Impact of diet-index selection and the digestion of prey hard remains on determining the diet of the Steller sea lion (Eumetopias jubatus)

Nine prey species (n = 7431) were fed to four captive female Steller sea lions (Eumetopias jubatus (Schreber, 1776)) in I I feeding trials over 75 days to investigate the effectiveness of different methods used to determine diet from prey hard remains. Trials aimed to replicate short (1-2 days) and long feeding bouts, and consisted of single species and mixed daily diets. Overall, 25.2% +/- 22.2% (mean +/- SD, range 0%-83%) otoliths were recovered, but recovery rates varied by species (ANOVA, P = 0.01) and were linearly related to otolith robustness (R-2 = 0.88). Squid beaks were recovered at higher frequencies (mean 96%) than the otoliths of all species. Enumerating both non-otolith skeletal structures and otoliths (together termed bones) increased species recovery rates by twofold, on average (P < 0.001), with increases up to 2.5 times for Pacific herring (Clupea pallasii Valenciennes in Cuvier and Valenciennes, 1847) and 3-4 times for salmonids. Using bones reduced interspecific differences (P = 0.08), but recovery varied among sea lions. Bones were distributed over more scats per meal (mean 2.9 scats, range 0-5) than otoliths (mean 1.9 scats, range 0-4). In three different 15-day mixed diet trials, biomass reconstruction (BR) indices performed better than frequency of occurrence indices in predicting diet fed. Applying our experimentally derived numerical correction factors (to account for species differences in complete prey digestion) further improved BR estimates, resulting in all 12 unweighted comparisons within 5% (for otoliths) and 12% (for bones) of the actual diet fed.