Unaccounted mortality in salmon fisheries: non-retention in gillnets and effects on estimates of spawners

P> Effective and sustainable natural resource management is enhanced when the consequences of exploitative practices are fully understood and acknowledged. Commercial fisheries devote considerable resources to maximize the harvest of target species and minimize interference with non-target stocks. Appropriately, bycatch and discard of non-target stocks are recognized as critical economic and conservation concerns. Few studies, however, have examined non-retention mortality in target stocks. Non-retention, where fish are engaged by fishing gear but not landed, is rarely quantified and the effects on stocks are unknown. Mortality due to non-retention may have important effects on the dynamics of exploited populations. We surveyed spawning populations of sockeye salmon Oncorhynchus nerka that had traversed commercial fisheries in Bristol Bay, Alaska, to estimate the incidence of non-retention in gillnets and the severity of injuries associated with entanglement. To better understand how gillnet injury affects spawning success, we tagged and monitored stream-spawning fish and applied a maximum likelihood model to mark-recapture data. A substantial portion (11-29%) of spawning sockeye salmon exhibited clear signs of past entanglement with commercial gillnets. Survival among such fish was significantly reduced. More than half of the fish that reach natal spawning grounds with fishery-related injuries fail to reproduce. This suggests that estimates of spawning stocks are inflated by 5-15% at minimum. Synthesis and applications. Our analyses indicate that non-retention in gillnet fisheries is an important and under-appreciated consequence of the exploitation of salmon. Stock estimates for exploited populations that do not account for non-retention mortality overestimate the number of reproductively viable fish. Unaccounted mortality and interannual variation in the magnitude of this mortality may prevent accurate estimates of viable spawners, confound our understanding of the relationship between stock size and recruitment, impede optimal management and obscure the ecosystem impacts of migratory stocks in coastal watersheds. Given the magnitude of non-retention in this fishery, explicit consideration of non-retention mortality may be warranted across a wide range of exploited populations.