Unintended Consequences of MSY Proxies for Defining Overfishing and Sustaining Fisheries

The foundations of fisheries science were built on defining overfishing as a fishery that removes more fish than biological production can replace. Early recognition of the importance of density-dependent production and compensatory population behavior led to the concept of maximum sustainable yield (MSY).  With sound theoretical basis, MSY and the fishing mortality that produces MSY (F_MSY) have been global standards for fishery management for decades.  Scientific critiques of MSY and its application to fisheries management (e.g., MSY as a target, equilibrium assumptions, ecosystem considerations) have galvanized the concept, refining its definition and improving its implementation.  A current debate is over the common practice of replacing F_MSY with more stable proxies that have nearly the same long-term yield at the expense of substantially less short-term yield and much greater stock biomass expectations.  As defined in the Magnuson Act, ‘overfishing’ means “a rate or level of fishing mortality that jeopardizes the capacity of a fishery to produce the maximum sustainable yield on a continuing basis” (i.e., F_MSY).  Many F_MSY proxies are used to define overfishing, because F_MSY is not well determined, but direct estimates of F_MSY and the uncertainty are more appropriate for implementing the national management strategy.  If catch limits are intended to represent stakeholder’s desired risk tolerance of overfishing, they should be based on F_MSY whenever possible rather than a proxy for F_MSY, because probability of overfishing is confounded by proxies.  MSY proxies fail to recognize all MSY properties and its unifying role in fishery science and management.