Variation in diet and activity of river otters (Lontra canadensis) by season and aquatic community
Keystone predators can impact many prey species, including those that are endangered. In order to assess the impact predators have on different prey populations, it is essential to identify the species being consumed in different types of aquatic communities, while accounting for possible seasonal variation in consumption. Here I use analysis of river otter (Lontra canadensis) scat to assess the impact otters have on prey populations, particularly endangered salmonids and migrating birds. I analyzed the prey composition of 1,411 river otter scats collected from 10 sites in Humboldt County, California, between 2011 and 2012. Analysis of prey items in scat divided study sites into four distinct clusters based on diet. Fish, particularly from the families Gasterosteidae (sticklebacks), Cottidae (sculpins), and Pholidae (gunnels), formed the main prey component, but crustaceans, birds (Anas sp. and Fulica sp.), amphibians, and insects were also main components of otter diet. Salmonids formed less than 5% of overall diet, but otters consumed the largest percentage of salmon during salmon spawning season at the inland cluster where salmonids spawn. Scat marking intensity varied between latrine sites, clusters, and seasons, with the most scats collected in the fall and the fewest in the winter/spring, except at the inland cluster where marking activity was reversed. Otters may be responding to seasonal migrations of endangered and threatened salmonids. Birds comprised 21% of diet and were eaten most frequently during the winter migration season, and no endangered bird species were found. Diet surveys of this type are useful for monitoring resource use by top predators in wetlands and other aquatic ecosystems.