An energetics model is implemented for lake trout, Salvelinus rzarnaycush, and applied to the Lake Michigan population. It includes an egestion function allowing any proportional mix of fish and invertebrates in the diet, a growth model accounting for both ontogenetis and seasonal changes in energy-density of predator and prey, a model for typical in situ swimming speed, and reproductive energy losses due to gametes shed. Gross conversion efficiency of energy by lake trout over their life (21 .$%) is about twice the efficiency of converting biomass to growth because they store large amounts of high-energy fats. Highest conversion efficiencies are obtained by relatively fast-growing individuals, and over half the annual energy assimilated by older age-classes may be shed as gametes. Sensitivity analysis indicates a general robustness of the model, especially for estimating consumption by fitting a known growth curve. Largest sensitivities were for the intercept and weight dependence coefficients of metabolism. Population biomass and associated predatory impact of a given cohort increase steadily for about 3.5 yr then decline steadily after fishing mortality becomes important in the fourth year in the lake. This slow response time precludes manipulation of lake trout stocking densities as a means to control short-term prey fluctuations. Predation by lake trout on alewife, Alosa pseudoharengus, has been increasing steadily since 1965 to about $ 400 t yr-' , and is projected to rise to almost 12 000 t . yr -' by 1990.Can. J. Fish. Aquat. Sci. Downloaded from www.nrcresearchpress.com by MCGILL UNIVERSITY on 12/04/14For personal use only. C.4N. J. FISH. AQLIAT. SCI.. VOE. 40. 1983 donnke augmente r6gulikrement pendant environ 3,5 ans, pour ensuite diminuer de m&me une Cois que la mortalit6 par +cAe est devenue importante dans le lac pendant la quatrikme annCe. A cause de cette lente rkponse, on ne p u t manipuler Ia densitt des peuplements de touladis d'un lac comme moyen de contrdle h court terme des tluctuations des proies. La predation du touladi sur le gaspaseau, Alosa pseudoharengus, a augment6 rt5gulikrement depuis 1965 pour atteindre environ 8 400 tm-an-' et on prkvoit qu'en 1990, elle aura atteint presque 12 008 tn~ean-'.

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We used a remotely operated submersible vehicle equipped with a color video camera to videotape the lake bed and document the distribution and abundance of burbot Lota lota on a 156-hectare portion of Julian's Reef in southwestern Lake Michigan. The substrates and bathymetry of the study area had been mapped recently by side-scan sonar. Burbot density determined from videotapes covering 6,900 m 2 of lake bed at depths of 23-41 m averaged 139 individuals/ hectare (range, 0-571/hectare). This density was substantially higher than the highest burbot density (59-95/hectare) reported in the literature. Burbot were present on the lake bed at depths of 23-36 m, but were most abundant near the crest of the reef at 23-28 m, where the water temperature was 8-13°C, their preferred summer temperature range. Substrates in that temperature range on the reef were bedrock, bedrock ridges, and bedrock and rubble. Burbot were most abundant on the bedrock and rubble. Small fish and macroinvertebrates typically eaten by burbot elsewhere in western Lake Michigan were distributed on the reef according to their summer preferred temperatures and were not seen in abundance where burbot density was highest. We saw no lake trout Salvelinus namaycush on Julian's Reef, although large numbers of juvenile lake trout have been stocked there annually and temperatures on the reef were in the preferred summer temperature range for lake trout.

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