Body Size, Food, and Temperature Affect Overwinter Survival of Age‐0 Bluegills

For many species of fish, size‐specific overwinter mortality is an important factor structuring year‐class strength. Protracted spawning by bluegill Lepomis macrochirus leads to extreme variation in individual size going into winter that could result in strong, size‐specific overwinter mortality, particularly in locations with limited food resources or long, cold winters. We performed laboratory trials to test the effects of winter temperature (4°C or 9°C) and food availability (food present or no food) on the survival of two size‐classes (20–30 or 50–60 mm total length) of young‐of‐year bluegills. Mortality was strongly size selective and appeared to be related to relative condition, suggesting that energy limitation was the primary mechanism of mortality. Fish of both sizes were less active at colder temperatures, leading to increased survival (presumably via reduced energy expenditure). Bluegills fed heavily in food treatments (wet weight/d consumed was typically 2–4% for both large and small fish in the warm treatment, 1–2.5% for small fish in the cold treatment, and 0.4–0.8% for large fish in the cold treatment) and experienced increased survival. However, small fish in all treatments had more than 55% mortality after 150 d, indicating that some of the mortality was not due to starvation. It appears that late‐spawned, small fish are unlikely to survive lengthy periods of winter conditions and will therefore be selected against at northern latitudes. Mechanisms other than overwinter mortality that lead to increased lifetime reproductive success may explain the persistence of late‐summer or fall spawning at these latitudes.