Summary
Two groups of largemouth bass, Micropterus salmoides, were reared in the laboratory. One group was reared on an artificial, passive diet (frozen brine shrimp) whereas the second was reared on a natural, active diet (cultured zooplankton). Observations on the development of feeding behaviour indicated that the motor patterns and duration (number of weeks in the behavioural repertoire) of the feeding acts did not differ between fry reared on the two diets. While feeding on their respective diets, natural-diet fry performed significantly more orientations and bites, the two major early feeding acts, than did the artificial-diet fry. When tested with live fish prey, fish reared on the natural diet performed fewer orientations and strikes and captured more prey per fry than did the artificial-diet fry. Natural-diet fry had a significantly better net efficiency (captures minus strikes minus orientations) than did artificial-diet fry. Diet, experience, and length (T.L.) of fry affected their predator efficiency significantly. We argue that providing hatchery-reared bass fry with an opportunity to prey on live forage fish once or twice before their release would enhance their survival and eventual recruitment into natural populations.
Methodology
Two groups of largemouth bass were studied. One group was reared at the Queen's University Biological Field Station located on Lake Opinicon, Chaffey's Locks, Ontario, Canada, and the other at the Queen's campus in Kingston, Ontario. Fish were collected as larvae of the same natural population from nests in Lake Opinicon and placed in separate wooden, 96-1, glass-fronted tanks at both locations. All tanks were supplied with a flow of either Lake Opinicon water (alkalinity 65 mg l-1, pH=7.7, Smol, 1984) or Lake Ontario water (alkalinity 67 mg l-1, pH=7.2, Johansen et al.,1984) and a natural photoperiod was maintained. A 2-cm layer of gravel covered the bottom of each tank. Water temperature was within 2° C of ambient at each location. Mean water temperatures were 20.63° C (S.E.=0.20, range 17-23) for the Queen's group and 20.65° C (S.E.=0.31, range 16-27) for the Opinicon group. The density of fry in each tank was initially 150 per tank but this was reduced over summer due to mortality. Five tanks of fry were maintained at each location.
The Opinicon group was fed live zooplankton. Plankton was collected weekly from the lake and cultured in large pools. A bucket of this enriched water was added to each of the rearing tanks three times per day. Copepods, cladocerans, and rotifers were the predominant organisms found in the zooplankton (for details see Brown, 1983). Previous studies had indicated that the zooplankton in the enriched water was similar to that in the lake (Brown & Colgan, 1984). No attempt was made to regulate either the abundance or type of zooplankton fed to the fry. Since largemouth bass fry suffer high mortality after 8 weeks of age if fed only plankton (pers. obs.), they were fed frozen brine shrimp once per day as well as the plankton from the eighth week onwards. The Opinicon group will be referred to as the natural-diet group.