Overwinter mortality is an important force structuring year-class strength of many fishes. Conventional wisdom is that overwinter mortality is primarily caused by starvation; however, recent research has demonstrated that piscivores continue to feed during winter and may also contribute to overwinter mortality of their prey populations. We conducted experiments in ten 0.04-ha earthen ponds in central Illinois to assess the effect of predation by largemouth bass Micropterus salmoides on survival of two size-classes (20-30 and 40-65 mm total length [TL]) of age-0 bluegills Lepomis macrochirus. Bluegills (400 fish/size-class) were stocked into each pond at the end of November. Half of the ponds also received five small (90-124 mm TL) and four large (166-192 mm TL) largemouth bass. Survival to the end of the 113-d experiment was higher for large bluegills than for small bluegills in all ponds. For both bluegill size-classes, the predator-present treatment had higher mortality than the predator-free treatment. Relative to the predator-free treatment, mortality of bluegills in the predator-present treatment increased by 16% for the large size-class and 49% for the small size-class. Mean length and relative condition (Kn) of both bluegill size-classes increased by the end of the experiment in both treatments, suggesting that the observed mortality was not caused by starvation. Further exploration is needed to elucidate why bluegills in the predator-free treatment suffered overwinter mortality despite the increase in Kn during the experiment. Length and Kn of largemouth bass increased (large size-class) or stayed the same (small size-class), suggesting that at least some of the predators foraged during winter. Our results indicate that size-specific overwinter mortality of bluegills occurs at the middle latitudes of the species' range. Further, winter predation can be an important component influencing size-specific overwinter survival and size-structured interactions between fishes.
ASJC Scopus subject areas
- Ecology, Evolution, Behavior and Systematics
- Aquatic Science