Juvenile Black sea bass (BSB) were caught on September 3rd (collection 1) and 21st 2021 (collection 2) via beach seine (30 × 2 meters) in Mumford Cove (41° 19' 25"N, 72° 01 '07"W), a shallow protected bay with extensive eelgrass cover in eastern Long Island Sound, USA.

Experiment 1 quantified juvenile BSB growth and lipid accumulation at constant food rations and 3 static temperature treatments. On November 5, 2021, we determined initial total length (TL) and wet weight (wW) on briefly anesthetized specimens (20 milligrams per liter MS-222) and selected 72 juveniles of similar size for the experiment (mean ± SD TL = 70.2 ± 5.5 millimeters (mm), wW = 5.2 ± 1.4 grams (g), 2ⁿᵈ collection). The remainder was euthanized as a baseline sample for subsequent lipid analyses (n = 32, mean ± SD TL = 85.6 ± 10.5 mm, wW = 8.8 ± 2.9 g). Each juvenile was placed into a round 20-liter (L) rearing container equipped with mesh-screened (250-micrometer (µm)) holes and a PVC shelter (15.2 x 3.8 centimeters internal diameter). We randomly placed each rearing container into one of nine 600 L recirculating tanks (8 containers per tank mean maximum stocking density = 0.07 kilograms per cubic meter (kg m⁻³) that were independently controlled for temperature (via LabView®, National Instruments™ triggering heaters or chillers) and assigned to one of three temperature treatments: 6, 12, 19° Celsius (C) (3 tanks per temperature treatment). Initially, all juveniles experienced 20°C for 7 days, then conditions were decreased by 0.5°C per day until target temperatures were reached. To achieve approximately similar cumulative temperature conditions, the total rearing period differed among treatments, i.e., 51 days (19°C treatment, 976 degree days), 71 days (12°C treatment, 936 degree days), and 99 days (6°C treatment, 783 degree days). The photoperiod was gradually decreased from 16L:8D to 9L:15D.

Response traits. Initial, monthly (Exp2 only), and final measurements of individual fish (TL, wW) were used to calculate total (final – initial), cumulative (end of month – initial), and/or serial (end of month – start of month) growth (e.g., cumulative and serial DTL for month 2 = TLd61 – TLd0 and TLd61 – TLd31), respectively) and average daily growth rates (growth/days in growth interval). Specific growth rates (SGR; % wW per day) were calculated similarly but used ln(wW) at each time period (e.g., 100*[ln(wWd61)-ln(wWd31)]). Growth efficiency (GE, %) was calculated cumulatively (Exp1, Exp2) and monthly (Exp2 only) as the change in body dry weight (DdWb, g) divided by total food consumed (DF, g) during a given time interval. (In Exp2, three GE values > 100% were excluded as outliers). Q₁₀ values (Exp1) were calculated for GR, SGR, and GE between 6-12°C and 12-19°C.

For each experiment, we also determined the lipid, lean, and ash dry weights of each surviving BSB juvenile and those of the baseline samples. Whole specimens were first transferred to -80°C for one week, then freeze-dried at -50°C for one week and re-measured for whole body dry weight (dWb, 0.001 g). Dried specimens were then loaded into pre-weighed Alundum medium-porosity extraction thimbles and transferred into a custom-designed Soxhlet apparatus, where they were bathed in petroleum ether for a total of 3.5 hours to extract all metabolically accessible lipids (15-minute cycles of bathing, flushing, and ether replacement). After extraction, thimbles were dried overnight at 60°C and re-weighed to determine DdW, which equaled the total lipid content (dWLipid, milligrams (mg)) of each specimen after accounting for any tissue loss during transfer from vial into thimble. Thimbles were then placed in a muffle furnace for 4 hours at 550°C and re-weighed, with DdW during this second step corresponding to a fish's lean mass (dWLean, mg), again after accounting for any tissue loss during transfer from vial into thimble. The difference between the final weight and the pre-weighed empty thimble equaled ash (dWAsh, mg), i.e., the inorganic fraction of each individual.