Effects of Body Size and Population Density on Overwinter Survival of Age-0 Pumpkinseeds

Abstract

We examined the effects of body size and population density on overwinter survivorship of age-0 pumpkinseed Lepomis gibbosus in a laboratory experiment. Pumpkinseeds were captured from a pond in central Ontario in mid-November 1992 and divided into two size-classes (20–24 mm and 30–37 mm total length). Fish were stocked at two biomass density levels (0.12–0.13 and 0.27–0.30 g/L) into four 22-L aquaria where they were maintained at 4°C without feeding. Large age-0 individuals survived significantly longer than their smaller cohorts, and there was also a significant density effect on time until death. Once the fish became active enough to monitor their movement, the number of fin and body movements by small individuals was significantly greater than those of the large individuals. Instantaneous rate of lipid loss showed a strong negative relationship with body size, and small individuals had a significantly higher percent lipid content at death than large individuals. The earlier death of the small fish occurred despite their prewinter weight-specific lipid content being significantly higher than that of the large fish, suggesting that metabolic allometry was a more important factor in their time until death than the quantity of lipid stored prior to winter. Fish held at high density displayed greater activity, earlier death, and a more rapid rate of lipid loss than those held at low density. These findings imply that in the north temperate zone, age-0 fish up to a certain size are unlikely to survive a given winter, regardless of the state of their lipid reserves; therefore sunfish progeny produced late in the season are unlikely to survive in locations with long winters. In high-density environments, they could also inhibit survival of larger, earlier-produced progeny.