Indirect effects of sea otter recovery on temperate reef fish

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The loss or recovery of apex predators can have profound positive or negative ecological and socio-economic impacts. Effects of predator depletion or recovery are frequently accompanied by time lags, which are often context-dependent. In temperate rocky reef ecosystems, sea otters (Enhydra lutris) trigger a cascade of direct and indirect effects driving transitions between kelp-depleted and kelp-dominated states. We quantified the indirect effects of sea otter recovery on copper rockfish (Sebastes caurinus) and kelp greenling (Hexagrammos decagrammos) along a gradient in sea otter occupation time on the central coast of British Columbia. We used hook-and-line surveys and underwater visual transects at shallow depths (2-25 m) across 20 rocky reef sites and a space-for-time substitution representing variation in sea otter occupation time. Overall, we found higher fishable biomass of copper rockfish at sites where sea otter occupation was greatest, and evidence that copper rockfish catch rates were higher in relatively larger kelp beds. However, copper rockfish fishable biomass was negatively correlated with canopy kelp stipe density, likely due to the successional transition as sea otters return to an area from the disturbance-tolerant annual kelp Nereocystis to the perennial kelp Macrocystis. Contrary to expectations, we found no evidence for an effect of sea otter occupation time or kelp metrics on the fishable biomass of kelp greenling. We found no evidence that sea otter occupation time or any other factor tested affected rockfish or greenling density observed on underwater visual surveys. Contrary to our hypothesis, we found no appreciable enrichment of the δ13C signature across the gradient of sea otter occupation time associated with an increase in the amount of kelp-derived carbon within fish. These data allow us to evaluate trade-offs elicited by sea otter recovery and devise management plans for temperate reef systems that incorporate realistic timelines for changes to kelp forest communities.
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