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Energy density and size of pelagic prey fishes in Lake Ontario, 1978-1990: Implications for salmonine energetics

Peter S. Rand·Brian F. Lantry·R. O'Gorman·Randall W. Owens·Donald J. Stewart
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Abstract

We describe dynamics of energy density and size of Lake Ontario alewife Alosa pseudoharengus and rainbow smelt Osmerus mordax , and we use a bioenergetics model of a common pelagic piscivore, chinook salmon Oncorhynchus tshawytscha , to demonstrate the effect of these factors on piscivore daily ration during 1978–1990. The energy density of alewives varied more than twofold between peaks in September (age 1) or October–November (age ≥2) and the lows in May (age 1) or July–September (age≥2). The previously described seasonal pattern of energy density of Lake Michigan alewives was similar except that energy density of older alewives (age≥3) was markedly higher in Lake Michigan. During 1978–1990, the spring energy density of Lake Ontario alewives peaked in 1979 (6,259 J/g wet weight), declined irregularly until 1985, and then remained stable through 1990 (at approximately 4,600 J/g). The initial decline may have been a density-dependent response to a burgeoning alewife population, but the lack of an increase in alewife condition in the late 1980s, when alewife biomass fell, suggests a decline in lake productivity. Energy density of rainbow smelt increased with age in Lake Ontario and condition was invariant during 1978–1990 despite a threefold change in rainbow smelt biomass. Rainbow smelt energy density was lower and fluctuated less seasonally in Lake Ontario than in Lake Michigan. Mean weight of alewives aged 2 and older dropped from 41 g in 1978 to 19 g in 1989 in Lake Ontario. Rainbow smelt aged 2 and older showed a drop in mean weight from 13–17 g in 1978–1982 to 8 g in 1990. This downward trend in mean size of alewives was correlated with the sizes of alewives consumed by Lake Ontario chinook salmon during 1983–1987. For adult chinook salmon to maintain a constant growth rate during 1978–1990, mean individual daily ration during June–October had to increase from a low of 2.2% body weight/d (or 1.5 prey fish/d) in 1979 to 3.1% body weight/d (3.7 prey fish/d) in 1988. This increase in forage demand may have caused the observed declines in individual condition of salmonines over this period.

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Peter S. Rand, Brian F. Lantry, R. O'Gorman, Randall W. Owens, Donald J. Stewart. 1994. Energy density and size of pelagic prey fishes in Lake Ontario, 1978-1990: Implications for salmonine energetics. https://doi.org/10.1577/1548-8659(1994)123%3C0519%3Aedasop%3E2.3.co%3B2

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