Unaccounted mortality in salmon fisheries: non-retention in gillnets and effects on estimates of spawners
Matthew R. Baker*1 and Daniel E. Schindler 1,2
1 School of Aquatic and Fishery Sciences, University of Washington, Box 355020, Seattle, WA 98195-5020, USA ; and
2 Department of Biology, University of Washington, Seattle, WA 98195-1800, USA
*Correspondence author. E-mail: firstname.lastname@example.org
Copyright Journal compilation © 2009 British Ecological Society
delayed mortality • ecosystem engineers • fishery-induced injury • mark–recapture analysis • natural resource management • Pacific salmon • population dynamics • stock-recruitment estimation
1. 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.
2. 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.
3. 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.
4. 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.
Full article here....
Some additional excerpts...
"We estimated the incidence and severity of injuries in fish returning to natal streams and the effect of such injuries on pre-spawning mortality. The findings suggest that gillnet injuries are common and, in many cases, inhibit spawning."
"It is clear that virtually all fish with moderate to severe gillnet injury fail to spawn. In the case of fish with minor injuries, the delay in stream entry, abbreviated stream residence and the inhibition of morphological traits associated with sexual maturation (fig. 1) suggest that even minor injuries delay maturation and reduce reproductive fitness."
"Pre-spawning mortality was highly correlated with and was likely facilitated by fungal infection, caused by Saprolegnia spp., a facultative parasite common in freshwater ecosystems. Saprolegnia spp. causes tissue damage, loss of epithelial integrity and osmoregulatoryfailure. It is associated with damaged epidermal tissue suggesting fish with gillnet injuries are particularly susceptible to such infections. Of 43 fish with fungal infection at the time of our tagging, only one successfully spawned. Many injured fish without Saprolegnia spp. at tagging presumably developed infections subsequently. Due to the close correlation between fungal infection and pre-spawning mortality, Saprolegnia spp. is likely to be the proximate cause of pre-spawningmortality in gillnet injured fish."
Must be why these guys didn't make it....