Summary
Analysis of the stomachs of 2435 pumpkinseeds (Lepomis gibbosus) showed that the diet was made up largely of chironomid larvae, molluscs, isopods, with lesser quantities of amphipods, Trichoptera larvae, and Ephemeroptera nymphs. Cladocera were only important to the very young fish. Age-class separation, in terms of taxonomic and prey-size categories, was minor, although the diet changed somewhat with age. Quantification of the diet overlaps between year-classes (by Levins index) revealed overlap levels that were moderate to high, varying seasonally between 0.5 and 1.1. They were greatest in July when the diversity of available prey organisms fell and October when several size-classes simultaneously consumed isopods, then at a peak in biomass. The feeding and spatial ecology of the pumpkinseed was compared with that of a closely related congener, the bluegill (L. macrochirus). The two species and their equivalent year-classes separated out partially on diet (the bluegill consuming a greater proportion of Cladocera and Tnchoptera larvae; the pumpkinseed, molluscs and isopods), and partly on habitat and the height at which they foraged in the water column. Ecological overlap values between equivalent year-classes of the two species were moderate, 0.52–0.57 (Levins scale), on the basis of diet alone as well as when the habitat component was incorporated. It is suggested, however, that other differences serve to reduce these levels.
Methodology
The work was carried out on the Lake Opinicon (Leeds County, Ont.) population of the two species. The study site was Cow Island Bay and the adiacent 400-mJong section of shoreline extending east towards the Queen's University Field Station. This inshore strip of sand-mud bottom is lightly to moderately weeded but bas heavily weeded Potamogeton pusillus-P. pectinatus sections at the head of the bay. The area supports large populations of the two species and a full range of age-classes of both. The work was carried out in the ice-free months (May-November) of 1971, 1.972, and 1973. There is virtually no winter feeding in these sunfishes (Keast 1968).
The fish were netted at monthly intervals over 12-day periods from the 15 to the 26 of each month' The time spread was to give as accurate a picture as possible of the prey organisms taken during the particular month. All collections were made between 1700-1900 and 0700-0900 to broadly match the major daily feeding periods of the two species (Keast and Welsh 1968), and were supplemented by night collections. The fish caught were allocated to year-classes on the basis of studies carried out in September*October (i.e. after completion of the summer's growth) in 1968 and 1969. This entailed measuring 3200 pumpkinseeds and 4500 bluegills and defining year-class limits by the reading of the scales of 350 of each. Year-class size distributions vary little from year to year in Lake Opinicon (Keast unpublished data). Regier (1962) provides an assessment of the scale method of aging in Lepomis. Size-distribution data for the individual summer months (Keast unpublished data) provided the basis for the allocation of fish to year classes.