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Can Contemporary Cisco Stocks Support Historical Levels of Yield in Lake Superior?

Prior to their collapse in the 1960s from overfishing of spawning stocks, cisco Coregonus artedi were the primary prey of most predators and target of the commercial fishery in Lake Superior. Annual yield averaged 5-million kg/year in 1912–1962, but declined steadily in the 1960s and 1970s. Since the decline, stock size and recruitment was low, until the early 1980s, when recruitment increased sharply. Stocks recovered in the 1980s and early 1990s because of production of larger year classes. Concurrent with recovery, commercial exploitation increased, especially in Wisconsin, although landings were smaller than historically because of fewer fishermen, fishery regulations, and market demand. The target of the current spawner-interception fishery is gravid females (95% of the catch) for the international caviar market, a situation that is ripe for overexploitation. For the last 25 years, recruitment has been weak and sporadic, and along with recently documented longevity (25 years), has been interpreted as conditions that existed historically. We question whether populations of a long-lived species could have sustained such high levels of harvest for 50 years (1912–1962) with sporadic recruitment. Alternatively, contemporary populations of cisco that exhibit sporadic recruitment may not be able to sustain contemporary yields, much less historical levels of yield. We propose to determine if contemporary cisco populations that exhibit 25-yr longevity and sporadic recruitment could have supported historical yields by developing a simulation model that considers combinations of reasonable life history profiles while accounting for observed historical harvest. We expect to find that 25-year longevity and sporadic recruitment evident in contemporary cisco populations is inconsistent with historical yields sustained over 50 years. Our findings will provide context for today’s observations of cisco life history and population dynamics and for managing contemporary fisheries that are declining in relative abundance and exhibiting single-age composition—both signs of overexploitation.

Our objective is to determine if historical yield of cisco in Lake Superior can be explained by contemporary recruitment dynamics and growth/longevity. If not, we will determine if historical yield was sustained by higher adult density or more frequent recruitment than that observed now. This would suggest that contemporary populations are characterized by low adult density that induces sporadic recruitment, which is more common and egregious at lower stock sizes. If so, we will confirm that contemporary populations are not close to full restoration and that sustained yields are far lower than those historically. More proactive management to control fisheries is needed to prevent population collapse.

We will use a stochastic age-structured multi-stock population model to determine if contemporary cisco life history and recruitment dynamics can explain yield statistics during 1912–1962 in Lake Superior. Spatial structure of cisco stock-recruitment will derive from prior BOTE-funded research (Rook et al. 2012, 2013). Model structure will be based on similar age-structured models for lake trout from BOTE- and Sea Grant-funded research (Nieland et al. 2011; Akins et al., In review), but will be parameterized with current life history parameters. To test the effect of sporadic recruitment on yield sustainability, we will compare multiple combinations of life history parameters on model fit to historical yield statistics. To test the effect of growth and longevity on yield sustainability, we will compare long and short longevity (slow and fast growth) on model fit to historical yield statistics. Likelihood of candidate models will be compared using an information-theoretic (AIC) approach.

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