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Satellite Tracking of Adult Female Steller Sea Lions in the Western-Central Aleutian Islands Reveals Diverse Foraging Behaviors

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Oct-Nov-Dec 2012
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Very little is understood about adult female Steller sea lion (Eumetopias jubatus) foraging behavior in the Aleutian Islands, particularly during winter for the western and central Aleutian Islands area. 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 gathering this information is especially crucial in the western and central Aleutian Islands where controversial large-scale commercial fisheries restrictions were enacted (see 2010 NOAA News Release). However, attaching satellite-telemetry instruments capable of recording diving behavior and tracking locations requires the safe capture and restraint of sea lions that may weigh more than 350 kilograms. Thus, to develop suitable techniques, a team from the National Marine Mammal Laboratory’s (NMML) Alaska Ecosystems Program (AEP), Alaska Department of Fish and Game Steller Sea Lion Program, and Vancouver Aquarium conducted a pilot project to test capture and handling methods in the more logistically tractable area of Southeast Alaska in November 2010. As reported in a previous AFSC Quarterly Report, October-December 2010, that effort resulted in three successful captures and the subsequent tracking of Steller sea lion movements for up to 253 days, longer than any previous satellite-telemetry instrument deployments on adult females. Based on that success, captures were next attempted in the western and central Aleutian Islands in November 2011, but the effort was confounded by weather and ocean conditions. The successful capture of one adult female (identified as “=24”) on Ulak Island in the central Aleutian Islands resulted in the subsequent tracking of her movements for a period of 175 days. During this time, “=24” spent most of her time foraging north of Semisopochnoi Island over the southern portion of Petrel Bank, where the potential prey field was described by a concurrent AFSC Fisheries Interaction Team Atka mackerel study (see AFSC Quarterly Report April-June 2012).

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Figure 1. GPS location data for five adult female Steller sea lions (Eumetopias jubatus) captured in the western and central Aleutian Islands during October 2012. Top left insert: study area; top right insert: movements of sea lion “=28” relative to ETOPO 2-ft gridded elevation data; bottom left insert: movements of sea lion “=27” relative to a mean composite of chlorophyll-a (mg/m3; Aqua MODIS, for the month of October. GPS data range through 2 January 2013. Click to enlarge.  

During October 2012, the Steller sea lion team returned to the western and central Aleutian Islands, where adult females with dependent pups were captured successfully at Attu, Agattu, and Alaid Islands in the western Aleutian Islands and at Amchitka and Ulak Islands in the central Aleutian Islands (Fig. 1). Satellite telemetry revealed these animals displayed an intriguing variety of foraging strategies within and among individuals. The sea lion captured at Cape Wrangell, Attu Island (identified as “=25” in Fig. 1), traveled among haulouts across the northern side of the island, and all of her foraging trips have been nearshore. In contrast, sea lion “=26,” captured at Alaid Island, moved around the Semichi Islands before making trips up to 48 km away to the Ingenstrem Rock area, where transmissions stopped after 19 days. Sea lion “=27,” captured at Cape Sabak on Agattu Island, has traveled among the Near Islands but mostly undertakes pelagic foraging trips lasting as long as 6 days and extending as far as 420 km south of Agattu Island. While at sea, this sea lion appeared to target areas associated with a mesoscale anti-cyclonic eddy south of the Near Islands (Fig. 1, inset). The last adult female captured in the western Aleutian Island area, “=28” captured on Amchitka Island (Fig. 2), spent 3 weeks post-release in areas within 31 km of the East Cape capture site but then made a 2-day trip off the western side of Semisopochnoi Island and, after returning to Amchitka, another 2-day trip south of Amchitka Pass. After a couple more trips to Semisopochnoi Island and Amchitka Pass, sea lion “=28” returned to Semisopochnoi Island, where she made trips over the southern portion of Petrel Bank, displaying movements similar to those of “=24” from the previous year, until transmissions ceased on 10 December. The adult female “=29,” captured on Ulak Island in the central Aleutian Islands, spent most of her time among the Delarof Islands (Ulak, Amatignak, and Unalga Islands) undertaking nearshore foraging trips until late December and, as of this writing, is taking long, looping trips south of Amchitka Pass, encircling the area previously visited by sea lion “=28” in November.

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Figure 2. Adult female Steller sea lion “=28” returning to the sea upon release after being captured on Amchitka Island, 25 October 2012. In view on the top of her head is the satellite-telemetry instrument, attached to track movements and record dive behavior. Photo by Dr. Vladimir Burkanov (NMFS MMPA research permit 14326).

Findings from tracking these six adult female Steller sea lions in the western and central Aleutian Islands will greatly improve our understanding of their foraging behavior and at-sea habitat use. Formal analysis of these data is forthcoming, but it is clear that adult female Steller sea lions in the western Aleutian Islands utilize pelagic (offshore) and benthic (on-shelf areas) habitats with foraging strategies that may key on definable habitat features both offshore (mesoscale eddies) and over shelf areas (e.g., Petrel Bank near Semisopochnoi Island). To date, none of these tracked adult females have relocated to regions away from their general area of capture.

By Brian Fadely and Michelle Lander


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