3.3 Energetic content of sea otter prey
Overall, sea otter prey had a similar makeup of protein and lipid content; sea urchins were the only prey group significantly different from other prey groups for lipid-to-protein ratio (Fig. 4, p < 0.01). Functional prey groups varied in their energy, lipid, and protein content across seasons (Table 2). Across all seasons, sea cucumbers exhibited lower energy than all other prey types (Fig. 5), and their energy and lipid varied significantly by season (Table 2). Sea urchins had significant variability in lipid content across seasons (Table 2). Clams exhibited a significant change in energy and lipid over seasons as well, but R values were low, which means the overall seasonal effect was low (Table 2). Crabs did not vary significantly across seasons or sex. Snails did not vary significantly across seasons. Pairwise comparisons for all prey groups and seasons revealed significant differences in energy for clams and sea cucumbers (Table 3). Lipid varied for clams, sea cucumbers, and sea urchins, whereas protein only varied between seasons for clams. Snails were not compared across seasons due to the small sample size.
When we compared year-round energetic changes in sea otter prey with diet proportion estimates, we found consumption rates of most functional prey groups did not correlate with energy density. Clam consumption, the major diet item for sea otters in this study, did not correspond with changes in energy (Fig. 6A) or lipid (Fig. 6C). Based on LaRoche et al. (2021), sea otters consumed more clams in the fall and winter months, when the energetic and lipid contents of clams were lower than the spring season. Sea cucumber consumption, the second most abundant diet contribution, did appear to correspond with changes in energy (Fig. 6B) and lipid (Fig. 6D). Sea otters decreased their consumption of sea cucumber in the fall and winter months, which corresponded with declining energetic value.