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 .