A new method to evaluate the nutritional composition of marine mammal diets from scats applied to harbor seals in the Gulf of Alaska

- Diets estimated using two different diet reconstruction indices were compared.
- Nutritional content was estimated using a prey nutritional database.
- Diet diversity was consistently lower in the breeding versus molting seasons.
- Despite changes in prey consumed, nutritional composition was relatively constant.
- Harbor seals can maintain nutritional input despite change in prey availability.

The nutritional profile of harbor seal (Phoca vitulina Linnaeus, 1758) diets was determined from scats using a new diet formulation software program and a prey nutritional database developed from proximate analyses of various prey found in Alaskan waters. Scats were collected from Tugidak Island, Alaska, during summers from 2001 to 2009 and prey composition of the diet was estimated using split-sample frequency of occurrence (ssFO) and biomass reconstruction (BR). While 60 different prey items were identified across the collection period, only 21 were found in > 10% of scats during at least one sampling period, and an average of 2.6 prey species were identified per scat. Diet diversity was consistently lower in the breeding season than in the molting season. The estimated relative importance of some prey items was significantly different between methods; however, there were no significant differences in the overall estimated protein, lipid, or gross energy composition of the diets. The diet formulation software allows evaluation of the nutritional composition of free-ranging diets using the non-invasive collection of scat. Results from combining the prey database and nutritional composition of diets across years suggest that harbor seals have flexible diets and can maintain nutritional input by taking advantage of seasonally or periodically abundant prey.