Summary
Field experiments were conducted over the summer of 1983 to determine food-evacuation rate and measure stomach fullness of bluegill sunfish (Lepomis macrochirus). A combination of these data gave estimates of daily food intake of fish. Fish were captured and held in large holding pens in Lake Opinicon, Ontario, Canada, and periodically subsampled over 24 h to monitor changes in mean weight of stomach contents. Stomach-evacuation rates obtained in this way increased significantly with water temperature over the range 10–25 °C. However, variation in stomach-evacuation rate was considerable, reducing the utility of water temperature as a predictor of stomach-evacuation rate. It is suggested that the simple field methods employed here to estimate stomach evacuation and food intake are more useful in the estimation of daily ration than the more commonly employed laboratory-based methods.
Methodology
I sampled a population of bluegills (L. macrochirus Rafinesque) in Cow Island Bay, Lake Opinicon (Leeds County, Ontario, Canada). This is a small (15 ha) freshwater eutrophic lake, with surface-water temperatures in May to October varying from 10 to 27°C and up to 3°C diurnally (unpublished data). The bay is less than 2 m deep and supports high densities of aquatic macrophytes. Large populations of juvenile bluegills feed in these areas during the summer months (May to late September). During this time, the fish grow rapidly and deposit lipid reserves used in overwintering (Booth and Keast 1986).
I pulled a 15-m seine net over a standard area of 150 m adjacent to the shoreline at 3-h intervals during four 24-h sampling periods (June 15–16, July 31 – August 1, August 21–22, and September 13–14, 1983). Fifteen bluegills 50–90 mm in length (corresponding to the 2-year-old length class; see Booth and Keast 1986) were removed from each net haul, measured, weighed, and then dissected to extract stomach contents. After identification and measurements of individual items, I dried stomach contents for 24 h at 60°C and measured dry weight of each sample.
Coincident with the four stomach-fullness collections made as above, and on three other occasions (July 5–6, 1983, October 21–22, 1983, and May 23–24, 1984), I netted from 75 to 120 2-year-old bluegills from a nearby area and transferred them to a large (2 X 2 X 1.5 m) holding pen submerged in shallow water in the bay. This pen was covered in fine (2 mm) plastic mesh, enabling lake water to circulate through it. Fifteen fish were immediately removed and stomach contents were extracted and processed as for stomach-fullness samples (above). I repeated this procedure at roughly 3-h intervals thereafter and recorded water temperature during this time. I conducted sampling in a manner that would minimize stress to fish by ensuring that fish were always immersed in lake water and handled gently; the fish did not appear to exhibit elevated respiration rates (but see Boisclair and Leggett 1988).