Summary
Handling time for prey of different sizes and morphological types was studied in the largemouth bass (Micropterus salmoides). Prey ranged in size from about 1/10 to 2/3 the length of the bass. Handling time increased rapidly with prey of increasing size. For prey of equivalent length, crayfish were the most time-consuming morphological type to handle and swallow, followed by bluegill (Lepomis macrochirus), yellow perch (Perca flavescens), brook stickleback (Culaea inconstans), bluntnose minnow (Pimephales notatus), and finally bullfrog tadpoles (Rana catesbeiana). The ratio of prey length to bass length that minimized handling time per unit weight of prey consumed, for prey types common in the diet of largemouth bass, was 0.22 for tadpoles, 0.24 for yellow perch and crayfish, and 0.29 for bluegill. Comparison of the experimentally derived optimum prey size based on handling time with that consumed by three natural populations showed that bass commonly chose prey of similar or smaller size than the optimal predicted. The reasons for this are discussed.
Methodology
Year I+, II+, and III+ largemouth bass ranging in fork length from 70 to 285 mm (18 in total: five of length 70-115 mm, four of 150-185 mm, and nine of 220-285 mm) were used. They were housed individually and acclimated to feeding routines for 2-3 weeks in 250- or 500-L aquaria. A continuous supply of lake water, temperature 19-22°C, was maintained.
Bluegill, pumpkinseed, yellow perch, bluntnose minnows, tadpoles, and crayfish are major prey species in the diets of largemouth bass in Ontario (Hamilton and Powles 1983; Keast 1985a). These species were used as prey in the present study. Since initial tests showed similar handling times for bluegill and pumpkinseed, only the former was used here. Sticklebacks were added to increase diversity. The largemouth bass and prey species were seined from Lake Opinicon, Ontario, and surrounding ponds.
Total numbers of individual handling time trials for each prey type were for bluegill, 138 (range in total length 26-96 mm); for yellow perch, 94 (34-135 rnm); for bluntnose minnows, 58 (28-64 mm); for stickleback, 104 (12-42 mm); for crayfish, 19 (35-70 mm); and for tadpoles, 61 (50- 105 mm). In each trial a single prey of measured length was fed to a largemouth bass of measured length. Prey up to a size larger than bass could swallow (about two-thirds bass fork length) were used and the maximum sized prey consumed was thereby determined. Handling time was defined as the time from initial capture to the time the prey was swallowed and gulping movements and pharyngeal constrictions ceased. To standardize the effect of hunger on handling time (see Werner 1974), only one or two (if prey were small) prey items were presented to each piscivore per day. When small prey items were used in experiments, bass were fed additional food items following the completion of trials to supplement their diet.
Handling time was expressed in terms of the ratio of prey total length to piscivore fork length as follows (Werner 1974):
handling time = 1 + a exp b (prey length/piscivore length)
where a and b are fitted constants determined by least-squares regression of log-transformed data. Such relationships were calculated for largemouth bass feeding on bluegill, yellow perch, bluntnose minnows, sticklebacks, crayfish, and tadpoles. The slopes of these regressions were compared using ANCOVA (SAS, Ray 1982a, 1982b).
Optimal prey size and type was considered as that prey that minimized handling time per unit wet weight of prey (determined from length-weight regressions) consumed. Optimal prey sizes were compared with the sizes of prey consumed in natural populations of largemouth bass (see below).
The field study was carried out in 1984 from 15 April to 10 September in three lakes, Lorimer (45°32' N, 79°58' W), Cranberry (45°38' N, 80°27' W), and Axe (45°20' N, 79°30' W), near the town of Parry Sound, Ontario. Largemouth bass were captured by trap nets with mesh size large enough to allow potential prey items to pass through. Judging from the advanced state of prey digestion relative to the length of time spent by bass in the traps, feeding within the traps was not significant. Advantages of trap nets are that they can be located in different habitats and depths, and used in places where gill nets cannot be used (e.g., dense weed beds). Trap nets also keep fish alive, permitting stomach contents to be removed (see below) without damage to the fish. Five trap nets were set and lifted daily during four 1-week periods from April to September in each lake.