4.2 Reproductive status
Female sea otters with pups had a more varied diet composition
than females without pups and males. The difference in the diet was the
largest shift among all tested metrics. Females with pups ate a higher
proportion of crabs, sea cucumbers, and sea urchins than sea otters
without pups. Sea cucumbers are the functional prey group with the
lowest energetic content, therefore females with pups are obtaining
fewer calories per gram of food consumed. This is likely due to females
with pups opting for a risk-averse strategy, instead of searching for
the highest quality prey and risking no success at all. One possibility
could be that a female with a pup would rather come up with prey every
dive instead of risking no success for a higher effort prey (such as
choosing a slow-moving sea cucumber laying the ocean floor, instead of
digging for a clam or a fast-moving crab). Success rates were high
across the POW region, but females with pups had the highest success
rate. Other studies have reported females to vary their diet according
to reproductive state. In California, tagged female sea otters switched
their foraging strategies and prey types consumed when they had no pup,
small pups, and large pups .
There were other sex-specific differences in prey type. Males were
observed to eat more snails than females, while females with pups were
never observed to eat snails. In previous studies, snail specialists
have been linked to poor overall body condition and higher death rates
due to disease . Because we did not follow individuals in this study, we
were unable to determine if there was a similar pattern in Southeast
Alaska sea otters. In addition, although the prevalence of geoduck clams
(Panopea generosa ) in diets was low in our study, with only seven
observations of geoduck clams consumed throughout the region, all of
these were consumed by males. Geoduck clams are higher risk prey because
of the increased effort needed to excavate them, thus, sea otters
generally make several dives to recover one geoduck clam . We did not
observe sea otters foraging for geoduck clams in the most recently
colonized zones, which matches previous studies in the same region .
These areas may have larger clams that reside in shallower areas (e.g.,
butter clams) creating a more efficient risk-reward ratio. Dietary
differences between males and females are worth noting, as males are
more likely to expand into new regions first . As new regions are being
recolonized, knowing the diet preferences of males can help to predict
invertebrate predation with relation to species that are of interest to
humans.
Lipid content and energetic gain per gram were lowest for females with
pups. There were no significant differences in dive or surface times for
females with pups vs. other age/sex classes, which could be an artifact
of eating less calorically-rich prey. However, females with pups have
the highest foraging success rate when compared to females without pups
and males, which could indicate that females with pups select less
energy-rich prey items over the risk of no success. Previous studies of
sea otter energetics showed that female sea otters with large pups
operate at an energetic deficit by the time a pup reaches weaning age .
In the present study, this deficit may be evidenced by the lower
energetic gain per gram for females with pups. However, there are a
multitude of other risks females with pups must contend with while
foraging. In areas where food is limited, females with large pups may
reach a maximum physical amount of time allowable for foraging . They
also encounter constraints with respect to allocating energy to nursing
vs. providing prey to their pup.