No abstract

We developed an energetics model for the alewife Alosa pseudoharengus to evaluate consumption and conversion efficiency processes. We estimated alewife standard metabolism (R) as a function of wet body weight (W, g) and water temperature (T, *C): R (g.g-i.d -•) = 0.0073 W-ø-2tSe ø.øs48r. We estimated maximum daily ration for 15-g, yearling alewives at 200C to be as much as 37% of their body weight in experiments of 24 h. Modeling simulations based on observed growth in Lake Michigan indicated that yearling alewives may attain close to 70% of their estimated maximum consumption rate during September and October. Simulation results indicated wide seasonal variations in consumption rates; almost 50% of the yeafly consumption by an individual adult alewife (age >-II) occurs in September and October, suggesting abundant food and possibly relaxed competition during that season. In contrast, adult alewives lose weight during the summer when stratification of Lake Michigan would permit orientation to water temperatures optimal for growth, suggesting that serious food limitations during that period may heighten competitive interactions. Over an annual cycle, adults converted only 1.3-2.8% of food consumed (wet weight) to body biomass; young of the year converted 5%. Conversion of energy consumed to body energy was higher than biomass conversion, but still relatively low for older age-classes--2.3-5.2% for adults and 12.7% for young of the year. Total annual consumption and conversion efficiency estimates were relatively insensitive to assumptions about seasonal dynamics of body energy density (j.g-l wet weight) but within seasons when energy density was changing rapidly, an assumption of constant energy density yielded errors of -45% to 104% for those variables. The alewife Alosa pseudoharengus has been one of the dominant fish species in lakes Michigan andHuron since about 1960, and in Lake Ontario since before 1900 (Smith 1970). Intense competition from, and perhaps predation on early life stages of other forage fishes by, the alewife may have contributed to the drastic declines and extinctions of native planktivorous fishes that have occurred in these lakes (Smith 1970; Crowder 1980 Crowder , 1986). The almost explosive recoveries of bloater Core-gonus hoyi, yellow perch Percafiavescens, and, to a lesser extent, rainbow smelt Osmerus mordax and deepwater sculpin Myoxocephalus thompsoni following a recent decline of alewives in Lake Michigan provide further evidence for strong interactions between alewives and other fishes (Jude and Tesar 1985; Wells 1985). Quantifying ecological processes such as predation rates and competitive interactions in systems as large as the Great Lakes is a difficult task. Models like the one de-643 J. Diana (University of Michigan, Ann Arbor) for providing us with his data on alewife energy density.

We evaluated the relative importance of starvation and predation as potential sources of mortality for larval bloater (Coregonus hoyi) in the laboratory. Fifty percent of starved bloater larvae survived 25 d after hatching; some individuals survived 35–40 d. Bloater larvae did not exhibit a point of no return, but length and weight after 30 d were inversely related to the delay in first feeding. Bloaters were swimming about half of the time within 1 h after hatching and almost continuously after 6 h. Twenty-one days after hatching, starved larvae were still swimming continuously at 0.9 cm∙s−1; fed larvae swam more than four times faster. Bloater larvae up to 30 d old were captured with about 80% success by 95- and 135-mm bloaters and 100-mm alewives (Alosa pseudoharengus). Susceptibility to predators generally decreased with increasing size, and starved larvae were more susceptible than similar sized fed larvae. Because of their swimming ability and resistance to starvation, it seems unlikely that bloater larvae starve to death in nature. However, starvation can contribute to mortality, as reduced growth and swimming capabilities may result in increased or prolonged susceptibility to predation.