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-'.

No abstract

American shad Alosa sapidissima from the Columbia River on the Pacific coast and the Delaware River on the Atlantic coast were reared from 3 June to 24 October 1986 in two adjacent hypalon-lined ponds. Although fish from the Columbia River were introduced into ponds 29 d after those from the Delaware River, they grew significantly faster and attained a greater final weight. Fish from the Columbia River also had lower mortalities at all test salinities and temperatures than fish from the Delaware River. Electrophoresis revealed allelic differences between the two stocks at one locus (creatine kinase). We conclude that the two stocks of American shad are sufficiently different so that managers should not introduce them to different river systems without careful consideration.

The ultimate upper lethal temperature of young-of-the-year lake whitefish, Coregonus clupeaformis, was 26.65 C; this value is closely similar to that reported for yearling bloaters, Coregonus hoyi (26.75 C) and young-of-the-year lake herring, Coregonus artedii (26.0 C).