Diversity in a Multi-Stock System: Temporal and Spatial Portfolio Effects in Lake Erie Walleye Production

Geographically distinct spawning stocks can generate population stability by increasing resiliency to environmental change. Stocks are likely to respond differently to environmental change due to variation in habitat-specific responses. The differential response among stocks may generate a “portfolio effect” by maximizing the ratio of spawning production to risk (i.e. poor production) through diversification. We compared larval walleye (Sander vitreus) production from three tributaries (Detroit, Maumee, and Sandusky Rivers) and the open-lake reef complex of western Lake Erie over several years (1994, 1995, 2011-2013). The reefs produced more larval walleye than all tributaries combined, but reef production was highly variable. In years when reef production was low, river production made substantial contributions, partially buffering reduced contribution from the reefs. The Detroit and Maumee Rivers produced relatively consistent numbers of larval walleye, whereas production from the Sandusky was more variable. Reef and river stocks responded differently to changing environmental conditions and climate, indicating that production risk is diversifiable.  Appropriate investments, such as stock-specific management and protection and restoration of in-river habitat may decrease variation in inter-annual production of larval walleye.