Age-Specific Movement Patterns of Sablefish in Alaska
December 15, 2004
Over 34,000 age 0–2 juvenile sablefish (Anoplopoma fimbria) were tagged and released in southeast Alaska waters during 1985–2005. The data set resulting from this tagging study was unusual because of its time span (20 years) and because age could be reliably inferred from release length (i.e., tagged and released fish were of known age); thus, age-specific movement patterns could be examined. The depth-and area-related recovery patterns supported the concepts that sablefish move to deeper water with age and migrate counterclockwise in the Gulf of Alaska. Availability to the fishery increased rapidly for fish of younger ages, peaked at age 5 to 6, and then gradually declined as sablefish moved deeper with age. Decreased availability with age may occur because of lower fishing effort in deep water and could have substantial implications for sablefish stock assessments because “dome-shaped” availability influences the reliability of abundance estimates. The area-related recovery pattern was not affected by year-class strength; i.e., there was no significant density-dependent relationship.
The general migration pattern of sablefish (Anoplopoma fimbria) in the northeast Pacific Ocean was deduced in the 1980s from several tagging studies (Bracken, 1983; Beamish and McFarlane, 1988; Fujioka et al., 1988;) and enlarged upon in further studies over the following two decades (Heifetz and Fujioka, 1991; Rutecki and Varosi, 1997; Kimura et al., 1998; Maloney, 2004). In southeast Alaska, juvenile sablefish that are spawned offshore appear in inshore waters in late summer or early fall and spend the first year or two of life in shallow coastal bays and inlets before moving into progressively deeper water. At the same time that they are moving into deeper water, many young sable-fish move north and west on a migration path that takes them across the Gulf of Alaska to the Aleutian Islands and Bering Sea. Eventually, most will return to the eastern Gulf of Alaska as adults.
The sablefish fishery in the Gulf of Alaska (GOA), eastern Bering Sea, and Aleutian Islands is managed by the National Marine Fisheries Service (NMFS) in cooperation with the North Pacific Fishery Management Council. Sablefish in these areas are assumed to belong to one population (Kimura et al., 1998), for which a total allowable catch is calculated each year and apportioned among six management areas. The annual quotas for each area are based on the distribution of biomass among the areas, which is estimated from longline surveys and commercial catches (Heifetz et al., 1997). Because sablefish are known to be migratory, estimates of the rates of migration between areas could affect the apportionment of quotas among management areas (Heifetz et al., 1997).
Migration rates between areas have been estimated from tag data by using fish-length classes in the modeling process (Heifetz and Fujioka, 1991). Although fish-length data are commonly available, actual age data are generally scarce. Age data are preferable to length data for estimating population age structure (Sigler, 1999), but sablefish are difficult to age, especially for ages greater than 5 or 6 years (Kimura and Lyons, 1991). Tagging of known-age juveniles before they leave coastal areas offers an opportunity to document age-specific movements. Age 0–2 (mostly age 1) sablefish have been tagged annually since 1985 in bays and inlets of southeast Alaska. The objective of our study was to determine movement patterns of sablefish based on these known-age fish, using a unique 20-year data set of age-specific mark-recapture data. Specifically, we determined 1) how the depth inhabited by sablefish changes with age; 2) how the area inhabited changes with age; 3) how availability to the primary fishery (longline) changes with age; and 4) whether there is a density-dependent effect of year-class strength on the extent of migration of young sable-fish. Results of objectives 1 and 2 largely confirmed the results of previous studies, whereas objectives 3 and 4 were new.