Unexploited Percid Populations of West Blue Lake, Manitoba, and their Interactions

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Abstract
West Blue Lake, Manitoba, essentially unaffected by man, sustains populations of yellow perch, Perca flavescens, and walleye, Stizostedion vitreum vitreum, which interact in two fundamental ways: perch are a major source of energy for walleye and the two species share common food organisms.Perch adults, juveniles and fry generally contributed 80–90% of the energy required for the annual walleye production (340 kg) and, on the basis of conversion experiments, provided for at least 1.7 kg∙ha−1 of the observed production of 2.1 kg∙ha−1. The brief summer growth period, combined with moderate mortality, caused biomass to reach a maximum in early fall at which time production ceased.Although walleye utilized yellow perch as its basic energy supply, behavioral differences tended to offer protection to juvenile and adult perch. Walleye were active at night, whereas older perch w ere inactive. Age 0 perch, during their epilimnetic phase, on the other hand, were active at night and vulnerable to walleye predation providing at least 18% of the observed walleye production.Walleye and yellow perch shared the same food resources although at different times and to different degrees. This competition was most intense between walleye (> 25 cm) and adult perch (> 12 cm). A main food item shared was amphipods. Most other food items (sticklebacks, mayflies, crayfish) were also shared but none in equal proportions by the two percids.Neither of the percid species showed a preponderance of older individuals and both showed evidence of strong and weak year-classes — walleye to a greater degree. Also, for walleye at least, production was only 2.1 kg∙ha−1, and biomass appeared to be stable, approximately 900 kg. Growth for both species was less than most other percid communities and was restricted to a brief, ≈ 80 day, period. Key words: unexploited, yellow perch, walleye, feeding, behavior, production, population structure

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