Shifts in the dietary diversity of the North American River Otter ( <i>Lontra canadensis</i> ) along habitat gradients in coastal rivers of Southern Oregon

Abstract North American river otters (Lontra canadensis) are generalist aquatic predators inhabiting both freshwater and coastal ecosystems that are capable of shifting their foraging strategy to use locally available prey. To understand how the diversity and composition of their diets change across a habitat gradient, we identified prey remains in river otter scat samples (n = 675) collected from both freshwater and estuarine areas of 6 coastal rivers in southern Oregon during the summers of 2023 and 2024. These rivers were spread across ∼120 km of coastline and varied substantially in their hydrology and tidal influence. The Shannon diversity of prey items in river otter scat increased in association with ocean proximity along the lengths of individual rivers, suggesting an increase in both the evenness and richness of prey taxa consumed in estuarine areas. Dietary dissimilarity between rivers was significantly higher in estuaries than in freshwater areas, suggesting that the complexity and variability of estuarine habitats drove an expansion of river otter dietary niches. A multivariate probit model demonstrated that while salinity and ocean proximity were the most important variables for explaining prey occurrence, other habitat variables (river flow volume and velocity) also have significant predictive importance. While overall dietary diversity increased, there was no notable change in the general types of prey taken—river otter diets shifted from a freshwater diet dominated by crayfish (60% frequency of occurrence [FO]), prickly sculpin (14% FO), and stickleback (14% FO) to an estuarine diet characterized by saddleback gunnel (23% FO), crab (21% FO), prickly sculpin (19% FO), and staghorn sculpin (10% FO). Salmonid smolts (genus Oncorhynchus) were an occasional prey item throughout all habitat types (2.6% FO). We conclude that both the alpha and beta diversity of river otter diets increase in ecologically complex estuarine habitats, but that this change is not driven by the expansion of their foraging strategy to include truly novel prey types.