Sea lion vibrissae indicate seasonal patterns in trophic consumption
Doll, A.C.1,2, Taras, B.D.3, Stricker, C.A.4, L.D. Rea5, M.B. Wunder1
1University of Colorado-Denver, Department of Integrative Biology, Denver, Colorado
2Denver Museum of Nature & Science, Department of Zoology, Denver, Colorado
3Alaska Department of Fish and Game, Division of Wildlife Conservation, Anchorage, Alaska
4US Geological Survey, Fort Collins Science Center, Denver, Colorado
5Institute of Northern Engineering, University of Alaska Fairbanks, Fairbanks, AK
The vibrissae of Steller sea lions grow continuously without regular shedding. Once grown, these keratinaceous tissues become fixed and thus can represent a multiyear record of dietary consumption. We measured stable isotopes of carbon and nitrogen in segments along the length of vibrissae collected from 12 adult sea lions in the central and western Aleutian Islands; both δ13C and δ15N exhibited an oscillating pattern. We paired this data with isotope data from groundfish species collected in the same region to estimate the proportional contribution of these potential prey items. Due to the number of potential prey species, the level of overlap in prey isotopes and the temporal (vs. population) nature of our sea lion data, traditional mixing models were not appropriate for this analysis. Therefore, we used a kernel density approach to describe the isotopic space occupied by each prey species and used these ‘utilization distributions’ to estimate a probability of each prey’s contribution to every vibrissae segment. While the level of overlap in prey isotope values remained too high to make species-level assessments of proportional contribution, we did identify a positive correlation between species of similar trophic levels. Assuming the oscillations in vibrissae isotope values represents an annual signal; our results suggest that Steller sea lions alternate between high trophic level prey in the winter months and low trophic level prey in the summer months. These findings suggest a system level pattern in prey availability which could aid fisheries managers seeking to reduce the role human-competition may play in current sea lion population declines.