Summer Site Fidelity, Movement Patterns, and Habitat of Acoustic-Tagged Pacific Halibut in An Alaskan Fjord

Small-scale information on fish movement patterns is important for designing spatial management regimes such as marine protected areas. Pacific halibut are commercially, ecologically, and culturally valuable in the North Pacific Ocean. Although Pacific halibut are known to exhibit a general site fidelity to summer foraging locations, little is known about movement scales during summer or linkages between movement patterns and landscape features. To rectify this knowledge gap, an acoustic telemetry study was conducted in Glacier Bay National Park, Alaska during 1991–1993 to examine fine-scale movement patterns of Pacific halibut during summer. Forty-three tagged Pacific halibut (mean total length = 133 cm, range 68–220 cm) were tracked by vessel daily to weekly during tracking periods of 4–6 months each year. Movement patterns were classified based on the slope of dispersion trajectories (mean squared distance vs. time) as “dispersive” (slope > 0) or “localized” (slope = 0). Kernel density estimation methods were used to define “core areas” (50% kernel utilization distribution) for localized movements of individual animals; GIS was used to characterize the habitat (depth, slope, habitat complexity) in core areas. The localized movement pattern was observed most often (82%). Animals with localized movement patterns were observed to reside in specific locations an average of 61 days; during this time the average displacement from the initial location was 1028 m. Some animals (16%) exhibited both localized and dispersive movements, while others (13%) exhibited only dispersive movements. Dispersive movements occurred over an average time period of 18 days, during which the average displacement from the initial location was 8023 m. The median core area size was 61 ha (range 2 – 491 ha). Overall, core areas were most often associated with depths <100 m, moderate to steep slopes, and moderate habitat complexity. Small core areas were associated with mounds or slopes with moderate habitat complexity while large core areas were associated with extensive, flat terrain with low habitat complexity. Summer site fidelity is indicated by the dominance of the localized movement pattern, where tagged animals remain in specific locations for long periods of time and can return to these areas following occasional extended forays. Information on the scale of localized and dispersive movement patterns, in addition to habitat specific core area sizes, is important information for the design of marine protected areas for halibut in glacial fjord ecosystems.