Estimating diet composition in sea lions: which technique to choose

Reliable estimates of diets are vital to monitor impacts of sea lion popu-lations on their ecosystems and their interactions with fisheries, and to understand the role of food to animal nutrition and health. Approaches include using (1) prey remnants in stomach contents, spews and scats; (2) prey DNA in scats; (3) fatty acid signatures in blubber; and (4) stable isotope ratios in predator’s tissue. Each methodology has particular ad-vantages and limitations, many of which can be assessed and improved through controlled captive feeding trials. Analysis of prey remnants from captive sea lion scats have shown significant variability in diges-tion between and within prey species, which, coupled with preferen-tial regurgitation and enumeration biases, can confound accurate diet quantification, but does not prevent spatial or temporal comparisons. Correction for partial digestion and use of additional structures besides otoliths can provide reliable prey size estimates. Prey DNA can be con-sistently isolated from soft remains in scats from captive sea lions, and with further development this approach may allow quantification of diet. Genetic methods can be expensive and representative of only one to tw