Juvenile Chum (Oncorhynchus keta) and Pink Salmon (O. gorbuscha) use of submerged vegetative habitats and the influence of an apex predator

Enclosed data and code are an archive for the manuscript accepted with Marine and Coastal Fisheries "Juvenile Chum (Oncorhynchus keta) and Pink Salmon (O. gorbuscha) use of submerged vegetative habitats and the influence of an apex predator". We include two scripts, one for each objective in the manuscript and the corresponding required datasets to run those analyses and generate figures and tables for the manuscript.Manuscript abstractAbstractObjectiveApex predator-initiated trophic cascades occur in many nearshore marine habitats that simultaneously serve as critical habitat and food sources for commercially and ecologically important species, including juvenile salmon (Oncorhynchus spp.). Yet the potential relationships among apex predators (e.g. sea otters Enhydra lutris), submerged aquatic vegetation, and juvenile salmon are not well understood. In Southeast Alaska, we investigated (1) juvenile salmon abundance in eelgrass meadows and understory kelp beds and (2) potential drivers of juvenile Chum (Oncorhynchus keta) and Pink Salmon (O. gorbuscha) abundance in eelgrass meadows.MethodsWe analyzed historic (1998-2007) beach seine surveys to compare juvenile salmon abundance in nearshore habitats. We then employed the same survey (2017, 2019) in eelgrass to quantify juvenile salmon abundance alongside the influence of sea otter density (no./km2), distance from anadromous stream (km), seasonality, sediment categorization, and aboveground eelgrass biomass (g/m2).ResultsWe found greater abundance of Chum Salmon in understory kelp compared to eelgrass, whereas Pink Salmon abundance was similar between habitats. In eelgrass, Chum Salmon abundance peaked in June and was positively associated with sea otter density. Pink Salmon abundance varied seasonally peaking in May. We found increased Pink Salmon abundance with increasing sea otter density, distance from anadromous stream and decreased abundance with increased eelgrass biomass.ConclusionGrowth and survival while juvenile salmon are out-migrating from streams and relying on nearshore vegetated habitats can determine if salmon recruit to fisheries as adults. Here, we highlight the use of multiple habitats, eelgrass and understory kelp, indicating both should be explored as critical nursery habitat. We present evidence of indirect effects of sea otters influencing the abundance of juvenile salmonids, with potential further implications as sea otter populations expand. Apex predators, quality of vegetated habitats, and their structuring roles in the nearshore are critical for informing adaptive coastal fisheries management.Impact statementEelgrass is often highlighted as a nursery habitat for juvenile salmon that provides foraging grounds and refuge from predation, but this research suggests that understory kelp should also be assessed as critical habitat. Additionally, the presence of juvenile salmon in the nearshore is modulated by sea otter density (no./km2), distance from anadromous stream (km), seasonality, and above ground eelgrass biomass (g/m2). We identified indirect effects of an apex predator influencing the abundance of juvenile Pink Salmon and to a lesser extent juvenile Chum Salmon and highlight implications for habitat quality as sea otters continue to expand spatially in Southeast Alaska.