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 2 quantified juvenile winter growth and lipid accumulation under seasonally varying food and temperature conditions over the course of 182 days (October 1, 2021 to April 1, 2022). Initial total length (TL, mean ± SD = 70.9 ± 5.4 millimeters (mm)) and wet weight (wW, mean ± SD = 4.4 ± 0.9 grams (g)) were determined on briefly anesthetized specimens, followed by randomly distributing 48 juveniles into individual 20-liter (L) rearing containers with mesh-screened holes, an air stone, and a PVC hide. An additional baseline sample (n = 25, mean ± SD TL = 71.4 ± 9.4 mm, wW = 4.7 ± 1.8g) was euthanized and stored at -20° Celsius (C). We then used six 700 L recirculating tanks to each house eight individual BSB rearing containers (mean maximum stocking density = 0.13 kilograms per cubic meter (kg m⁻³). Tanks (n = 2 tanks per treatment; n = 8 containers per tank) were randomly assigned to one of three seasonally varying food treatments (Otohime C1 pellets) designed to simulate three phases of contrasting overwinter food availability for BSB juveniles. The 'Winter dip' treatment was fed 5% body wW 3× weekly during Phase 1 ("Fall": October, November), 2% wW 3× weekly during Phase 2 ("Winter": December – February) and again 5% body wW 3× weekly during Phase 3 ("Spring": March). This was to simulate a scenario of good onshore feeding conditions for juveniles in fall and spring, but poorer feeding conditions at their offshore overwintering habitats. Conversely, our 'Winter pulse' treatment was to simulate poor fall and spring feeding conditions for juveniles onshore by feeding 1% wW 3× weekly during Phase 1 and 3, but 2% wW during Phase 2, thereby simulating the same offshore winter feeding conditions (Fig.2A of Zavell et al. (in review)). Last, we included a 'Constant' treatment as control, which fed 2% wW 3× weekly throughout the entire six months of rearing (Phases 1-3). Food rations were based on both Cotton (2002), who showed no difference in growth at rations of 5 - 7.5% wW, but reduced growth at 2.5% wW and Watanabe et al. (2021) who report ideal feeding rates between 3 – 5% wW. For all treatments, rations were individually recalculated after the 1ˢᵗ of each month, based on re-measured wW and TL on briefly anesthetized fish.

Temperature conditions in Exp2 were recorded in 30 minute intervals using HOBO temperature loggers (Onset MX^®).

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 = TL_d61 – TL_d0 and TL_d61 – TL_d31), 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(wW_d61)-ln(wW_d31)]). Growth efficiency (GE, %) was calculated cumulatively (Exp1, Exp2) and monthly (Exp2 only) as the change in body dry weight (DdW_b, 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 (dW_b, 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 (dW_Lipid, 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 (dW_Lean, 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 (dW_Ash, mg), i.e., the inorganic fraction of each individual.