Cessation of a salmon decline with control of parasites

Abstract

The resilience of coastal social-ecological systems may depend on adaptive responses to aquaculture disease outbreaks that can threaten wild and farm fish. A nine-year study of parasitic sea lice (Lepeophtheirus salmonis) and pink salmon (Oncorhynchus gorbuscha) from Pacific Canada indicates that adaptive changes in parasite management on salmon farms have yielded positive conservation outcomes. After four years of sea lice epizootics and wild salmon population decline, parasiticide application on salmon farms was adapted to the timing of wild salmon migrations. Winter treatment of farm fish with parasiticides, prior to the out-migration of wild juvenile salmon, has reduced epizootics of wild salmon without significantly increasing the annual number of treatments. Levels of parasites on wild juvenile salmon significantly influence the growth rate of affected salmon populations, suggesting that these changes in management have had positive outcomes for wild salmon populations. These adaptive changes have not occurred through formal adaptive management, but rather, through multi-stakeholder processes arising from a contentious scientific and public debate. Despite the apparent success of parasite control on salmon farms in the study region, there remain concerns about the long-term sustainability of this approach because of the unknown ecological effects of parasticides and the potential for parasite resistance to chemical treatments.

Lessons from sea louse and salmon epidemiology

“…The natural process of migratory allopatry, where uninfected juvenile salmon are separated from adult salmon and protected from sea lice by migration [39], can be interrupted by the presence of sea louse reservoirs on salmon farms. When these reservoirs were removed by fallowing or parasiticide treatment of farmed salmon, epizootics of sea lice on wild juvenile salmon declined [40], helping to verify model predictions.…”

“…In spawner-recruitment models, a population's growth rate during sea louse epidemics can be compared to the population growth rate of the same population before or after the epidemic, or to nearby, unexposed salmon populations (figure 5b). Studies on wild pink (Oncorhynchus gorbuscha) [13,24,40] and coho (Oncorhynchus kisutch) [25] salmon in Pacific Canada found evidence that population growth rates are depressed in populations exposed to sea lice outbreaks associated with salmon aquaculture. Similarly, studies on post-smolt wild sea trout (Salmo trutta) in Ireland found that infection levels were higher if the fish were captured in bays that also contained salmon farms [46].…”

“…Incorporating information on variability in infection pressure can increase statistical power of such tests (figure 5c). For example, productivity of wild pink and coho salmon has been found to negatively correlate with time-varying regional sea louse abundance on farmed or wild salmon [26,40].…”

Quantifying the contribution of sea lice from aquaculture to declining annual returns in a wild Atlantic salmon population

“…Sea cage aquaculture causes sea lice on sympatric wild fish to increase (Frazer 2009). Marine survival of wild pink salmon has been related negatively to lice density on farmed salmon (Marty et al 2010, Krkošek et al 2011 and to observed lice infestation on out-migrating juvenile wild fish (Peacock et al 2013). The negative impact of sea lice on salmonid survival appears to be exacerbated by warmer environmental conditions (Bateman et al 2016, Shephard et al 2016).…”

“…The contribution of sea lice to overall marine survival of wild Atlantic salmon re mains an important knowledge gap, particularly in the context of changing oceanographic conditions and the long-term decline of many populations. Parsing out coastal sea lice effects might contribute to understanding of changing high-seas marine survival, and possibly guide management of lice on salmon farms to reduce impacts on wild populations (Peacock et al 2013).…”