To elucidate prey preferences, we quantified stomach contents of 1,231 salmonines collected from inshore (21 m or shallower) southeastern Lake Michigan during 1973–1982. Predators ate 12 species of fish. Alewife Alosa pseudoharengus made up 48–79% by weight of the diet of brown trout Salmo trutta, chinook salmon Oncorhynchus tshawytscha, coho salmon Oncorhynchus kisutch, lake trout Salvelinus namaycush, and rainbow trout Salmo gairdneri. Alewives eaten ranged from 23 to 245 mm total length; 46% were 150–200 mm. Rainbow smelt Osmerus mordax eaten were 21–245 mm long and made up 4–11% of the salmoninesˈ prey. There were significant direct linear relationships between lengths of alewives and rainbow smelt eaten and lengths of the five predators. Alewives are currently declining in Lake Michigan. If their population collapses, there should be a shift to alternative prey species. We have seen no such shift through 1982, although more recent data of other investigators show a decline in the importance of alewife in salmonine diets. Diets of the midwater‐feeding chinook and coho salmon were heavily dominated by the pelagic alewife, whereas brown and lake trout diets were more diverse. This suggests that trout should have better survival and growth than salmon, because trout would be able to utilize the more benthic yellow perch, Perca flavescens, rainbow smelt, and, to some degree, bloater Coregonus hoyi. The latter species are becoming more abundant with the decline in alewife. Under the current salmonine stocking regime, alewives will continue to supply a lower and variable portion of the salmonine diet, and predatory pressure on alewife should lead to increases in endemic prey speciesˈ populations.

Concerns have been expressed that increasing predatory pressure by salmonids may change the forage fish community of Lake Michigan. A decline in alewife (Alosa pseudoharengus), the major forage fish, would be a forewarning of such a change. Our 1973–82 surveys showed that alewife declined 86% in 1980–82; concurrently, rainbow smelt (Osmerus mordax) and yellow perch (Perca flavescens) increased threefold and fivefold, respectively. Spottail shiner (Notropis hudsonius) and trout-perch (Percopsis omiscomaycus) showed no long-term changes. Bloater (Coregonus hoyi) increased 24-fold since commercial fishing was prohibited in 1976. These trends imply that (1) alewife declined from predatory pressure, (2) alewife suppressed rainbow smelt and yellow perch, presumably by preying on their pelagic larvae, (3) alewife did not substantially affect spottail shiner and trout-perch, presumably because their demersal larvae escaped alewife predation, and (4) overfishing, more than the alewife, suppressed bloater.

Alewife (Alosa pseudoharengus) and yellow perch (Perca flavescens) abundances, estimated from monthly gillnet and trawl catches at two transects, were monitored before (1973–74) and during (1975–82) operation of the D. C. Cook Nuclear Power Plant, southeastern Lake Michigan. Intervention analysis, a technique which accounts for autocorrelated observations, and analysis of variance (ANOVA) were applied to the monitoring data to assess any plant impact beginning in 1975. Both analyses disclosed no significant power plant impacts except for gillnetted yellow perch adults. The ANOVA indicated a significant decrease in abundance at the plant-discharge transect relative to the reference transect as plant operation began, which established a plant effect; intervention analysis showed no change. When April and May catches (months of low abundance) were deleted, this plant effect was insignificant. Monte Carlo simulation showed that as the first-order autoregressive coefficient increased positively, type I error of the ANOVA F-test increased. However, ANOVA was more powerful than intervention analysis when a first-order autoregressive component was included. Impact assessment based only on ANOVA can result in detection of impact when actually there was no effect (type I error) when observations are serially correlated (lack independence).

Intrusion of cold (usually < 10 °C), hypolimnetic water into the Lake Michigan nearshore zone (≤ 15 m) substantially changed local abundance and distribution of larval alewife (Alosa pseudoharengus) during the summers of 1977–79. During upwelling, larval fish densities were reduced to below 1000 larvae/1000 m3; when no upwelling occurred densities commonly exceeded 5000 larvae/1000 m3. Widespread distribution to depths of at least 15 m was found in the absence of upwelling, while during upwelling larvae were more concentrated in water ≤ 6 m deep. Additionally, at depths > 3 m during upwelling, alewife larvae were most abundant in bottom strata, a trend not observed in the absence of upwelling. Postulated mechanisms for reduced larval alewife abundance in inshore areas during upwelling include mortality of larvae due to thermal shock, and offshore or alongshore movement, either actively by larvae and potential spawning adults, or passively by larvae.Key words: Alosa pseudoharengus larvae, distribution, abundance, Lake Michigan, upwelling