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Groundfish Assessment Program

Sablefish Maturity Cruise Successfully Completed

  see caption
  Figure 1. The fishing and scientific crew of the sablefish maturity cruise conducted aboard the F/V Gold Rush.

AFSC biologists Jim Stark and Katy Echave completed a 12-day cruise in December off Kodiak Island, Alaska, aboard the chartered commercial fishing trawler Gold Rush as part of a special study to obtain biological specimens to improve knowledge about maturation of sablefish, Anoplopoma fimbria, in Alaska waters. The study, a joint effort between the AFSC's Auke Bay Laboratories and RACE Division, is designed to provide updated and more accurate estimates of sablefish age and length at maturity, which is critical to the stock management models used to set the overfishing limit and maximum sustainable yield. Sablefish (also known as black cod) is a deepwater groundfish common in the Gulf of Alaska which brings more than $100 million dollars per year to the commercial fleet.

In order to maintain sustainable fishery quotas, fishery managers need reliable estimates of the stock's female spawning biomass, an estimate that depends upon the age and length-at-50% maturity of females. For any species, maturity must be assessed from specimens just about to spawn. Available maturity estimates made nearly 25 years ago were based on crude maturity classifications collected from fish during the summer months when maturity is difficult to assess. The prespawning period for sablefish is December. Key information about the maturity of sablefish also has been difficult to obtain because adult sablefish inhabit the deep waters of the continental slope and shelf gullies at depths from 200 to 1,000 m.

With Stark as chief scientist of the cruise, the crew of the Gold Rush conducted 41 successful bottom trawls (Fig. 1). Working in the Pacific waters off of Kodiak is challenging, and the stormy seas in December made narrow windows for successful fishing. Trawling was conducted in outer Chiniak Bay, Chiniak Gully, Marmot Bay, Marmot Gully, Portlock Flats, and Amatuli Trough south of Kodiak. Trawling also occurred in the northern Kodiak areas of Shelikof Strait, Uyak Bay, Uganik Bay and Viekoda Bay. The crew took every advantage to safely fish during good weather day and night and to navigate the vessel between the fishing grounds and port.

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  Figure 2. NOAA scientists will examine female sablefish ovaries like this one to gather data on maturity levels and number of eggs each female produces. Research was conducted for the first time in winter, when determining maturity is straight forward since females have developed eggs in time for the annual spawning cycle.

Ovaries and otoliths were collected from 385 female sablefish ranging in size from 37 to 88 cm. Information on length, weight, the appearance of the ovary, and associated ocean conditions were collected as well. The ovaries will be examined to determine fecundity and egg size (Fig. 2). Satellite tags were successfully attached to five large sablefish as part of a tagging study to monitor sablefish movements during the spawning season. The tags are set to release from the fish in mid January and early February and upon surfacing will transmit data on location, depth, temperature and acceleration.

In order to accurately determine the maturity, egg sizes, and fecundities of each specimen the ovary samples will be processed for microscopic histological examination. The development and maturity of each specimen will be classified using the slides of ovary tissue and a compound microscope. Overall sablefish maturity will be estimated as a function of total body length and age with a statistical model to estimate the age (A50) and length at 50% maturity (L50). Environmental data collected during each haul also will be evaluated to determine if there are any significant correlations between sablefish maturity and depth, seawater temperature, and ambient light intensity. The number of eggs (fecundity) will be determined within each female categorized by size, assigned maturity classification, and age. Future work also will include developing a visual classification of maturity based upon photographs of ovaries correlated with the actual microscopic determination of maturity.

This project was designed to provide for improved sustainability in the fishery quotas which benefits consumers, fishermen, processors, U.S. balance of payments, shipyards and the fishing community. The sablefish population in Alaska is healthy and the fishing quota is managed conservatively. The new information on maturity and reproduction will fine-tune these estimates and help continue to protect the fishery and the renewable resource upon which it depends.

By Wayne Palsson and Jim Stark (RACE Division), and Katy Echave and Cara Rodgveller (ABL)


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