The implications of chemosensory feeding and associated ictalurid morphology were investigated through a seasonal study of the diets of the brown bullhead (Ictalurus nebulosus) and yellow bullhead (I. natalis) relative to food availability. Based on the attributes of chemosensory feeding (little discrimination between prey types) and bullhead morphology (poor sight, broad mouth), it was predicted that bullheads would be food generalists and opportunists, and that age-class diets would overlap. These predictions were generally supported for the brown bullhead. Some food types (amphipods) were harvested when most abundant; however, others (chironomid larvae) were consistently taken, and a few potential prey types were underutilized. The yellow bullhead, by contrast, was a food generalist only when young. Adults were specialized piscivores and crayfish feeders. The brown bullhead showed greater weight and length increases than co-occurring visually feeding centrarchids with diets of similar calorific values. Because of the similarity in calorific intake among species, a greater metabolic efficiency in brown bullheads was indicated. Other workers have shown that brown bullheads have lower standard and active metabolisms than some fish species.
Lake Opinicon, Leeds County, Ontario, is a small mesotrophic lake of surface area 7.87 km'. with a mean depth of 4.88 m and a maximum depth of 9.15 m. The study site was in Cow Island Bay and the 2000-m long section of shoreline adjacent to the Queen's University Biology Station. The area is diversified with weedy, sandy, and rocky inshore sections and slopes gently outwards to a depth of 7 m, about 300 m offshore.
The fish were netted at monthly intervals over 14-day periods from the 15th to the 28th of each month from May to November 1974. Seine nets were used in the inshore area and gill nets were set offshore in water 1.5-3.0 m deep. The netting was carried out between 2 100- 2400 and 0500-0800 as ictalurids are nocturnal feeders (Darnell and Meierotto 1962; Keast and Welsh 1968). Fish were allocated to age-classes defined in September-October of the year before the study, at the conclusion of the summer growth period. Year-class limits were determined by sectioning the dorsal spines of 70 brown bullheads and 40 yellow bullheads (for technique see Martzolf 1955). As fewer yellow bullheads were obtained, aging data were less complete. Year-class limits appeared, however, to be similar to those of the brown bullhead. The following morphological measurements were made (Table I): total and standard body length, internal mouth width (distance across the inside of the open mouth), and external eye dimensions (rostralcaudal and lateral - medial planes). Eye diameter was considered to be the average of the measurements on the two planes. From the eye dimensions, retinal areas were calculated following Teichmann ( 1954) and were regarded as half the area of a sphere (7~ d') with the stated eye dimensions. Total retinal area for the two eyes was twice this value. Gill-raker numbers on the first arch were counted. Gill-raker spacing was measured in year 0 bullheads, the only age group for which Cladocera are a major prey. Spacing was taken as the distance between the base of the longest gill raker and the one dorsal to it. The pharyngeal pads of brown and yellow bullheads (year 0 individuals) were dissected out and drawn to scale with the aid of a camera lucida. Diets were determined by stomach content analysis. Most prey organisms were sorted to order and family, and amphipods, isopods, Cladocera, and molluscs were identified to genus. Abundances and total wet weight were obtained for each group, and body lengths and widths were measured for each organism. Mollusca and Trichoptera larvae were weighed less the shell or tube.