Pacific Halibut in Glacier Bay National Park, Alaska

The Pacific halibut is a large (up to 3 meters long) predatory fish in the flatfish family. Glacier Bay National Park is the site of extensive and controversial commercial halibut fisheries that began before the park was established in 1925. These fisheries continue despite prohibitive regulations, including the Wilderness Act and National Park Service regulations. Today, more than 70 commercial boats (1991 and 1992 data) harvest between 136,200 and 181,600 kilograms of Pacific halibut per year within the park.

Commercial halibut fishing in Alaska began as a fishery open to all people and was managed by controlling its duration. The duration of the open fishing season has gradually been shortened, until in 1994 the openings were only a one- to two-day "derby." The large numbers of vessels fishing the waters of Glacier Bay National Park resulted in an additional conflict for the National Park Service, which severely limits the numbers of boats other than commercial fishing vessels permitted to enter Glacier Bay in order to reduce negative impacts on humpback whales and other park resources. In 1995 an individual fishing quota system replaced the derby-style fishery. This system was predicted to result in greater local resource use by fishing vessels, and preliminary data bear out this prediction. Because Glacier Bay is near many fishing communities and its waters are relatively protected, this new quota system may cause the bay to experience increases in fishing activity and more conflicts between visitors and fishing vessels. In addition, unlike the derby-style fishery, the individual fishing quota system permits fishing throughout the summer, which is when both whale abundance and visitor attendance also peak. The majority of previous studies on Pacific halibut have been directed toward maintaining maximum sustainable yield. In contrast, the National Park Service is directed to manage its resources in such a manner as to maintain their natural state and to provide for visitor enjoyment, which is why 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.

To analyze the Pacific halibut's diet, we examined the stomach contents of 947 sport-caught fishes. Content analysis revealed a shift in diet as halibut mature, from small crustaceans consumed by young halibut to fish eaten by large, mature halibut. Two foraging modes by halibuts were revealed by analysis of stomachs with multiple prey items. Individual halibut often exhibited only prey items found during active foraging (for example, large numbers of juvenile crabs) or else only prey items associated with sit-and-wait predation (such as walleye pollock).

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 that movement is relatively unrestricted among areas (Quinn et al. 1985). In our own studies of movement patterns, we internally implanted long-life sonic tags in 97 halibut in Glacier Bay and individually wire-tagged more than 1,500 halibut (Fig. 1). Results from the sonic- tagging study indicate that home range patterns shift during the course of halibut development. Juvenile fish move widely but often still remain within the Glacier Bay area, whereas large, sexually mature individuals have much smaller home ranges--often less than 0.5 square kilometers--and many mature fish stay in the same area both during the course of a year and from one year to another with little simultaneous spatial overlap (Fig. 2). Larger fish occasionally alter their pattern of small home ranges to travel more widely before returning to a relatively sedentary pattern. A few larger fish, though, appear never to establish home ranges.

Wire-tagging data corroborate the apparent site fidelity of adult Pacific halibut. Of the halibut originally wire-tagged in Glacier Bay and then recaptured later, more than 95% were recaptured within Glacier Bay, with an additional 3% caught in the adjoining Icy Strait area. Individuals have been recaptured 5 years after tagging within a few hundred meters of their original capture and release location.

Sonic-tracking data also indicate that although some individuals leave during the winter, many appear to remain within Glacier Bay. Researchers have hypothesized that halibut may spawn only in certain areas off the outer coast during the winter (Skud 1977; St-Pierre 1984). The presence of reproductively mature individuals within Glacier Bay during winter may indicate either that Pacific halibut do not spawn every year or that spawning can also occur within the bay.

To study the species' habitat selection and distribution patterns in Glacier Bay, we have sampled Pacific halibut by setting 149 research long-lines with 400 hooks each throughout the bay (Fig. 3). Long-lining is a method of fishing that uses a piece of ground line (often kilometers in length) to which short (0.5 to 1 meter long) gangion lines with baited hooks are attached at intervals of 1 to 10 meters. Results of these studies and those from sonic-tracked individuals suggest halibut engage in two broad patterns of habitat choice and dispersion. The first pattern seems to depend on the developmental stage of the fish, with larger individuals distributed relatively uniformly in deeper water and smaller individuals preferring shallow water and areas of steep topographic relief in a much more aggregated distribution pattern. The second observed pattern, which reflects changes in distribution along the length of Glacier Bay's recently deglaciated fjord system, is characterized by decreased halibut abundance with greater proximity to glacial termini. Sampling of water conditions--salinity, temperature, and the amount of silt and phytoplankton (indicated by chlorophyll a)--indicates that this pattern is due to oceanographic conditions rather than the habitat's physical or biological successional processes. The presence of tidewater glaciers dramatically affects oceanographic conditions, and Glacier Bay, with its many tidewater glaciers, is a profound example of this. Tremendous amounts of cold, freshwater flow and as much as 6 centimeters of siltation a day come down into the bay from the tidewater glaciers. Comparisons between long-lining data and sonic-tag data indicate that many larger individuals are not caught by long-lines.

As a result of investigations into Pacific halibut ecological relationships, we hypothesize that the way halibut in Glacier Bay hunt depends on their life stage, and that these foraging modes determine their movement patterns, distribution, and catchability. Juveniles probably range widely, actively searching for areas of high prey abundance, where they are easily caught on long-lines. Many adults probably establish small, discrete home ranges where they wait for large fish or invertebrates; rarely does a long-line come close enough for these individuals to be captured.

We plan to continue our studies on long-term site fidelity, to test the hypotheses on foraging modes, and to further our understanding of the distribution and abundance of halibut predators and prey. If future regulatory fishing closures occur, we also hope to use them as experiments to examine the effects of local depletion or broader-scale ecological factors in fished and unfished areas.

Our research results have management implications for the Pacific halibut. As stated, the studies at Glacier Bay indicate that halibut have much smaller home ranges and greater site fidelity than previously thought. These findings, coupled with the potential increase in local commercial fishing due to the individual fishing quota system and a rise in halibut sport fishing in southeast Alaska, indicate a potential for local resource depletion of this species. On the other hand, our hypotheses about differing foraging patterns and catch- ability in Pacific halibut suggest that this species' behaviors may help protect larger halibut from commercial long-line fishing but not from trawling with nets. This "behavioral refugium" could act to buffer the population from the collapses experienced by other fisheries that have been managed based on maximum sustain- able yield models and catch-per-unit effort (Ludwig et al. 1993; Rosenberg et al. 1993).