Temperature- and ration-dependent winter growth in northern-stock Black Sea Bass juveniles

Objective: The northern stock of Black Sea Bass Centropristis striata has spatially expanded over the past decade, potentially due to warming northwest Atlantic Shelf waters affecting overwintering.Methods: To gather empirical data on temperature-dependent energetics, we quantified winter growth and lipid accumulation in juveniles from Long Island Sound using two experiments.Result: Experiment 1 measured individual length growth (GR), weight-specific growth (SGR), growth efficiency, and lipid content at constant food levels and three static temperatures (6, 12, 19 degrees C), resulting in decreasing GR from 0.24 mm/day at 19 degrees C (SGR = 0.89%/day) to 0.15 mm/day at 12 degrees C (0.54%/day) to 0.04 mm/day at 6 degrees C (0.17%/day). Even at the coldest temperature, most juveniles sustained positive GRs and SGRs; hence, the species' true thermal growth minimum may be below 6 degrees C. Lipid accumulation was greatest at 12 degrees C, which is close to what overwintering juveniles likely encounter offshore. Experiment 2 measured the same traits but combined a representative thermal overwinter profile (20 degrees C -> 13 degrees C, October-March) with seasonally varying rations designed to mimic low and high levels of food availability offshore. Monthly GR and SGR responded in the direction of seasonal food level changes. The "winter pulse" consumption average of 1.7%/feeding elicited a mean GR of 0.15 mm/day and SGR of 0.55%/day, whereas the "winter dip" consumption average of 3.8%/feeding yielded faster GR (0.20 mm/day) and SGR (0.71%/day). Growth efficiency ranged between 15% and 30% and was inversely related to food consumption. In both experiments, juveniles disproportionally accumulated lipid over lean mass, with lipid proportions tripling in experiment 2 from 4% at 65 mm to 12% at 120 mm.Conclusion: As inshore winter waters continue to warm, the energetic trade-offs of overwinter offshore migration are likely to shift, potentially leading to a year-round inshore Black Sea Bass presence.