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Juvenile Steller Sea Lion Studies

Figure 1, see caption
Figure 1.  Locations of Steller sea lion research activities undertaken by NMML during the November 2003 juvenile capture cruise aboard the M/V Tiglax.

Researchers from the Alaska Ecosystem Program of the National Marine Mammal Laboratory (NMML) conducted a cruise throughout the eastern Aleutian Islands and western Gulf of Alaska during 2-19 November 2003 to capture endangered juvenile Steller sea lions. Data obtained from this cruise contributes to our continuing effort to understand the foraging ecology and health status of sea lions as they become independent of maternal input and in relation to seasonal changes in oceanic conditions and prey abundance. Ultimately, this will allow testing of hypotheses for mechanisms underlying population declines and the efficacy of fisheries management actions.

Operating off the Alaska Maritime National Wildlife Refuge research vessel Tiglax, NMML researchers captured 13 sea lions on land with hoop nets or in water using scuba divers at eight locations between Unalaska and Kodiak Islands (Fig. 1 above).

Captured sea lions were 5, 17, or 29 months old, covering ages likely representing a transition from maternal dependence to independent foraging. Samples obtained from captured sea lions will provide information on their health, body condition, weaning status, diet, and genetic background. In addition to the analyses conducted by NMML, samples were collected for collaborators at the Alaska Department of Fish and Game (ADF&G), Alaska Sea Life Center, and the University of California at Davis. Collaborators from the University of Washington outfitted 10 sea lions with satellite-relayed data loggers, which will provide movement and detailed dive data via satellite telemetry.

In addition to these captures, scats for food habits analysis were collected from three sites, and 24 sea lions marked as pups and 5 marked as juveniles from previous capture trips were observed. Data collected will be utilized in estimating juvenile survival rates and to infer long-term movements. Ages of resighted sea lions ranged from 5 months to 3 years old. There were few incidental observations of other marine mammal species and no observations of killer whales. The only whales observed were humpback whales, mostly off the southeastern side of Kodiak Island. In this area we conducted a cold water test of a new hydrophone for the Pacific Marine Environmental Laboratory and Northwest Fisheries Science Center (NWFSC) that will eventually be used for recording geological and biological sounds in the North Pacific Ocean.

Program staff conducted a preliminary study into the feasibility of using multibeam sonar to study Steller sea lion foraging behavior. The work was conducted 13-20 November 2003 in Prince William Sound. Participants included scientists from Canadian Department of Fisheries and Oceans, University of Alaska Anchorage, U.S. Geological Survey, Anchorage, ADF&G, and Captain Mark Hottman and crew of the ADF&G research vessel Solstice. In the past two decades, technical innovations have played an essential role in the study of free-ranging pinnipeds and cetaceans in marine ecosystems. In particular, the study of foraging ecology has increasingly relied on time-depth recorders (TDRs), satellite dive recorders (SDRs), camera recorders, stomach temperature sensors, and similar innovations to track animals and to infer what they are doing while at sea. All of these technologies fall short of the ideal of placing marine mammal foraging behavior in its surrounding prey distribution.

One technology that offers some promise of observing both the predator’s behavior and the surrounding prey is multibeam imaging sonar, which has been used to document the interactions of killer whales and puffins with schooling herring. Our purpose was to test multibeam sonar as a possible tool for studying the foraging behavior and trophic interactions of Steller sea lions and their fish prey. In addition to the high likelihood that sea lions would be visible in the Simrad SM-2000 multibeam, this unit has also been calibrated for quantitative fisheries work and is scheduled to become a part of the inventory of acoustic instruments used for fisheries assessment in the AFSC.

Our objectives were

  1. To determine whether Steller sea lions would present an easily identifiable sonar signature that could be viewed, recorded, and followed against various backgrounds in a 200-kHz multibeam sonar.
  2. To determine the feasibility of actively tracking Steller sea lions (and possibly other marine predators) during foraging dives with multibeam sonar.
  3. To determine if both sea lions and fish prey could be simultaneously observed with multibeam sonar during these foraging dives.

We used a Simrad SM-2000 multibeam sonar mounted on the side of a 25-ft whaler. The direction of the ensonified plane was controlled manually by an operator outside of the cabin. Most of the trip was beset by strong, freezing winds and spray that kept small boat operations to near shore. Only a few useful images of sea lions diving near the haulouts were obtained until the final day. On 20 November the wind dropped, allowing us to spend 8 hours searhing for VHF-equipped target animals (none were found) and surveying for fish concentrations in the vicinity of the Dutch Group (60.77N, 147.82W) where satellite telemetry indicated recent sea lion foraging. We recorded dives of chance-encountered Steller sea lions into fish schools at the bottom in 70-120 m of water. The sea lions were clearly visible until entering the schools, where they were lost against the strong reflectance of the fish and rocky bottom. The technology clearly has potential in the study of marine mammal foraging behavior, but some critical technological development is needed for reliable tracking underwater. In particular, acoustic transponders would enable a target animal to be identified by sonar in all backgrounds, and robust sonar controls (pan and tilt) for tracking a target animal would enable the recording of complete dives and bouts of dives.

By Brian Fadely and Ward Testa.


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