EFFECTS OF WINTERKILL ON POPULATION-STRUCTURE, BODY SIZE, AND PREY CONSUMPTION PATTERNS OF PUMPKINSEED IN ISOLATED BEAVER PONDS

Beaver ponds are a ubiquitous, but rarely studied, aquatic habitat that are subject to hypoxia and other catastrophic events that can affect fish populations. We examined the population dynamics and feeding ecology of pumpkinseeds (Lepomis gibbosus) inhabiting two isolated Ontario beaver ponds and adjacent Lake Opinicon to determine the persistence of and reasons for small body size in pond populations. Pumpkinseed populations fluctuated drastically in both ponds as a result of winterkills (from hypoxia and a decline in water level). These winterkills eliminated as many as 96% of the older pumpkinseeds and resulted in populations consisting mainly of age 0-2 individuals. Pond pumpkinseeds rarely survived to age 5, whereas lake conspecifics lived to age 8 or older. The diet of pond pumpkinseeds consisted mainly of chironomids, ephemeropterans, odonates, and molluscs, whereas lake pumpkinseeds ate mainly molluscs. The difference in diets was attributed mainly to differences in prey availability in the two habitats. Despite lower prey biomass and higher population density, pond pumpkinseeds grew as fast as, or faster than, their lake conspecifics owing mainly to early warming of water in the spring and the absence of bluegills and other major competitors. We conclude that small body size in pond pumpkinseed populations is due to high annual mortality and not stunted growth.