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Science

Stock Assessments

Annual Bottom Trawl Survey

Each year since 1975, the Alaska Fisheries Science Center (AFSC) conducts an eastern Bering Sea bottom trawl survey. The purpose of this survey is to collect data on the distribution and abundance of crab, groundfish, and other benthic resources in the eastern Bering Sea. These fishery-independent data are used to estimate population abundances for the management of commercially important species in the region. This annual survey and data collection are essential for proper assessment and management of target stocks and non-target species. Without the bottom trawl survey and associated collection of data, there would be: 1) a decrease in the precision of biomass estimates resulting in increased uncertainty regarding population dynamics; 2) increased annual variation in catch limit recommendations resulting in a higher risk of overfishing; 3) a loss of other important biological information including age, growth, and spatial distribution; and 4) foregone economic opportunities as a result of an increase in precautionary management. An interactive data map showing the eastern Bering Sea shelf bottom trawl survey catch per unit effort for commercially important crab species from 1975 to the present can be found at http://www.afsc.noaa.gov/maps/crabmap/crabmap.html.

Crab Plan Team and Scientific and Statistical Committee

Data collected from the annual eastern Bering Sea bottom trawl survey (combined with other data sources like observer and fishery-dependent information) make it possible for stock assessment scientists to build detailed population dynamics models for each of the harvested crab species. These models are used to provide biologically-based quota recommendations that ensure commercial fishing remains at a level to sustain the population over the long-term.

An annual stock assessment and fishery evaluation (SAFE) report is prepared that provides the current biological, ecosystem, and economic data associated with each target crab species. The Crab Plan Team (CPT) and Scientific and Statistical Committee (SSC) review each assessment and modeling data and make recommendations for biological reference points for each of the stocks. The CPT and SSC are composed of leading scientists in biology, economics, statistics, and social science and are made up of individuals from the Alaska Fisheries Science Center (AFSC), Alaska Department of Fish and Game (ADF&G), University of Alaska Fairbanks (UAF), University of Washington (UW), and the NMFS Alaska Regional Office (AKRO). The purpose of the review is to assess the scientific validity of the stock assessment, including any assumptions, methods, results and conclusions. Specific aspects of the review will vary, but may include: quality of the data collected or used for the assessment, appropriateness of the analyses, validity of the results and conclusions, and appropriateness of the scope of the assessment (e.g., were all relevant data and information considered). The primary function of both the CPT and SSC is to provide the North Pacific Fishery Management Council (Council) with the best available scientific information, including scientifically based recommendations regarding appropriate harvest levels and measures for the conservation and management of the Bering Sea and Aleutian Islands (BSAI) king and Tanner crab fisheries. All recommendations must be designed to prevent overfishing while achieving optimum yield (National Standard 1). All recommendations must also be scientifically based (National Standard 2), drawing upon the CPT's and SSC's expertise is the areas of regulatory management, natural and social science, mathematics and statistics. The annual BSAI King and Tanner Crab SAFE report provides the Council with a summary of the most recent biological condition of the crab stocks and the social and economic condition of the fishing and processing industries. The SAFE report summarizes the best available scientific information concerning the past, present and possible future condition of the crab stocks and fisheries, along with ecosystem concerns. Because the Council strives to use the best available scientific and commercial data and analyses when making regulatory decisions, scientific peer review is an integral component of the process for ensuring the quality and integrity of the scientific assessments that are used to determine biologically acceptable catch limits for each of the commercially harvested crab species. More information on the Crab Plan Team and the SSC can be found on the North Pacific Council's website at http://www.npfmc.org/fishery-management-plan-team/bsai-crab-plan-team/ and http://www.npfmc.org/scientific-and-statistical-committee/. Links to the most recent King and Tanner crab SAFE Reports can be found at http://www.npfmc.org/safe-stock-assessment-and-fishery-evaluation-reports/.

Tier System

As part of the annual stock assessment review process, status determination criteria for commercial crab stocks are annually calculated using a five-tier system that accommodates varying levels of uncertainty of information. This tier system allows the incorporation of new scientific information and provides the ability to improve status determination criteria as new information becomes available. Under this system, overfishing and overfished criteria and acceptable biological catch (ABC) levels are formulated annually. For crab stocks, the annual catch limit (ACL) is equal to the ABC. For crab stocks, the overfishing limit (OFL) equals maximum sustainable yield (MSY) for that stock. Each crab stock is annually assessed to determine its status and whether: 1) overfishing is occurring; 2) the stock is overfished or approaching an overfished condition; and 3) the catch (all directed and non-directed fishery removals including retained and discards) has exceeded the ACL. For Tiers 1 through 3, reliable estimates of biomass, biomass at MSY, the fishing rate necessary to achieve MSY, and other essential life-history information are available. Tier 4 is for stocks where essential life history and other information are insufficient to achieve Tier 3. Tier 5 stocks have no reliable estimates of biomass and only historical catch data is available. A more detailed description of the status determination criteria and the five-tier system can be found in the federal King and Tanner Crab Fishery Management Plan at http://www.npfmc.org/wp-content/PDFdocuments/fmp/CrabFMPOct11.pdf.

Uncertainty

In setting annual total allowable catch levels (TACs), the Alaska Department of Fish and Game (ADF&G) takes the following factors into consideration: 1) whether the ACL for a stock was exceeded in the previous year; 2) stock status relative to the OFL and ABC; 3) estimates of exploitable biomass; 4) estimates of recruitment; 5) estimates of thresholds; 6) market and other economic considerations; 7) any additional factors pertaining to the health and status of the stock; and 8) additional uncertainty. Additional uncertainty includes both management uncertainty and scientific uncertainty. Management uncertainty encompasses the uncertainty in the ability of managers to constrain catch such that the ACL is not exceeded and the ability to quantify the true catch amount from all sources. Scientific uncertainty that is not already accounted for in the ABC including uncertainty in bycatch mortality, estimates and trends in size composition, shell condition, molt status, reproductive condition, spatial distribution as well as environmental conditions, fishery performance, and fleet behavior. ADF&G establishes the annual TAC for each crab stock at a level sufficiently below the ACL to ensure that the sum of the catch and assessment of additional uncertainty do not exceed an ACL. More information on the dual management system for commercial crab stock as well as on the incorporation of uncertainty into the TAC-setting process can be found in the King and Tanner Crab Fishery Management Plan at http://www.npfmc.org/wp-content/PDFdocuments/fmp/CrabFMPOct11.pdf.

ABSC Science Symposiums

Every year in September, in conjunction with the fall Crab Plan Team meeting, ABSC hosts a Crab Science Symposium for its members and all other interested stakeholders and members of the public. Crab scientists, researchers, and managers are invited to provide presentations and answer questions on the latest science and relevant topics to Bering Sea crab harvesters. Past Symposiums include regular presentations on and preliminary results from the NMFS bottom trawl survey. They have also included presentations on such topics as: the size-fecundity relationship in Bristol Bay red king crab; the life cycle of the male snow crab; results from handling mortality experiments for snow crab; ocean acidification and impacts to Bristol Bay red king crab; hatchery production of red and blue king crab; and predation and habitat use of red king crab around Kodiak Island.

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