2.3 Energy content nutritional analysis
We measured energetic value and proximate composition (the proportion of
protein content, lipid content, moisture, and percent ash) for sea otter
prey items. We used previously established methods to measure energy
density using a Parr 6725 semi-micro bomb calorimeter. Standards and
replicates were used to confirm consistent calorimeter readings. Lipid
content was determined using previously established methods using a
sulfo-phospho-vanillin colorimetric analysis. Protein content was
estimated by multiplying total nitrogen content by 6.25, which accounts
for the nitrogen content of protein . Nitrogen content was measured with
a FlashSmart elemental analyzer coupled to a Delta-V continuous-flow
isotope ratio mass spectrometer (Thermo Scientific, Waltham,
Massachusetts, USA). Carbohydrate content was not assessed as it is
assumed to be negligible in marine invertebrates . Ash content was
processed at 600°C and measured with a LECO Thermogravimetric Analyzer
701 (TGA) dryer. Only samples with > 1 g of dried material
were able to be combusted for percent ash content.