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NOAA Technical Memorandum NMFS-AFSC-321

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Aerial and ship-based surveys of Steller sea lions (Eumetopias jubatus) conducted in Alaska in June-July 2013 through 2015, and an update on the status and trend of the western distinct population segment in Alaska


Surveys conducted during June-July 2013 through 2015 provided strong evidence that both the western and eastern distinct population segments (DPSs) of Steller sea lion (Eumetopias jubatus) continued to increase in overall abundance in Alaska. Between 2003 and 2015, counts of western DPS non-pups (adults and juveniles) and pups in Alaska increased at average rates of 2.25% per year (95% credible interval of 1.16%-3.29% per year) and 2.26% per year (1.43%-3.03% per year), respectively. However, there was considerable regional variability in trends across the western DPS, with increases in each of the four NMFS survey regions east of Samalga Pass (~170° W in the Aleutian Islands; 3.80% per year for pups overall in the four regions) being offset somewhat by decreases in the two regions to the west (overall, -1.90% per year). Additionally, within the central Aleutian Islands, recent trends varied east and west of Tanaga Pass (~177°W): in two sub-regions to the east, trends were stable or weakly positive, while in three sub-regions to the west, trends were strongly negative. In southeast Alaska (eastern DPS of Steller sea lion), non-pup and pup counts increased at 2.29% per year (1.59%-2.97% per year) and 3.26% per year (2.66%-3.89% per year), respectively, between 1985 and 2015.

The distribution of marked animals during the breeding season (along with other demographic data) indicates that there is a small net annual inter-DPS movement from southeast Alaska (eastern DPS) to the western DPS (~80 sea lions total; ~30 to the central Gulf of Alaska and ~50 to the eastern Gulf of Alaska), but a much larger net inter-regional movement within the western DPS from the central to the eastern Gulf of Alaska (~1,200 sea lions). The estimated inter-regional movement within the western DPS represents between 11% and 13% of the non-pups counted in the two regions in 2015 (after accounting for sightability) and likely inflated the non-pup trend in the eastern and depressed it in the central Gulf of Alaska. Therefore, it may be more appropriate to estimate non-pup trends in a single eastern/central Gulf of Alaska region. Average net inter-DPS movement represents a very small fraction (≤ 0.3%) of the total 2015 survey non-pup count in the combined eastern and central Gulf of Alaska, southeast Alaska, or the western DPS overall, and thus, had only a small influence on trends in these regions.

The recovery plan for the western DPS of Steller sea lion (NMFS 2008) refers to a start year of 2000 for the 15-year and 30-year periods used in the down-listing (to threatened status) and de-listing (to recovered status) criteria, respectively, since this was when the lowest survey non-pup count was obtained and when recovery of the overall stock was thought to have begun. However, modeling of all observed (survey) counts (including for the first time data collected at terrestrial sites in the eastern Bering Sea) indicates that the western DPS in Alaska likely had its lowest abundance in 2003, and not in 2000. If observed non-pup counts continue to increase in the western DPS (in both Alaska and Russia) through 2018, then the DPS-wide and the first of two regional demographic criteria for down-listing could be satisfied. However, persistent decreasing trends in the western Aleutians and the western half of the central Aleutian Islands may preclude it from satisfying the second regional demographic down-listing criterion. Furthermore, satisfying the demographic de-listing criteria by 2033 appears to be unlikely given the magnitude of the recent decline in the western Aleutians (-61% for non-pups between 2003 and 2015).

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