4.3 Seasonal diet shifts
Sea otters consume prey with higher percent lipid in the fall and winter months. This change in nutrient composition could be to compensate for colder temperatures in winter months and the need to obtain more calories to metabolize for warmth. Average sea surface temperatures (SST) for nearby Ketchikan range from a low of 6°C in the winter to a high of 14°C in the summer . Currently, to our knowledge, there are no published studies that test the density and characteristics of sea otter fur across seasons, so it is unknown if the fur is thicker and can add additional warmth in the winter months. Sea otters may need to increase the consumption of lipids to assist in thermoregulation. The low end of the water temperature range in which a sea otter can remain in a thermoneutral zone (i.e., a physiological state whereby the animal maintains its normal core body temperature without metabolic heat production or active cooling) is about 15°C . This temperature is slightly above the typical summer SST on POW, meaning that at temperatures below this critical level, sea otters must consume more energy to generate additional heat. Previous studies have shown that sea otters adjust their foraging patterns to environmental conditions, which could affect diet composition.
Based on sea otter diet estimates from LaRoche, et al (2021), clam consumption in spring and summer was lower than fall and winter. This change, which does not correlate with seasonal changes in energy or lipid content, could be due to paralytic shellfish poisoning (PSP). PSP is a toxin in algae that blooms in the spring and summer months along the Pacific coast . Studies have shown that sea otters will still eat bivalves that have PSP toxins present, but will avoid bivalves with very high amounts, as well as only consuming the foot and discarding the siphon, which usually has the highest concentration of PSP .
Sea otter consumption of sea cucumbers across seasons positively corresponded with sea cucumber total energy and lipid content. Sea cucumbers are broadcast spawners. They move into shallow waters in the late spring to begin spawning in the summer months . During the fall and winter months, they retreat to deeper water. Their highest percent lipid and caloric content were observed in the summer when they are preparing to spawn. Estimates from stable isotope analyses show the highest diet proportion in the summer, with spring slightly lower, and a drastic drop in the fall and winter months. This correlates with sea cucumber life history. The visual foraging observations show higher consumption in the summer than the spring. This correlation can be due to increased caloric content. Additionally, their shallow-water summer habitat makes them more easily obtainable for sea otters. At other times of the year, sea cucumbers inhabit depths up to 250 m, which is outside of a sea otter’s diving ability .