Effects of temperature, ration, and fish size on growth of juvenile black rockfish, Sebastes melanops

SynopsisYoung Sebastes melanops live as pelagic larvae and juveniles in offshore waters, recruiting to the nearshore environment at an age of approximately six months. In the summer upwelling season, juveniles may be captured in tidepools and shallow coastal waters, where temperatures may be as low as 8°C, or in shallow estuarine habitats where temperatures reach 18°C. This study was conducted to determine the effects of temperature, ration, and fish size upon growth. Juveniles 35 to 93 mm standard length were acclimated to 7°, 12°, and 18°C and provided four daily rations (nominally 0, 25, 50, and 100% maximum ration); growth and food consumption were monitored over 57 days.Growth in length ranged from −0.023 to 0.314 mm per day, relative growth in weight ranged from −0.689 to 1.495% body weight per day, and gross conversion efficiencies ranged from −13 to 21% among treatments. Under starvation conditions, weight loss increased with increasing temperature. At rations expressed as percent maximum at a given temperature, growth increased with increasing temperature. Maximum relative ration (% body weight per day), however, decreased with decreasing temperature; at equivalent relative rations, growth did not differ significantly among temperatures. Evacuation rates determined at the experimental temperatures suggest that daily ration is limited by digestion rate and that temperature therefore affects energy turnover.Growth was analyzed with multiple regression models with ration, temperature, and initial weight as independent variables. Relative growth increased with increasing initial weight. The model predicts that relative maintenance rations decrease with increasing size. The percentage of full ration represented by maintenance ration is relatively greater for small fish at low temperatures, suggesting a basis for ontogenetic change in thermal optima for growth. The results of this study suggest that thermal environment may have an important effect upon first year growth of juvenile S. melanops.

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