2.4 Statistical analysis of sea otter prey
To test our first objective (investigate the macronutrient quality of
sea otter prey across seasons), we calculated the percent protein and
lipid of each prey group using the energy equivalents of 9.5 kcal/g for
lipid and 5.7 kcal/g for protein . We used PRIMER v7 with a one-way
analysis of similarity (ANOSIM) procedure with season as factors for
each prey group, excluding crabs (α = 0.01). Snails had too few samples
to conduct pairwise seasonal comparisons. Mussels were excluded from
results because they were a very small portion (< 1%) of the
sea otter diet around POW. Crabs were analyzed separately with season
and sex as factors, using a two-way ANOSIM (α = 0.01).
To test our second objective (compare diet composition and prey
macronutrient composition across different sea otter metrics), we
calculated regional-level concentrations of macronutrient composition of
sea otter diets using established methods . In brief, first, we
converted the dry mass average (kcal, lipid, and protein) for each
functional prey group to a wet mass value. Second, using the proportion
of diet (also in wet mass) from SOFA outputs for each functional prey
group and the average prey value (kcal, lipid, and protein), we
calculated an average for each prey and macronutrient and added all
individual groups together. Finally, we divided this newly calculated
wet mass by dry mass to get the nutrient composition of dry mass.
Statistical comparisons between sea otter metrics tested were not
possible because we were not able to study known individual sea otters
and all data were used at a regional scale (Western POW and surrounding
islands).
Due to weather and light limitations, it was only possible to conduct
visual foraging observations during spring and summer. To estimate
year-round diets, we used results from stable isotope analysis . Diets
estimates were made using stable isotope (δ13C and
δ15N) analysis of both sea otter vibrissae and the
prey present around POW using a Bayesian model to estimate percent of
total diet for functional prey groups. These diet estimates were used in
the calculations for seasonal sea otter macronutrient contribution and
comparison to changes in sea otter prey. All datasets are archived in a
publicly accessible database with the Knowledge Network for
Biocomplexity .