCR (mean = 0.87) was excellent and not significantly different. Coefficient of variation of weight (mean = 28.1 + 2.3%) was not significantly different between NT density treatments. In a simulated air-shipping trial, 59 dph fingerlings from both NT density treatments placed in oxygenated bags containing seawater (biomass loading = 10 g/L) and held in insulated shipping boxes for 24 h showed ~100% survival, even though dissolved oxygen declined (mean = 1.73 mg/L). To evaluate tolerance to acute crowding, 59 dph fingerlings from both NT density treatments were placed in 10-L seawater tanks (biomass loading = 25 g/L) and held under static conditions with oxygenation and zero feeding. After 72 h, survival was excellent (mean = 99.7%) and not significantly different between NT density treatments, even though mean TAN and pH reached 57.3 ppm and 8.06, respectively. The results demonstrate that stocking post-metamorphic stage black sea bass in NTs at high densities of 4.5 to 6.5 fish/L does not affect survival, growth variation or FCR to the 60 dph transport-ready stage fingerling stage, although growth was slightly depressed at the higher density. Fingerlings produced under these high NT stocking densities were robust and showed excellent survival during simulated air-shipping trials and resistance to acute crowding.