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Status of Stocks & Multispecies Assessment Program

A Generalized Size-Based Assessment Modeling Approach for Alaskan Crab Stocks (GMACS)

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Spring 2015
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A number of AFSC scientists contributed to a review of the developments on a General Model for Alaska Crab Stocks (GMACS).  An external panel of three reviewers from the “Center of Independent Experts” (CIE) were presented with a wide range of materials leading up to a meeting at the AFSC from 29 June to 3 July. Dr. Martin Dorn chaired the review and preparations and presentations were prepared by Jim Ianelli (AFSC), Jie Zheng (Alaska Department of Fish and Game) and Darcy Webber (AFSC contractor) as the lead analysts. Drs. William Stockhausen and Robert Foy (remotely) also participated and provided input on crab assessment processes. The terms of reference for the review were:

  1. Evaluation of functional forms, estimation approaches, and diagnostics used in GMACS including uncertainty characterization and satisfying required assessment elements.
  2. Evaluate application of GMACS to the Bristol Bay red king crab (BBRKC) stock. Specifically, comment on what important features are missing relative to the current assessment approach?
  3. Evaluation of GMACS as a flexible assessment modeling tool, i.e., potential application to other crab stocks e.g., Tanner crab, data poor stocks (Tier 4), and stocks with unique data sets (e.g., snow crab and Bristol Bay red king crab cooperative survey data).
  4. Evaluation of utility of the modeling framework as a community supported modeling approach and practicality for managing into the future.
  5. Recommendations for further improvements and comment on the general applicability to the fishery management questions (risk assessment and management strategy evaluations [MSE} options).  
refer to caption
Figure 1. Example plot for four model alternatives in GMACS invoked in the R package by the call “plot-ssb(M)” where M is a “list” of lists for model output. For more information see the site

The software has been actively developed through collaborative arrangements with post-doctoral researchers at the University of Washington and other agencies as coordinated through REFM staff. The package includes an integrated set of diagnostic utilities for plotting and evaluating fits. Presently there are two approaches to simulating data for testing estimation methods. Also, utilities designed to facilitate creating crab stock assessments for the North Pacific Fishery Management Council (NPFMC) are included. In particular, the ability to quickly compare among alternative models was considered a high priority should the Council wish to evaluate an ensemble of models (e.g., Fig. 1). For the review, a case study application to BBRKC was compared alongside the model that was designed specifically for the BBRKC stock. While the project is slightly behind schedule, (it was hoped that the model would be presented as an alternative for BBRKC at the start of 2015), the review should provide critically useful guidance going forward in preparation for the September NPFMC crab plan team meetings. The collaborative development of the software can be tracked and monitored at

By Jim Ianelli


Estimating Vulnerability of Eastern Bering Sea Stocks to Climate Change

NOAA Fisheries scientists Paul Spencer, Mark Nelson, (NOAA Fisheries Office of Sustainable Fisheries), Anne Hollowed, and Mike Sigler are leading a project this summer to estimate the vulnerability of eastern Bering Sea stocks to climate change. The objectives of the project are to apply a rapid, semi-quantitative methodology to estimate the relative vulnerabilities of 35 stocks in the EBS, where vulnerability is a function of the estimated sensitivity of a stock to climate change and its estimated exposure to climate change. More than 40 scientists are contributing to this project, representing a variety of organizations including the AFSC, Northwest Fisheries Science Center, Alaska Department of Fish and Game, International Pacific Halibut Commission, Pacific Marine Environmental Laboratory, and several universities. Workshops were held at AFSC during the week of July 13th to present the methodology for obtaining preliminary estimates of the sensitivity of stocks to climate change, and an additional workshop is scheduled for August 27-28 to discuss and refine the preliminary estimates. The results of this project can assist in identifying data gaps and research priorities for EBS stocks, as well as informing strategies for adapting to climate change. 

By Paul Spencer



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