Abstract.-Over the past 20 years, a restructuring of the commercial fishing industry on Michigan's upper Great Lakes has resulted from a shift in the state's Great Lakes management policy. The state's policy stresses recreational rather than commercial fishing, implementation of limited entry, delineation of zone management, an effort to rehabilitate lake trout (Salvelinus namaycush), which entailed conversion from traditional gill nets to trap nets, and litigation by tribal entities.
Catch and effort data, reported obligatorily by commercial fishermen and supplemented by seasonal age and size data collected by the state, have allowed calculation of modality, age distribution, growth, and catch quotas for lake whitefish (Coregonus clupeaformis). Trends in commercial yield during the 1980s show peak whitefish catches around 1985 in management zones WFM00 and WFM01, followed by decreasing harvests through 1988. Catches have been higher in zones WFM06 and WFM08 during the last few years than earlier in the decade.
Annual total mortality rates during 1983-88 were high for whitefish stocks in management units WFM00 (0.77-0.88) and WFM01 (0.73-0.86), but they were low in units WFM06 (0.45-0.51) and WFM08 (0.45-0.48). Whitefish stocks in zones WFM00 and WFM01 have sustained themselves at some level short of collapse despite high total mortality rates. Apparently collapse has been averted because an adequate spawning biomass composed of large, older whitefish was distributed in depths unavailable to trap nets for much of the fishing season, and because conditions in northern Green Bay have been consistently favorable for whitefish reproduction. Age distribution in landed catches was dependent on time of year and gear. Whitefish catches in zones WFM06 and WFM08 included larger proportions of older fish, and fish larger than 500 mm were significantly heavier when compared to catches of fish from the two northern zones.
Yields were simulated under minimum size limits (MSL) of 432 mm, 457 mm, and 483 mm in each management unit. The balance between the costs and benefits of various MSLs cannot be adequately evaluated without field testing. Contradictions between calculated catch quotas and reported yields may exist due to the use of 3-year averages for parameters used in models.