Spatial and temporal distribution of juvenile Steller sea lion (Eumetopias jubatus) predation events in the Gulf of Alaska

Graduation date: 2014The first objective of this study was to quantify the intensity of space use of 70 juvenile (12-26 months old) Steller sea lions (SSLs) from the western Distinct Population Segment (DPS) in the Kenai Fjord(KF)/Prince William Sound (PWS) region of Alaska as derived from externally attached ARGOS satellite transmitter tags. A Bayesian state-space model (SSM) approach was used to process and interpolate the ARGOS-based locations. The resulting tracks were then used to quantify the spatio-temporal distribution of SSLs by way of a gridded utilization distribution (UD) in the study area. Each of six combined two-month UDs had an average of 24 ± 4.16 animals (range: 20-30) contributing around 5,090 ±1,879 filtered locations (range: 2,757-7,593). Results of the bimonthly UDs show juvenile SSL remained within the KF/PWS study region during the tracking period, with the exception of one individual, TJ16 (male), who crossed the 144° W Meridian separating the western and eastern DPS. Furthermore, results revealed that juveniles exhibit different spatial distributions depending on the time of year. They remain confined to a relatively smaller area (6,500-12,075 km²) in the 100% UD months of March through August compared to a larger area of 11,300-22,575 km² in the 100% UD months September through February. In general, in winter (November-April) 90% of observations fell within 30km of the nearest haul-out, whereas in summer (May-October) 90% fell within 20km. A multifactor analysis of covariance (MANCOVA) revealed a significantly higher percent time spent at sea from November through April than May through October (n=63; age as covariate F₁=3.796, p=0.056; season as factor F₁=13.147, p=0.001). Thus, while age contributed to the observed variance, the main effect was clearly by season. The second objective of this study was to characterize the density effects of predator-prey interactions based on the spatial distribution of actual juvenile Steller sea lion predation events in relation to their utilization distribution. Fifteen predation events were detected from archival Life History Transmitter (LHX) tags implanted into a subset of n=36 of the 70 juvenile SSLs. A Bayesian SSM approach was used to process and interpolate the ARGOS locations received post-mortem from LHX tags. The processed tracks were then used to extrapolate the most likely locations of predation events. All 15 predation events occurred in the western DPS from 2008 through 2013. We expected predation events to occur uniformly in all areas for non-specialized predators, and more often in areas of high utilization such as near rookeries and haul-outs for predators that are specialized on SSLs and focus hunting efforts on high use areas. Results suggest the opposite, with four of the 15 predation events occurred in the 91-100% UD, or areas of the lowest relative frequency distribution of animal locations, 3 events occurred in the 81-90% UD, and only 1 event in the 21-30% UD corresponding to an area of higher relative frequency distribution of animal locations. Seven of the 15 predation events occurred outside the UD range, and therefore in areas likely of the lowest relative frequency distribution of animal locations. The winter predation rate was twice that of summer, with 10 predation events during the winter periods (November-April), and 5 events during the summer periods (May-October). Six predation events occurred in January-February. One predation event occurred in March-April, 2 events in May-June, 1 event in July-August, 2 events in September-October, and 3 events in November-December. A Rayleigh test for circular distribution showed a significant difference from uniformity with a mean ordinal date of January 6th (for n=7 animals ≤ 22 months of age), p=0.016). Studying predation in the marine environment comes with many challenges due to limitations in locating and tracking highly mobile marine predators over a long period of time and over a wide geographic range. Previous Steller sea lion studies looking at predation have inherent biases due to only sampling during the summer months and only near rookeries and haul-outs. Our study shows that we can detect predation all year round, including winter, and far away from rookeries and haul-outs. Since juvenile survival is low, and thus poses a significant risk for population-level survival, increased knowledge of the specifics of juvenile SSL space utilization patterns is of vital importance to assessing the importance of predation risk as one of the main drivers of their survivability. This is the first study to relate actual predation events to the intensity of space use of juvenile SSLs in a declining population in the KF/PWS region of Alaska