An investigation into the possible relationship between killer whale (Orcinus orca) predation and the continuing decline of the Steller sea lion (Eumetopias jubatus) population

Foraging ecology of marine mammals is often poorly understood because of the inherent difficulties in studying them in their natural environment. Increasingly, fatty acid signature analysis of blubber is being used to evaluate dietary preferences; therefore it is important to fully understand the biochemical composition of this tissue. The purpose of the present study was to describe the characteristics of killer whale blubber and skin, with the goal of ultimately applying this information to explore the feeding ecology of this species, and in particular how killer whales might be impacting Steller sea lions. Full depth blubber and skin samples were obtained from fresh-dead captive (n=8) and freeranging (n=2) killer whales. The entire blubber depth was divided into skin plus six equal depth sections. Gross lipid and water content of each layer was determined, lipid classes assessed, and fatty acid composition analyzed. Statistical analyses consisted of univariate and multivariate techniques, including classification and regression tree (CART) analysis. Blubber and skin separated into three groupings during CART analysis for all animals. Skin separated completely from all blubber layers and the outer blubber layers (A, B, C) were distinct from inner layers (D, E, F). Water content of captive whales averaged less than 17.5 ± 6.8% across layers B – F and gross lipid content averaged 70.7 ± 11.9% across layers B-F. In general, lipid content of the skin and layer A was lower than that of deeper layers. The lowest lipid content values from lateral body sites were seen in the sample taken from an adult wild whale with lipid contents averaging 63.6 ± 3.9% and protein averaging 11.9 ± 2.3%. Lipid content of the deepest blubber layer of the one wild neonate (W 2) was also low (22.9%), while skin composition was indistinguishable from adult whales. Lipid class analysis of blubber and skin suggests that, while blubber is composed primarily of triglycerides (74.2 – 98.4%), there can be substantial quantities of wax esters in the skin (mean: 18.2%, range: 8.9 – 36.6%) and the outer blubber layers A (mean: 27.6%, range: 3.2 – 53.2%) and B (mean: 19.3%, range: 1.3 – 28.3%). Deeper blubber layers (D-F) generally contained < 2% wax esters. Fatty acid composition of blubber and skin changed dramatically with depth for several saturated, monounsaturated, and polyunsaturated fatty acids. Further investigation will be required to determine whether this species has true layering or if a discontinuous gradient is present through the blubber. Changes in fatty acid composition could be a result of the different roles that blubber plays in marine mammals, including buoyancy, thermoregulation, structural support, and energy storage. The application of fatty acid signature analysis will require the acquisition of blubber samples from the deeper portions of the blubber layer. Stable isotope analysis of skin samples indicated different feeding habits for the different ecotypes of killer whales found in the region. Using IsoSource analysis, Prince William Sound (PWS) resident whales fed exclusively on fish – consistent with observational data. PWS residents consumed significant proportions of Chinook salmon (22.4 ± 19.0 %), sablefish (19.3 ± 12.4%), Pacific cod (19.1 ± 10.3%), and arrowtooth flounder (16.3 ± 16.2%) as well as