Pacific Halibut in Glacier Bay National Park, Alaska
Philip N. Hooge
Spencer J. Taggart
Glacier Bay National Park is the site of extensive and controversial
commercial fisheries which began before the Park was established in 1925.
These fisheries  continue
despite regulations prohibiting them, including the Wilderness Act and
National Park Service (NPS) regulations. Today, more than 70 commercial
boats (1991 and 1992 data) harvest between 300,000 and 500,000 lb. of
Pacific halibut (Hippoglossus stenolepis) per year within the Park.
Commercial halibut fishing in Alaska began as a fishery open to all
individuals and was managed by controlling the duration of the fishery.
The duration of the openings has gradually been shortened until in 1994
the openings were only a 1-2 day "derby." The large numbers of
vessels fishing the Park resulted in an additional conflict for the NPS,
which severely limits the numbers of other types of boats permitted to
enter the Bay in order to reduce impacts on humpback whales and other Park
resources. In 1995, an Individual Fishing Quota (IFQ) system was initiated
to replace the derby-style fishery. The IFQ system was predicted to result
in greater local resource use by fishing vessels, and preliminary data
bear out this prediction. Because Glacier Bay is in close proximity to
many fishing communities and the waters are relatively protected, the Bay
may experience increases in fishing activity and conflicts between
visitors and fishing vessels. In addition, fishing can now occur
throughout the summer, which is also the peak of both whale abundance and
visitor attendance.
The International Pacific Halibut Commission (IPHC), an international
research and regulatory agency, is responsible for managing the fishery
for maximum sustainable yield. The majority of studies conducted by the
IPHC have been directed toward this effort. In contrast, the NPS is
directed to manage its resources in such a manner as to maintain their
natural state and to provide for visitor enjoyment. Given the potential
resource conflicts and the differing management guidelines of the NPS and
the IPHC, Glacier Bay Field Station's research efforts have been directed
at such basic ecological questions as diet, home range, site fidelity,
habitat selection, distribution patterns, and the relationships between
halibut and other species.
The Pacific halibut is a large (up to 3 m) predatory fish in the
flatfish family (Pleuronectidae). We examined the stomach contents of 947
sport-caught halibut. Analysis of these contents revealed an ontogenetic
shift in diet from small crustaceans to fish as halibut mature. There is
also a dichotomy in the co-occurrence of stomach content items;
individuals often either exhibit only items that can be found during
active foraging (e.g. large numbers of juvenile crabs) or only items that
are associated with sit-and-wait predation (e.g. walleye pollock).
Photo (1) Sonic-tags being internally implanted in a
Pacific halibut.
 Long
distance movements of Pacific halibut have been emphasized in previous
studies (Skud 1977; St-Pierre 1984) and most
population models developed for this species assume relatively
unrestricted movements between areas (Quinn et al. 1985).
Long-life sonic tags were internally implanted in 97 halibut in Glacier
Bay and more than 1,500 were individually wire tagged. Results from these
two studies indicate an ontogenetic shift in home range patterns; juvenile
fish move widely but often still within the Glacier Bay area, whereas
large sexually mature individuals exhibit much smaller home ranges. Many
of these mature individuals have home ranges, which are often less than
0.5 km2, and exhibit both within-year and between-year site fidelity.
These home ranges often have little simultaneous spatial overlap. Larger
individuals have been observed occasionally to alter their pattern of
small home ranges and to travel more widely before returning to a
relatively sedentary pattern; a few larger individuals appear never to
establish home ranges.
Wire-tagging data provide a separate corroborating line of evidence for
site fidelity of adult Pacific halibut. Of the halibut originally
wire-tagged in Glacier Bay and then recaptured at a later time, more than
95% were recaptured within Glacier Bay with an additional 3% caught in the
adjoining Icy Strait area. Individuals have been recaptured five years
after tagging within a few hundred meters of their original capture and
release location.
Sonic-tracking data also indicate that some individuals leave during
the winter, but many appear to remain within the Bay. Spawning of halibut
has been hypothesized to occur only at spawning areas off the outer coast
during the winter (Skud 1977; St-Pierre 1984). The
presence of reproductively mature individuals within the Bay during this
time may indicate either that Pacific halibut do not spawn every year or
that spawning can occur within the Bay.
Photo (2): Research long-lines being retrieved in front
of the Grand Pacific tide-water glacier.
We have sampled halibut by setting 149 research long-lines with 400
hooks each throughout Glacier Bay. Results of these studies and from
sonic-tracking individuals suggest that there are two broad patterns of
habitat choice and dispersion. The first pattern appears to be
ontogenetic, with larger individuals exhibiting a relatively uniform
distribution pattern and a preference for deeper water; smaller
individuals seem to prefer shallow water and areas of steep topographical
relief in a much more aggregated distribution pattern. The second observed
pattern reflects changes in distribution along the length of Glacier Bay's
recently deglaciated fjord system. This pattern is characterized by
decreased halibut abundance with greater proximity to glacial termini.
Initial indications from sampling salinity, temperature, and the amount of
silt and phytoplankton (indicated by chlorophyll a) in the water are that
this pattern is due to oceanographic conditions rather than successional
processes.
Comparisons between long-lining data and sonic-tag data indicate that
many larger individuals are not caught by long-lines and are found in
areas in which catch per unit effort (CPUE) with long-lines is low.
As a result of our studies, we have hypothesized that halibut at
Glacier Bay exhibit two foraging modes that lie at two ends of an
ontogenetic trajectory and which underlie movement patterns, distribution
and catchability. These dichotomous modes are active foraging and
sit-and-wait predation. Juveniles probably establish wide-ranging movement
patterns, actively searching for areas of high prey abundance where they
are easily caught on long-lines. Many adults probably establish small
non-overlapping home ranges where they wait for large fish or
invertebrates; rarely does a long-line come within enough proximity for
these sit-and-wait individuals to be captured.
The focus of our current research is to continue our studies on
long-term site fidelity, to test the hypotheses on foraging modes, and to
understand the distribution and abundance of halibut predators and prey.
If in the future regulatory fishing closures occur, we also hope to test
experimentally for local depletion or broader scale ecological effects in
fished versus unfished areas.
The studies at Glacier Bay indicate that halibut have much smaller home
ranges and greater site fidelity than previously thought. These facts,
coupled with the potential increase in local commercial fishing due to the
IFQ system and a rise in halibut sport fishing in Southeast Alaska,
indicate a potential for local resource depletion. On the other hand, our
hypotheses about differing foraging patterns and catchability suggest that
there may be a behavioral refugium from commercial long-line fishing (but
not from trawling with nets) in Pacific halibut. This behavioral refugium
could act to buffer the population from the collapses experienced by other
fisheries that have been managed based on maximum sustainable yield models
and CPUE (Ludwig et al. 1993; Rosenberg et al. 1993).
Ludwig, D., Hilborn, R. and Walters, C. 1993. Uncertainty, resource
exploitation, and conservation: lessons from history. Science 260: 17-36.
Quinn, T. J., Deriso, R. B. and Hoag, S. H. 1985. Methods of population
assessment of Pacific halibut. International Pacific Halibut Commission
Science Report 72.
Rosenberg, A. A. et al. 1993. Achieving sustainable use of renewable
resources. Science 262: 828-829.
Skud, B. E. 1977. Drift migration and intermingling of pacific halibut
stocks. International Pacific Halibut Commission Science Report 63.
St-Pierre, G. 1984. Spawning locations and season for Pacific halibut.
International Pacific Halibut Commission Science Report 70. |
