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Article Excerpt
Introduction
The Gulf of Alaska seamounts are a group of undersea mountains of volcanic origin rising from the ocean floor at depths of 3,200--4,000 m to within 400--800 m of the surface (Fig. 1). They are located in the eastern and central Gulf of Alaska at distances of 270-465 km offshore. Half of the major seamounts in this group fail within the U.S. Exclusive Economic Zone and half are outside; all are separated from the continental slope by waters from 2,900 to about 5,000 m in depth.
[FIGURE 1 OMITTED]
NOAA's National Marine Fisheries Service (NMFS) first conducted exploratory fishing on nine Gulf of Alaska (GOA) seamounts in June and July 1979 (Hughes, 1981). Using trawls and traps, they found that sablefish, Anoplopoma fimbria, were the dominant finfish on each of the seamounts and that trap catch rates of sablefish were higher than those from NMFS survey sites off southeastern Alaska. There were more than twice as many males as females, and nearly all sablefish were ripe, spawning, or recently spent. However, only older and larger fish were caught on the seamounts, suggesting that the seamount populations are maintained by the migration of mature fish from the continental slope rather than by local recruitment.
Tagged sablefish released in the Bering Sea, Aleutian Islands (BSAI) region, and the western and central GOA have been recovered on GOA seamounts, verifying the occurrence of slope to seamount migration (Shaw and Parks, 1997). Of 99 tagged sablefish released on 5 GOA seamounts in 1979, 5 have been recovered on the seamount where they were tagged, and none have been recovered elsewhere.
NMFS revisited seven of the seamounts sampled in 1979 and one (Murray Seamount) that was not sampled in 1979. Sablefish were tagged and released on the seamounts during 1999-2002 to determine the extent, if any, of emigration from the seamounts back to the continental slope and movement between seamounts. A second objective was to gather biological information from the seamount sablefish populations.
Materials and Methods
Seamount sampling was conducted aboard chartered commercial fishing vessels during NMFS longline surveys in 1999-2002. These longline surveys, conducted annually, sample stations along the continental slope of the Bering Sea and Aleutian Islands (alternate years) and the GOA (every year) to determine the relative abundance and size composition of commercially important species. During the survey each year, several thousand sablefish are tagged and released to provide information on growth and movement rates. Three seamounts per year were sampled during the first week of July. Giacomini, Surveyor, and Pratt Seamounts were sampled in 1999. In 2000, Surveyor and Pratt were revisited and Welker Seamount was sampled for the first time. Surveyor was sampled for the third time in 2001, Welker for the second time, and Dickens Seamount for the first time. Patton, Murray, and Durgin Seamounts were sampled in 2002. A total of 12 samples were taken at 8 seamounts from 1999 to 2002 (Table 1).
Standard survey longline gear and techniques (Rutecki (1)) were used on the seamounts. At each seamount, 2 sets of 80 skates each with a total of 7,200 hooks were set and retrieved. Two exceptions to this were Giacomini Seamount in 1999 and Dickens Seamount in 2001, where only 120 skates were set. Some skates were also lost on Dickens Seamount due to entanglement in rough substrate. Catch data were recorded on a handheld electronic data logger during gear retrieval and subsequently downloaded to a computer. Length frequency and sex data were collected with a bar-code-based measuring board and a bar-code reader/data storage device (Sigler, 1994). Usually, all sablefish that were not either tagged or sampled for otoliths were measured and sexed. Sablefish lengths were combined by year to ensure adequate sample sizes.
Starting with the fifth skate of each set, the first three sablefish of every tenth skate were set aside for age and condition sampling. These fish were weighed and measured, their stage of sexual maturity noted, and their otoliths collected and stored in a 50% ethyl alcohol solution for later ageing in the laboratory. Otoliths were read by experienced readers of the Age and Growth Program at the NMFS Alaska Fisheries Science Center (AFSC) in Seattle, Wash., using the ageing methods of Beamish and Chilton (1982). A total of 440 sablefish were aged. The first 150 sablefish of each set, except for fish set aside for otolith extraction or fish that were damaged during retrieval, were tagged with anchor tags, measured for length, and released. Release and recovery data discussed here were obtained from the NMFS Sablefish Tag Database, described in detail by Fujioka et al. (1988).
Results
Catch
Sablefish were the most numerous finfish on 3 of the 12 seamount visits between 1999 and 2002 (Welker Seamount in 2000 and 2001, and Dickens Seamount in 2001) (Table 2). Giant grenadier, Albatrossia pectoralis, dominated catches on the other nine visits. Pacific grenadier, Coryphaenoides acrolepis, were present in substantial numbers on several seamounts and replaced sablefish as the second-most common species on Murray Seamount in 2002. A list of scientific and common names of fish and crustaceans caught on the seamounts during 1999-2002 may be found in Table 3. Catch per unit of effort (CPUE), calculated as number of sablefish per 100 hooks, is shown for each seamount and year in Table 4.
CPUE varied greatly between seamounts, ranging from highs of 30.0 and...
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