National Marine Mammal Laboratory (NMML)
Alaska Ecosystems Program
First Winter Capture and Tracking of Adult Female Steller Sea Lions in Southeast Alaska
In November 2010, three adult female Steller sea lions (Eumetopias jubatus) were successfully captured and handled in Southeast Alaska, the first time adult female Steller sea lions have been captured, tagged, and subsequently tracked using satellite telemetry in Southeast Alaska in the winter. This successful project resulted from a collaboration of researchers—from the Alaska Ecosystems Program (AEP), Alaska Department of Fish and Game, and Vancouver Aquarium—who developed techniques that will be used in new studies to address critical Steller sea lion and commercial fishery management research needs in the western and central Aleutian Islands.
Figure 1. The locations of November 2010 captures of three adult female Steller sea lions (Eumetopias jubatus) and their subsequent at-sea GPS locations through early January 2011.
Understanding where marine mammals forage to obtain energy needed for growth and reproduction is necessary to evaluate the potential for competition with other predators (including humans) for resources and is a fundamental requirement for informed ecosystem-based fisheries management. In the Aleutian Islands where Steller sea lions are listed as an endangered species, very little is understood of adult female sea lion foraging behavior outside of the June-July breeding season when most foraging trips occur close to rookeries. Where adult females access prey resources to maintain pregnancy during winter is one of the fundamental questions central to understanding whether commercial groundfish fisheries have indirect impacts on those sea lion populations. However, the minimal information on adult female foraging behavior in winter comes from only six adult females captured in the eastern Aleutian Islands through the eastern Gulf of Alaska in 1990-96.
Because of their size and behavior, the method of choice for capturing adult female Steller sea lions has been through the use of sedatives delivered with darts fired from rifle-like dart-projectors. In Alaska, limited numbers of adult females were captured using this method during the 1990s; but because their unique otariid physiology limited sedative drug options, and their haul-out location on rocky sites in close proximity to water encouraged water entry prior to sedation, these captures were associated with a high mortality risk. As research attention became focused on understanding juvenile sea lion foraging ecology and health status, underwater capture techniques perfected during the 2000s greatly increased the number of juvenile sea lions that could be safely captured and handled, but this technique was only successful with animals less than 4 years old. For our study, we built on the recent success of a California sea lion study—by researchers from the NMML California Current Ecosystems Program, Vancouver Aquarium, and Marine Mammal Center—that utilized a novel combination of sedatives. In contrast to previously used sedatives, this new combination appears to be well tolerated by sea lions and is quickly reversible through the administration of reversal drugs.
The objective of our study was, thus, to perfect techniques to allow safe capture and handling of adult female Steller sea lions and to apply state-of-the-art tracking instrumentation and sampling techniques to assess foraging behavior and health status and condition. Our team of nine researchers (including two veterinarians) used the chartered vessel Norseman to attempt captures at three locations: Benjamin Island in Lynn Canal and Southwest Brothers Island and Sail Island in Frederick Sound (Fig. 1). Of six adult females darted, three were captured and handled. Two were sedated within 10 minutes on land at the point of capture, and a third entered the water but returned to shore prior to sedation. All three sedated animals were transferred to inhalable anesthesia, and multiple samples and measurements were collected to assess their health status and condition. Each was outfitted with a Mk10-AF fast-location GPS/Argos-location and dive-recording transmitter manufactured by Wildlife Computers (Redmond, WA). All three rapidly and fully recovered from sedation after injection with reversal agents. Three other sea lions were darted but moved into the water within minutes. In a distinct difference from studies in the 1990s, however, we were able to locate, follow, and confirm survival of all three animals from a skiff. Two animals were followed for over an hour and were observed to fully recover from the sedation effects. The third was followed, given reversal agents via dart injection while in the water, and then rapidly swam away from the skiff; this animal was subsequently observed hauling out on land fully recovered.
Because no adult female Steller sea lions have previously been tracked in Southeast Alaska during winter, these data will provide new insights into how mothers with dependent pups navigate and forage in these waters. Locations determined from GPS positions transmitted via Argos over the first 60 days of deployment show distinctly different behaviors associated with habitat use in Southeast Alaska among the three sea lions (Fig. 1). In contrast to data collected in summer, when adult females are constrained to provisioning newborn pups and typically undertake foraging trips within 20 nautical miles (nmi) of rookeries, data from these three females show that sea lions travel much greater distances and utilize much more habitat in winter. Sea lion 61079 captured at Benjamin Island has spent most of her time within Favorite Channel, with some forays into Stephens Passage. The two sea lions captured at haulouts less than 15 km apart in Frederick Sound displayed contrasting movements. Sea lion 61083 from Sail Island spends most of her time in southern Stephens Passage and Port Houghton but has also made two trips down Frederick Sound to the Petersburg area and spent some time in Wrangell Narrows. In contrast, sea lion 61111 captured at Southwest Brothers Island has mostly moved between that location and various other haulouts in Frederick Sound near Kupreanof Island. But this sea lion also undertook a longer journey north through Chatham Strait, through Peril Strait to the outer coast, and north to White Sisters (a summertime rookery) before heading south around the southern tip of Baranof Island and back into Frederick Sound.
This pilot study developed methods that will be applied to our planned captures of adult females in the western Aleutian Islands later in 2011. Compared to methods used in studies of adult female Steller sea lions in the 1990s, the new sedative combination used in this study improved capture efficiency and animal safety. Data on adult female foraging behavior in Southeast Alaska and the information on health status and condition obtained in this study will be useful contrasts to data collected from sea lions captured in the western Aleutian Islands.
By Brian Fadely