Investigations of southern sea otter foraging ecology at the northern range extent

Sea otters (Enhydra lutris) are vital keystone predators within coastal ecosystems. Following near extinction during the fur trade, population recovery has proceeded slowly in California with southern sea otters currently ranging from Pigeon Point in the north to Point Conception in the south. Decades of research have revealed consistent evidence of inverse relationships between sea otter density and prey resources. Furthermore, population density is known to influence foraging effort and success rates, such that individuals in populations nearing carrying capacity spend more time foraging, with lower rates of energy return. These relationships likely play a role in species recovery and range expansion, but little is known about these dynamics at the northern range edge. Yearly census data comprise the majority of information regarding otters in this region, where a historic lack of range expansion has limited population growth. / We are collecting fine-scale census and foraging data of sea otters at Año Nuevo State Park to determine seasonal abundance, diet composition, diet diversity, and energy intake rates. We compare data from this study with previously collected data throughout the current range, including high- and low-density populations, to evaluate the status of southern sea otters at the northern range extent. / Preliminary results suggest sea otter diet at Año Nuevo is primarily composed of crabs, urchins, clams, and infaunal worms. These prey items are indicative of a mixed substrate habitat, with both rocky reef and sandy-bottom areas. Sea otters appear to be sourcing prey from both habitat types. Prey capture rate is 61.9%, and diet diversity (using Shannon-Wiener index) appears to be higher than other low sea otter density sites (H=1.727). Monthly surveys show seasonal movement in sea otter habitat use, which we hypothesize may be coupled with seasonal presence of kelp. Since data collection started in Fall 2019, our current foraging data set is limited in number (n=660 foraging dives) and spans just over one season. As we continue collecting data through the next year we will be able to quantify energy intake rates and improve our ability to make comparisons with previously published values. Further data collection will elucidate the status of Año Nuevo sea otters relative to carrying capacity and provide insight into potential reasons behind the historic lack of northward range expansion. This information may be vital to management agencies for refining conservation methods and priorities aiding in sea otter population recovery