Juvenile Black sea bass (BSB, Centropristis striata) were caught on September 9th via beach seine (30 × 2 meters (m)) in Mumford Cove (41°19'25"N 72°01'07"W), a shallow protected bay with extensive eelgrass cover in eastern Long Island Sound (LIS).

Experiment. This study quantified juvenile BSB growth and lipid accumulation at constant food rations and over a seasonal temperature profile for Long Island Sound over eight months (10/12/2022 – 5/16/2023). On October 12th, 2022, we determined initial total length (TL) and wet weight (wW) on briefly anesthetized specimens (20 milligrams per liter MS-222) and randomly selected 63 individuals for the experiment. The remaining 35 fish were euthanized as a baseline sample for subsequent lipid analysis. Each juvenile was randomly distributed to an individual 20-liter (L) rearing container with mesh-screened holes, an air stone, and a PVC hide. A total of 9 individual rearing containers were allocated to one of 6 larger 600-liter flow-through tanks (4208-liters per hour; fully replaced 7x hour). Fish were fed Otohime C2 (51% protein, 11% fat, 3.5% fiber, 15% ash) at a target ration of 5% of their wW 3x per week and uneaten food was siphoned 24 hours after feeding. Temperature was recorded every 30 minutes by a HOBO logger in each individual tank (Standard Deviation between tanks = 0.17°C). At the beginning of the experiment, mean daily water temperatures began at 18 degrees Celsius (°C) before rising ~ 5.5-6°C in early February and rising to ~11°C in May. Photoperiod was changed by 30 minutes weekly, to mimic local conditions. At the start of the experiment in October, the photoperiod was 11.5L:12.5D before dropping to a low of 9L:15D in December and rising to 14.5L:9.5D in May of 2023. Throughout the experiment, specimens were briefly anesthetized every 20 to 40 days and re-measured in length (TL; millimeters (mm)) and weight (wW; grams (g)) to calculate monthly absolute and specific growth rates. At the end of the experiment, all surviving fish were euthanized, measured for TL and wW, and frozen for lipid extractions. Throughout the experiment, 10 specimens escaped their rearing containers and were not included in subsequent analyses.

Response traits. Initial, monthly, 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_d0and TL_d61 – TL_d31), respectively) and average daily growth rates (growth/days in growth interval). Specific growth rates (SGR; % wW d^-1) were calculated similarly but used ln(wW) at each time period (e.g., 100*[ln(wW_d61)-ln(wW_d31)]).

Lipid, lean, and ash dry weights of each surviving BSB juvenile and those of the baseline samples were quantified via proximate analysis. 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 h 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, 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. Total energy content (ED; Kcal; Brett and Groves 1979) was then calculated as (dW_Lean * 4.8) + (dW_Lipid * 9.45) and total energy density (ED) was calculated as EC / dW.