Changes in Diet and Body Condition of Lake Whitefish inSouthern Lake Michigan Associated with Changes in Benthos

—We evaluated the long-term trends of the benthic macroinvertebrate community (1980–1999) and biological attributes of lake whitefish Coregonus clupeaformis (1985–1999) in southeastern Lake Michigan. We also determined what food types were important to lake whitefish in an area where the amphipod Diporeia had not yet declined in 1998 and how the diet of lake whitefish changed as Diporeia declined during 1999–2000. Zebra mussels Dreissena polymorpha invaded the study area in 1992; Diporeia began to decline in 1993 and was nearly absent by 1999. The body condition of lake whitefish decreased after 1993 and remained low thereafter. The length at age and weight at age of lake whitefish was lower in 1992–1999 than in 1985–1991. After declines of Diporeia off the city of Muskegon, Michigan, between 1998 and 1999–2000, the proportion of Diporeia in the diet by weight fell from 70% to 25% and the percent occurrence decreased from 81% to 45%. In contrast, the proportion of lake whitefish that ate other prey, such as Mysis relicta (an opossum shrimp), ostracods, oligochaetes, and zooplankton, increased in the same period. At sites south of Muskegon, where the density of Diporeia has been low since 1998, chironomids, zebra mussels, and fingernail clams (Shaeriidae family) were the most important diet items of lake whitefish. Decreases in body condition and growth are associated with the loss of the high-energy prey resource Diporeia, the consumption of prey with lower energy content, such as zebra mussels, and possible density-dependence. Commercial harvests of lake whitefish will probably decrease because of low body condition and growth. Future management may require changes in harvest quotas, size restrictions, and depth restrictions as zebra mussel-related impacts spread northward in Lake Michigan. Historically, lake whitefish Coregonus clupeaformis were a mainstay of the commercial fishery in Lake Michigan and other Laurentian Great Lakes (Fleischer 1992). A fishery collapse in Lake Michigan in the 1950s was attributed to pollution, overfishing, and pressure from sea lampreys Petromyzon marinus (Wells and McLain 1973). Lake whitefish stocks began to recover in Lake Michigan in the 1970s, and catches currently equal or * Corresponding author: [email protected] Received October 24, 2000; accepted April 16, 2001 exceed past levels (Baldwin et al. 2000). During 1990–1998, the average annual commercial harvest of lake whitefish from Lake Michigan was 3,305 metric tons, which accounted for nearly 40% of the total commercial catch in Lake Michigan (P. Schneeberger, Michigan Department of Natural Resources,unpublished data; Baldwin et al. 2000). Zebra mussels Dreissena polymorpha invaded Lake Michigan in 1989 (Marsden et al. 1993) and quickly spread throughout nearshore areas of the southern basin (Nalepa et al. 1998). The benthic amphipod Diporeia began to decline in 1993 in southern Lake Michigan, perhaps because zebra 877 LAKE WHITEFISH AND BENTHOS FIGURE 1.—Map of Lake Michigan showing sampling locations for lake whitefish and macroinvertebrates in 1985–2000. mussels filtering limited food availability (Nalepa et al. 1998; 2000). Diporeia also declined after invasions by zebra mussels and quagga mussels Dreissena bugensis in Lake Ontario (Hoyle et al. 1999) and Lake Erie (Dermott and Kerec 1997). Diporeia are a trophic link between pelagic primary production and fish production in Lake Michigan (Gardner et al. 1985). Many fish species, including lake whitefish, that historically ate Diporeia could be adversely affected if this prey becomes scarce (Ihssen et al. 1981; Jude et al. 1981; Hoyle et al. 1999). Our first objective was to quantify long-term trends in the benthic macroinvertebrate community (1980–1999) and biological attributes of lake whitefish (1985–1999) in southeastern Lake Michigan. We expected that body condition, length at age, and weight at age of lake whitefish would decrease in association with changes in the benthos related to Dreissena invasion (Hoyle et al. 1999). Our second objective was to quantify the diet of lake whitefish relative to the abundance of Diporeia in southeastern Lake Michigan in 1998–2000. We expected that Diporeia would be an important component of the diet of lake whitefish and that, as Diporeia decreased, alternative prey (e.g., mollusks) would be eaten (Hoyle et al. 1999).