Using grizzlies to assess harvest-ecosystem trade-offs in salmon fisheries

A new study, published in the journal PLoS Biology by scientists at Raincoast and the University of California-Santa Cruz has found that providing more spawning salmon to grizzlies also benefits salmon, the ecosystem and in several cases even fisheries yields.  

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Citation:

Levi T., Darimont C.T, MacDuffee M., Mangel M., Paquet P., et al. 2012. Using Grizzly Bears to Assess Harvest-Ecosystem Tradeoffs in Salmon Fisheries. PLoS Biol 10(4)

The density, number of cubs and even the size of grizzlies is strongly coupled to salmon abundance. The more salmon that are available to grizzlies, the less of each fish a bear eats, and the more carcass remains go to the ecosystem and other wildlife. Conversely, when salmon abundance is low, bears will have fewer cubs (or none at all), more of each fish gets eaten by the bear and less salmon goes to the ecosystem and other species.

While this makes sense, we found that letting more salmon past the nets and hooks of fisherman also contributed to more salmon abundance in the long term. Ultimately the study uses grizzlies as surrogates for salmon ecosystem function and evaluates trade-offs between grizzlies, salmon and fisheries. Only in the Fraser River watershed, where grizzly populations are highly threatened, was there a cost to fishery yields to help recover grizzlies.

Abstract

Combining salmon biomass and stock-recruitment data with stable isotope analysis, we assess potential tradeoffs between fishery yields and bear population densities for six sockeye salmon stocks in Bristol Bay, Alaska, and British Columbia (BC), Canada. For the coastal stocks, we find that both bear densities and fishery yields would increase substantially if ecosystem allocations of salmon increase from currently applied lower to upper goals and beyond. This aligning of benefits comes at a potential cost, however, with the possibility of forgoing harvests in low productivity years.

In contrast, we detect acute tradeoffs between bear densities and fishery yields in interior stocks within the Fraser River, BC, where biomass from other salmon species is low. There, increasing salmon allocations to ecosystems would benefit threatened bear populations at the cost of reduced long-term yields. To resolve this conflict, we propose an EBFM goal that values fisheries and bears (and by extension, the ecosystem) equally. At such targets, ecosystem benefits are unexpectedly large compared with losses in fishery yields. To explore other management options, we generate tradeoff curves that provide stock-specific accounting of the expected loss to fishers and gain to bears as more salmon escape the fishery. Our approach, modified to suit multiple scenarios, provides a generalizable method to resolve conflicts over shared resources in other systems.