UC Santa Cruz

Elkhorn Slough, a small estuary located in Monterey Bay, California, has lost 50% of its salt marsh in the last 70 years (Van Dyke & Wasson 2005) and is the only estuary within the range of the threatened southern sea otter (Enhydra lutris nereis) that has a resident sea otter population. Recent studies suggest that estuaries provide important habitat for the recovery of southern sea otters (Hughes et al. 2013; Lindsey 2016; Eby et al. 2017; Silliman et al. 2018). Using habitat suitability modeling, this study identified important physical features of salt marsh tidal creeks for sea otter use to guide restoration projects of salt marsh habitat in Elkhorn Slough. Using ArcGIS, six tidal creek salt marsh predictor variables (channel order, width, bank slope, elevation, geometric efficiency and distance to main channel) were calculated and spatially joined to sea otter location data from abundance surveys from September 2013 through September 2017. Five generalized additive models (GAMs), a full model and four other models using subsets of data based on sea otter behavior (resting or foraging) and demographic status (female with pups vs. male and females without pups), were created with different combinations of predictor variables and evaluated against their respective null models using AIC values, in R. Results suggest that sea otter behavior influences salt marsh use more than sea otter status. Sea otters primarily use salt marsh as resting habitat and prefer resting in tidal creeks that are wider (>10 m), with elevations between 0.3 m to 2 m NAVD 88, have shallow bank slopes (10 to 20 degrees) and are close to the main channel (<300 m). The foraging models could be significantly improved in the future by including prey distributions and other habitat types, but suggest that foraging sea otters prefer feeding in flooded salt marsh or in deeper creeks (<0 m NAVD 88) close to the main channel. These methods can be used to assess salt marsh habitat suitability and identify critical habitat for sea otters in other estuaries that may soon be re-colonized.