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Status of Stocks & Multispecies Assessment Program

Summer Workshops

The busy field season coincided with a busy season for analysts to hone skills and obtain new ideas. Three major workshops (funded by outside sources) were coordinated by REFM Division staff: one on the spatial structure and dynamics of pollock in the Bering Sea, another on the practical considerations for natural mortality specifications in stock assessments, and the third on management strategy evaluations for southern bluefin tuna. The tuna workshop involved refining the technical aspects of developing an operating model (conditioned on available data) from which alternative management approaches can be tested. The other two workshops are summarized below.

Workshop on spatial structure and dynamics of walleye pollock in the Bering Sea:

A 4-day workshop was hosted by the AFSC with outside funding from the Pollock Conservation Cooperative Research Center was convened 7-10 July 2009 to synthesize relevant information and modeling of the spatial structure and dynamics of the walleye pollock population in the Bering Sea. The synthesis was needed to address issues related to ecosystem effects of one of the world's largest fisheries on a finer temporal and spatial scale than is currently available.

Workshop sessions reviewed empirical information from research surveys, the commercial fleet, and factors that influence the spatial distribution of pollock on seasonal and daily scales. Oceanographic and environmental conditions related pollock food and predators to possible regime shifts and forces affecting early life survival. Discussions throughout the week centered on a number of questions including:

  • What mechanisms best explain pollock spatial distribution? What are the tradeoffs of different movement model approaches (i.e., estimation features, scalability, data requirements)?
  • Are technological issues for tagging studies prohibitive? What is the ideal field study to determine movement? What type of tag, what type of recovery effort and other design issues (e.g., sample sizes, locations, cost, logistics) should be considered? How have simulation studies shown the importance of tagging data? How would conventional tagging improve understanding of spatial processes? Can directed fishing where data are missing improve seasonal spawning distribution information?
  • Has spatial modeling resolved retrospective patterns? Will spatial modeling reduce uncertainty (relevant for ABC/ACL specification)? How should current evidence of pollock movement be used to modify management approaches? How can spatially explicit modeling aid in operating model development to test management strategies?

Recommendations arising from these discussion points included:

  • Continue model developments by disaggregating the present assessment model (e.g., developing seasonally and spatially disaggregated models where movement between areas may be included). This could form the basis of an operating model to test the simpler model that is presently used for management. One specification might be to have a single area-model but with fisheries spatially and seasonally defined.
  • Conduct a synthesis of available information on population structure of pollock and develop a conceptual model of population structure (e.g., along the lines done by Kotenev and Glubokov 2007).
  • Evaluate the implication of movement and fishery patterns on underlying stock-recruitment patterns.
  • Examine the consistency of spawning aggregations year after year—are they the same group of fish? Can micro-constituents or other method be used to identify a signature for these fish? For example, do Bogoslof spawning fish reside on the shelf during other times of year? Examine spawning throughout the year, including the fall when roe is less of a target fishery.
  • Promote further funding to determine whether tagging pollock is feasible. Such a pilot project should be designed to begin testing hypotheses on movement and spatial structure for EBS pollock. Also, consider the use of temperature recorders with the tag (technology has made them inexpensive and about the size of a dime).
  • Extend the cooperative program for acoustic data loggers aboard commercial vessels to do additional directed transects to help fill in gaps on the seasonal distributions. Use caution when adding spatially explicit recommendations for catch levels for stocks that are relatively mobile since the uncertainty for spatially-disaggregated estimates is likely to increase. (For unit stocks, spatially explicit management recommendations generally occur for relatively stationary species).

Workshop on natural mortality in stock assessment applications:

As part of the national program to improve interaction among fisheries stock assessment scientists, the Office of Science and Technology (through the Stock Assessment Methods working group) held a workshop at the AFSC on 11-13 August 2009. The meeting drew international experts and interested colleagues from each science center (25 NOAA participants) and representatives from universities and other agencies (18 participants). A large set of literature was compiled and made available for the workshop; the report of discussions and extended abstracts is in preparation.

The terms of reference for the workshop were: 1) identify and compare alternative methods of estimating natural mortality rates for conducting stock assessments, 2) make recommendations for best practices for estimating natural mortality rates, 3) provide examples of natural mortality rate estimation, and 4) prepare a document addressing these recommendations that will be used to guide future assessments by NMFS.

Professor Kai Lorenzen (from Imperial College, London, UK) was guest speaker and provided talks on approaches he has used in a variety of settings and proposed a "Best Ad-hoc Mortality Model" (BAMM). This approach ties life-history specific characteristics (e.g., the onset of sexual maturity) to physiological changes that affect mortality rates with age.

The workshop reviewed a variety of other dimensions related to natural mortality including how rates may change over time and the impact of factors such as changes in predator abundances and environmental conditions. Gender differences in natural mortality rates were also highlighted in a number of the presentations and in discussions.

In terms of information, a set of categories was proposed for approaches based on available information. For low information species, natural mortality estimates are often derived from life history correlates and are typically fixed. For stocks with moderate information, estimates may be available from numbers-at-age (e.g., via a catch curve analysis) or from tagging data and may consider gender differences.

For data-rich stocks, mortality estimates may be derived from integrated analysis using size and/or age data providing that a fleet (or survey) can be adequately modeled with asymptotic selectivity, potentially with time-specific patterns. For the highest level of information available, natural mortality would have information that reliably links trophic interactions through ecosystem analyses.

By Jim Ianelli

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