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.