Lake trout Salvelinus namaycush in nearshore waters of Lake Michigan grow faster than lake trout residing offshore on Sheboygan Reef, which is in midlake. We examined the stomachs of lake trout, spanning ages 1 through 16, caught in both nearshore and offshore environments of Lake Michigan during 1994 and 1995 to determine whether diet differences may be responsible for the difference in growth rate. A comparison of the diets, coupled with bioenergetics modeling, indicated that juvenile lake trout on Sheboygan Reef experienced slow growth due to low food availability rather than to cold water temperatures. The availability of appropriatesize prey appeared to regulate lake trout growth. Small prey fish were probably not readily available to small (200-to 399-mm total length) lake trout on Sheboygan Reef, a substantial portion of whose diet consisted of invertebrates; in contrast, nearshore juveniles had a nearly 100% fish diet. Growth rate on the reef remained slow through intermediate lake trout sizes (400-599 mm total length), presumably due to low availability of rainbow smelt Osmerus mordax on the reef. Once lake trout achieved total lengths of approximately 600 mm, they grew slightly faster on Sheboygan Reef than near shore, indicating that large (Ͼ170-mm total length) prey fish were readily available to lake trout in the reef area. On a wet-weight basis, alewife Alosa pseudoharengus dominated the diet of large (Ն600 mm total length) lake trout from both the nearshore and offshore regions of the lake, although bloater Coregonus hoyi composed over 30% of the diet on Sheboygan Reef and in southeastern nearshore Lake Michigan. Size of alewife prey increased with lake trout size. The bloater population currently represents the bulk of the biomass of the adult prey fish community, so our diet analysis suggests that large lake trout are continuing to select alewives.

Diets of salmonines in Lake Michigan have been dominated by alewives Alosa pseudoharengus since the 1960s, and information on alewife population dynamics is critical to the management of salmonine fisheries. We monitored alewife size at age and condition (K) at several different locations in Lake Michigan during fall 1984–2001. Alewives were aged by enumerating annuli on otoliths. The results indicated that alewife length at age did not trend upward or downward between 1984 and the late 1990s but decreased from the late 1990s to 2001. Alewife weight at age was relatively constant between 1984 and the mid‐1990s but decreased from the mid‐1990s to 2001. Mean condition for a given alewife age was, on average, 13.7% higher during 1984–1994 than during 1995–2001. This decline in alewife condition was not a density‐dependent response by the alewife population because alewife abundance trended neither upward nor downward during 1984–2001. The decline in alewife condition was possibly due to the lakewide decrease in the abundance of Diporeia spp. during the 1990s. Apparently, the availability of the large‐bodied invertebrates Diporeia and Mysis spp. was an important regulator of adult alewife growth because alewives attained a substantially larger size in Lake Michigan, where these invertebrates were relatively important constituents of the adult alewife diet, than in Lake Ontario, where these invertebrates were not readily eaten by adult alewives. For age‐2 or older females, mean length was 2–9 mm greater than for males. Alewife size at age and condition were slightly higher on the eastern side of Lake Michigan than on the western side.

The Northern Refuge was established in 1985 as part of the lake trout Salvelinus namaycush rehabilitation effort for Lake Michigan. To evaluate progress toward lake trout rehabilitation in the Northern Refuge, we conducted annual (1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008) gill-net surveys in the fall to assess the adult population and beam trawl surveys in the spring to assess naturally reproduced age-0 lake trout. Our criteria for evaluating progress included the density of ''wild'' age-0 fish within the Northern Refuge, the proportion of wild fish within the adult population, density of spawners, adult survival, growth, and wounding rate by sea lampreys Petromyzon marinus. No wild age-0 lake trout were caught in the Northern Refuge during 1991-2008. Overall, wild lake trout did not recruit to the adult population to any detectable degree. The mean density of spawning lake trout decreased from 45 fish 305 m of gill net À1 d À1 during 1991-1999 to only 4 fish 305 m À1 d À1 during 2000-2008. Although the sea lamprey wounding rate more than doubled between these two time periods, catch curve analysis revealed that mortality of adult lake trout actually decreased between the two periods. Therefore, the 90% decrease in abundance of spawning lake trout between the two periods could not be attributed to increased sea lamprey predation but instead was probably due in part to the reduced lake trout stocking rate during 1995-2005. The paucity of natural reproduction in the Northern Refuge during 1991-2008 most likely resulted from alewife Alosa pseudoharengus interference with lake trout reproduction and from the relatively low lake trout spawner density during 2000-2008. Our results suggest that the annual stocking rate of lake trout yearlings should be increased to at least 250,000 fish/reef to achieve greater densities of spawners.