ALASKA ECOSYSTEM RESEARCH PROGRAM:
Juvenile Steller Sea Lion Studies
![Figure 1, see caption](images/nmml_ssl_fig1.jpg)
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 predators
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
- 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.
- To determine the feasibility of actively tracking Steller sea lions (and
possibly other marine predators) during foraging dives with multibeam sonar.
- 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 (60.77°N,
147.82°W) 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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