Population declines of Steller sea lions (Eumetopias juhatus) in western Alaska (west of 144°W) may be a result of reduced juvenile survival. We used satellite telemetry to study the at‐sea distribution and movement patterns of pup (1.6–11.9 mo) and juvenile (12.0–35.1 mo) Steller sea lions. We studied trip distance, duration, and interhaul‐out movements of sea lions in relation to age, sex, and month of year in the decreasing western population (WP; Prince William Sound, Kodiak, Aleutian Islands, Alaska) and the increasing eastern population (EP; Southeast Alaska). We deployed 103 satellite transmitters (29 WP; 74 EP) on sea lions between 1998 and 2001. Round trip distance and duration increased with age, trip distance was greater in the WP than the EP, trip duration was greater for females than males, and haul‐out use was clustered. Changes in round trip distance and duration occurred from April to June for all age classes studied indicating that the annual timing of weaning may be less variable than the age of weaning. Overall, 90% of round trips were ≤ 15 km from haul‐outs and 84% were <20 h, indicating nearshore areas adjacent to haulouts are critical to the developing juvenile.

We hypothesized that: (1) Steller sea lion Eumetopias jubatus diet choice is a function of prey availability, (2) sea lions move to take advantage of times and locations of seasonal prey concentrations and (3) the number present depends on the amount of prey available (numerical response). Over 3 yr, typically on a quarterly basis, in Frederick Sound, SE Alaska, multiple measurements were taken of Steller sea lion abundance (aerial surveys), diet (scats), dive behavior (satellite telemetry) and prey availability and caloric density (nearshore, pelagic and demersal fish surveys). We found that Steller sea lions shifted diet composition in response to changes in prey availability of pollock Theragra chalcogramma, hake Merluccius productus, herring Clupea pallasi and salmon Oncorhynchus spp. They selected intermediate-sized fish and avoided small (<10 cm) and large (> 60 cm) fish, and moved between areas as prey became available seasonally. The number of sea lions present depended on the amount of prey available; a standing biomass of 500 to 1700 t of prey in a nonbreeding area such as Frederick Sound, depending on species composition, can attract and sustain about 500 sea lions. Pollock was more frequent in sea lion diet in inside waters of SE Alaska -including Frederick Sound, Stephens Passage and Lynn Canal -than anywhere else in Alaska and contributed ~1⁄ 3 of the dietary energy in Frederick Sound. This finding implies that a diet with substantial year-round contributions from less nutritious, but abundant prey such as pollock can form part of a healthy diet as long as more nutritious prey such as herring, salmon or eulachon Thaleichthys pacificus also are consumed. Our study supports the conclusion that the Steller sea lion is an opportunistic marine predator with a flexible foraging strategy that selects abundant, accessible prey and shifts among seasonally available species.

Development of competent diving ability is critical to obtaining nutritional independence in marine mammals such as Steller sea lions (SSLs), Eumetopias jubatus (Schreber, 1776). We studied diving performance in pup (75) and juvenile (36) SSLs using satellite data recorders. In general, dives by SSLs were brief and shallow. Overall, 82.3% of dives were <2 min long and 86.9% of dives were <10 m deep. Long (>5 min) and deep dives (>100 m) constituted only 2.49% and 0.77%, respectively, of total dives. We used linear mixed-effects models to investigate the relationships between the response variables maximum-daily-depth, time-at-depth, mean-dive-duration, dive rate, and time-at-sea and the predictor variables age, sex, population (eastern and western Alaska populations), time-of-day, and month-of-year. All response variables except dive rate were positively related (P < 0.05) to age. Dive rate declined (P < 0.001) with age. Time-of-day, month, population, sex, and some first-order interactions were all significantly (P < 0.05) related to some measure of diving performance. With large samples we were able to identify significant relationships between the response variables and the predictor variables, even though the total amount of variation explained by the models was low, because most dives were short and shallow regardless of age, sex, population, time-of-day, or month-of-year. Depths and durations of dives by juvenile animals increased throughout the range of ages studied and were similar to or greater than those previously reported for juveniles and adult females. We expect maximum depths and durations to continue to increase with age until body mass plateaus at about 10 years of age. Therefore, we expect older animals to be more efficient foragers, as they would have greater aerobic dive limits as well as more experience locating and capturing prey.

Climate change-driven alterations in Arctic environments can influence habitat availability, species distributions and interactions, and the breeding, foraging, and health of marine mammals. Phocine distemper virus (PDV), which has caused extensive mortality in Atlantic seals, was confirmed in sea otters in the North Pacific Ocean in 2004, raising the question of whether reductions in sea ice could increase contact between Arctic and sub-Arctic marine mammals and lead to viral transmission across the Arctic Ocean. Using data on PDV exposure and infection and animal movement in sympatric seal, sea lion, and sea otter species sampled in the North Pacific Ocean from 2001–2016, we investigated the timing of PDV introduction, risk factors associated with PDV emergence, and patterns of transmission following introduction. We identified widespread exposure to and infection with PDV across the North Pacific Ocean beginning in 2003 with a second peak of PDV exposure and infection in 2009; viral transmission across sympatric marine mammal species; and association of PDV exposure and infection with reductions in Arctic sea ice extent. Peaks of PDV exposure and infection following 2003 may reflect additional viral introductions among the diverse marine mammals in the North Pacific Ocean linked to change in Arctic sea ice extent.

During the 1990s, the Steller sea lion (Eumetopias jubatus Schreber) Western Alaska stock (WS) suffered steep population decline while the Eastern Alaska stock (ES) steadily increased. One bottom-up forcing hypothesis explaining this decline predicted lactating adult female foraging behavior would be different between stocks. To investigate this effect, we monitored 11 ES females at two breeding rookeries using satellite dive recorders (SDR) during the early breeding seasons of 1992-1993, examined their behavior with respect to prey, physiological limitations, and habitat, and made limited comparisons to observations of WS female behavior reported in the literature. ES females were not operating at the extremes of ability, with most diving within the limits of aerobic metabolism, less than one-quarter of possible foraging time during trips spent submerged and most foraging trips requiring less than one-half the lipid store fasting ability of dependent pups. Thus, females may have some capacity to alter behavior to accommodate future changes in foraging conditions, but the extent of this plasticity is unknown. Because recent work suggests WS recovery is impeded by low natality, future studies should test differences between reproductive and non-reproductive mature females in order to properly assess the contribution of foraging ecology to SSL population dynamics.