LaRoche NL et al. (2023), Macronutrient composition of sea otter diet wit...

ECB-ART-51436

Ecol Evol 2023 May 01;135:e10042. doi: 10.1002/ece3.10042.

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Macronutrient composition of sea otter diet with respect to recolonization, life history, and season in southern Southeast Alaska.

LaRoche NL , King SL , Fergusson EA , Eckert GL , Pearson HC .

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The sea otter (Enhydra lutris) population of Southeast Alaska has been growing at a higher rate than other regions along the Pacific coast. While good for the recovery of this endangered species, rapid population growth of this apex predator can create a human-wildlife conflict, negatively impacting commercial and subsistence fishing. Previous foraging studies throughout the sea otter range have shown they will reduce invertebrate prey biomass when recolonizing an area. The goal of this study was to examine and quantify the energy content of sea otter diets through direct foraging observations and prey collection. Our study area, Prince of Wales Island in southern Southeast Alaska, exhibits a gradient of sea otter recolonization, thus providing a natural experiment to test diet change in regions with different recolonization histories. Sea otter prey items were collected in three seasons (spring, summer, and winter) to measure caloric value and lipid and protein content. We observed 3523 sea otter dives during the spring and summer. A majority of the sea otter diet consisted of clams. Sea otters in newly recolonized areas had lower diet diversity, higher energetic intake rates (EIR, kcal/min), and prey had higher energy content (kcal/g). Females with pups had the highest diet diversity and the lowest EIR. Sea otter EIR were higher in the fall and winter vs. spring and summer. Sea cucumber energy and lipid content appeared to correspond with times when sea otters consumed the highest proportion of sea cucumbers. These caloric variations are an important component of understanding ecosystem-level effects sea otters have in the nearshore environment.

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	FIGURE 1. Sea otter visual foraging observations were made within three foraging zones (shaded areas with zone numbers listed) on Prince of Wales Island in southern Southeast Alaska. Each zone was designated by time‐since‐recolonization based on US Fish and Wildlife Service aerial surveys. Prey sample sites (1, Craig and 2, Soda Bay, near Hydaburg) are in yellow.
	FIGURE 2. Proportion of diet from biomass estimates for time‐since‐recolonization zones (a) and sea otter reproductive status/sex (b). Zones are based on US Fish and Wildlife Service aerial surveys. Zone 1 is the area occupied for >30 years, Zone 2 is the area occupied >15 years, and Zone 3 is the area occupied >7 years.
	FIGURE 3. Comparison of the energy, lipid, and protein concentrations in population‐level diets for time‐since‐recolonization zone (a), sea otter reproductive status/sex (b), and season (c). All calculations are made from the wet mass (as a sea otter would eat the item) and converted to dry mass for comparison. Zones are based on US Fish and Wildlife Service aerial surveys. Zone 1 is the area occupied for >30 years, Zone 2 is the area occupied ≤30 and ≥15 years, and Zone 3 is the area occupied <15 and >7 years.
	FIGURE 4. Mean (±SE) proportion of energy in each functional prey group derived from lipid and protein.
	FIGURE 5. Seasonal whole‐body nutrition of functional prey groups of sea otters in dry mass: (a) mean (±SE) energy content in kilocalories per dry gram; (b) mean (±SE) % lipid content in dry grams; (c) mean (±SE) % protein content in dry grams.
	FIGURE 6. Seasonal variation in the nutrition of sea otter prey (green bars are clams, and blue bars are sea cucumbers. Error bars are standard deviation) and frequency of occurrence of that prey species in the diet (e, f, from LaRoche et al., 2021). a and b are energy density (kilocalorie per dry gram), and c and d are lipid content (percent per dry gram). Prey samples were not collected in Fall.

References [+] :

Chinn, THE HIGH COST OF MOTHERHOOD: END-LACTATION SYNDROME IN SOUTHERN SEA OTTERS (ENHYDRA LUTRIS NEREIS) ON THE CENTRAL CALIFORNIA COAST, USA. 2016, Pubmed