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Groundfish Assessment Program

Western Groundfish Conference

Kalei Shotwell (presenter), Jon Heifetz, and Dana Hanselman from the Auke Bay Laboratory (ABL) gave a presentation on their rockfish research at the 14th Western Groundfish Conference in Newport, Oregon, held on 30 January to 3 February 2006. The presentation summarized research conducted on adult rockfish benthic habitat associations in the Gulf of Alaska.

Many species of commercially valuable rockfish (Sebastes spp.) inhabit waters on the outer continental shelf and upper slope in the Gulf of Alaska, typically between depths of 100 m to 500 m. The benthic habitat requirements and spatial distribution of these rockfish species are relatively unknown. Information regarding benthic habitat use would improve current stock assessments and provide baseline information for an ecosystem approach to management. Several study areas in the Gulf of Alaska have recently been mapped with high resolution multibeam bathymetry and backscatter to generate detailed benthic habitat maps. Large populations of rockfish have been surveyed and fished within these mapped areas.

The objectives of ABL's rockfish research were to delineate habitat on two resolutions of classification, generate density estimates for rockfish, and identify useful predictors of rockfish distribution for use in stock assessments. The ABL researchers used a modified deep-water marine benthic habitat scheme to classify habit. The larger scale used multibeam imagery for determining benthic habitat, while the smaller scale used direct observations of seafloor features from submersible video.

In summer 2005, 40 dive transects were completed on three mapped sites (Albatross Bank, Cape Ommaney, and Hazy Islands) in the Gulf of Alaska using the two-person submersible Delta. Scientists observed schooling behavior in dusky rockfish typically in the water column and observed Pacific ocean perch in very large schools over soft bottom. Numerous gravid females were found in cobbles and small boulder fields. Solitary individuals such as shortraker, rougheye, redbanded, and sharpchin rockfish were encountered in fairly distinct habitat types.

The researchers identified habitat types with high densities of rockfish species within each classification method. In general, the multibeam classifications were too broad to be consistent with the high variability of the finer scale video classifications. However, when the two classification methods were compared using generalized features (i.e., boulder, sand, mud), densities for some species were more consistent, particularly on hard substrates such as boulders and cobble. Depth was also considered as an additional factor for determining species distributions. Major commercial rockfish species were clearly delineated by depth, occupying specific ranges. This finding was consistent with previous studies on adult rockfish distributions. The researchers concluded that more general habitat classifications such as substrate hardness combined with depth might be a better predictor for rockfish species distributions over large areas.

By Kalei Shotwell

2006 Sablefish Longline Survey

The AFSC has conducted an annual longline survey of sablefish and other groundfish off Alaska since 1987. The survey is a joint effort involving ABL and the Resource Assessment and Conservation Engineering (RACE) Division. Beginning in 1996, biennial sampling of the Aleutian Islands region and eastern Bering Sea was added.

The 2006 survey is scheduled 1 June to 1 September, using the chartered fishing vessel Alaskan Leader. The 2006 survey area covers the upper continental slope and selected gullies of the eastern Aleutian Islands and the Gulf of Alaska. The eastern Aleutian Islands will be sampled first, followed by the western Gulf of Alaska. The survey vessel then will travel to Dixon Entrance in early July and sample the eastern and central Gulf of Alaska.

Several special projects will be conducted during the 2006 survey. A marine mammal observer will observe killer whale depredation on the longline during the first two survey legs in the western Gulf of Alaska. A second observer will study sperm whale depredation in the eastern and central Gulf of Alaska, and there are tentative plans to tag sperm whales with bioacoustical tags during this time. Seabird observations and counts will be conducted throughout the survey to help address where and when certain seabird species occur in Alaska waters. A giant grenadier reproductive biology study will be conducted during the Southeast leg. Finally, a 2-day experiment will be conducted off Yakutat to collect genetic tissues of rougheye rockfish (Sebastes aleutianus) and to investigate depth distribution patterns of "light" and "dark" color phases of rougheye rockfish.

By Chris Lunsford


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