Growth, Density, and Interspecific Influences on Pumpkinseed Sunfish Life Histories

Abstract

I collected data on 27 pumpkinseed (Lepomis gibbosus) populations in eastern and central Ontario and conducted a transplant experiment with one of these populations to test growth and competition—related predictions of several life history models. The predictions are that early maturity and high gonadal investment will occur: (1) in large and fast—growing juveniles; (2) in populations with low adult: juvenile growth ratios; (3) in populations with low density; and (4) in the absence of bluegill sunfish (Lepomis macrochirus), a competing congener. Predictions 1 and 4 were fully supported and Prediction 2 was partly supported, but Prediction 3 was not. The earliest maturing juveniles within populations were significantly larger than conspecifics of the same age that did not mature. Females from a lake population that were transplanted into a fishless pond exhibited both significantly faster growth and a significantly higher gonad to body mass ratio (gonado—stomatic index) that females living in the lake. At the population level, juvenile growth was significantly correlated with age at maturity (r = 0.52), but not with gonadostomatic index, whereas the Adult : Juvenile Growth Ratio showed a significant, negative correlation wih gonadosomatic index (r = 0.66), but not with mean age at maturity. Mean age at maturity showed a stronger correlation with the indicator of life—span and adult survival rate (r = 0.74), suggesting that mortality is more important than growth in the shaping of reproductive life histories of pumkinseed. High population density was associated with early maturity and high gonadal investment, the opposite of what was predicted. However, pumpkinseed populations that co—occurred with bluegills did mature significantly later and at a larger size and tended to have a lower gonadal investment than populations living in waterbodies without bluegills. This difference in reproductive patterns in the presence of bluegill is consistent with two—stage life history theory and may be the result of a direct effect of bluegills on pumpkinseed growth and survivorship, as well as differences in environmental conditions were the two species do or not co—occur.