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Framework science peer review of stock assessment methods for the southern Gulf snow crab stock

Regional Advisory Process – Gulf

November 21-25, 2011
Moncton, NB

Chairperson: Jean-Claude Brêthes


Snow crab, Chionoecetes opilio, has been commercially exploited in the southern Gulf of St. Lawrence since the mid 1960s. The fishery grew quickly from 1966, peaking at 33,400 t in 1982 and dropped to the lowest catch of 8,900 t in 1990, subsequently fluctuating with peaks in 1995 of 22,750 t and in 2005 of 36,200 t. The stock assessment of the southern Gulf snow crab resource is based on indicators of abundance (commercial biomass), reproductive potential (numerical abundance of mature females), and recruitment derived from observations collected with a fishery-independent trawl survey (Hébert et al. 2010) and geostatistical methods (Conan et al. 1988). The area of the southern Gulf of St. Lawrence sampled by the survey (referred to as the polygon), which began in 1989, has increased over time, reaching its current size of 44,302 km² in 2006 (Moriyasu et al. 2009). The analytical models for estimating the biomass of commercial-sized adult male crab were reviewed during a framework meeting in 2005 (DFO 2006) and the agreed approach has been used since for the assessment and to re-analyze the historical time series back to 1989 (Surette et al. 2007; Moriyasu et al. 2009).

Following on a decline in the estimated exploitable biomass that raised concerns beginning in 2009, a collective of traditional snow crab fishers from zone 12 (pre-2003) and others challenged the assessment methods used by DFO to estimate the biomass of snow crab in the southern Gulf. Further, in February and March 2011, independent analyses of the trawl survey data reported by an industry consultant hired by the collective of fishing associations above suggested that the biomass estimates in 2010 were higher than those reported by DFO, and critiqued the reconstruction of the assessment time series prior to 1999. The independent analyses used different analytical assumptions and different estimation areas from those used by DFO and which had been discussed and agreed to at the framework peer review meeting of 2005. As a result of these challenges and in keeping with the DFO science objective of providing robust and scientifically defensible advice, a framework peer review meeting is being organized to review and develop recommendations for the assessment methods and analytical approaches to be used to assess the status of the snow crab stock of the southern Gulf of St. Lawrence.

Meeting Objectives

The objectives of the framework science meeting are to review the following assessment methodologies questions:

  1. Review of current sampling design, the estimation model and approach to estimate the biomass of commercial-sized male crab from the southern Gulf of St. Lawrence,
  2. Review of the definition of the polygon for which the biomass will be estimated,
  3. Review of the methods for reconstructing a homogenous time series of biomasses and abundances from 1989 to present
  4. Review of the best approach to estimate and include uncertainty of the biomass estimates within the four management areas based on a survey designed to estimate biomass for the entire southern Gulf.

Details of the assessment methodologies to be reviewed are provided in Appendix 1a.

The framework review will recommend the approaches for the best treatment of data currently collected, best treatment for historical reconstruction, and recommendations for improvements to design and assessment methods. Where possible, the recommended approaches and models will be applied to the stock assessment based on data collected in 2011 which will be peer reviewed in February 2012.

Expected Publications

The meeting will not generate advice. The review of approaches and the recommendations for the best assessment methods will be documented in a proceedings report. The recommendations contained in the proceedings will be put into practice for the 2011 stock assessment and must be finalized in time for the regional advisory process peer review meeting of February 2012. As such, the proceedings document is to be completed within two months of the date of the framework meeting.

Working papers or reports that are presented for review may be upgraded to research documents where considered appropriate. The supporting research documents are expected within four months of the date of the meeting.

When finalized, the products from the meeting will be posted on the DFO Canadian Science Advisory Secretariat website.


The meeting will be highly technical in nature and the discussions and review will require participants that are familiar with a broad range of quantitative assessment and modeling techniques. Participation will be by invitation.

To contribute materials and analyses and to assist in the framework review, participation is expected from:


Conan, G.Y., M. Moriyasu, E. Wade, and M. Comeau. 1988. Assessment and spatial distribution surveys of snow crab stocks by geostatistics. ICES C.M. 1988/K:10, 23 pp.

DFO. 2006. Proceedings of the Assessment Framework Workshop on the southern Gulf of St. Lawrence snow crab (Areas 12, E, F and 19), Gulf Regional Advisory Process; 11-14 October 2005. DFO Can. Sci. Advis. Sec. Proceed. Ser. 2006/042.

Hébert, M., E. Wade, M. Biron, P. DeGrâce, J.-F. Landry and/et M. Moriyasu. 2010. The 2009 assessment of snow crab, Chionoecetes opilio, stock in the southern Gulf of St. Lawrence (Areas 12, 19, 12E and 12F). DFO Can. Sci. Advis. Sec. Res. Doc. 2010/091. vi + 85 p.

Moriyasu, M., E. Wade, M. Hébert, and M. Biron. 2008. Review of the survey and analytical protocols used for estimating abundance indices of southern Gulf of St. Lawrence snow crab from 1988 to 2006. DFO Can. Sci. Advis. Sec. Res. Doc. 2008/069.

Surette, T.J., Marcotte, D., and Wade, E. 2007. Predicting snow crab (Chionoecetes opilio) abundance using kriging with external drift with depth as a covariate. Can. Tech. Rep. Fish. Aquat. Sci. 2763: vi + 33 p.

Appendix 1a. Detailed points to be reviewed during the framework meeting

1)  Review of the current sampling design, estimation model and approach to estimate the biomass of commercial-sized adult male crab from the southern Gulf of St. Lawrence

  1. Review of sampling design
    1. Current design based on principles of geostatistics of samples within defined quadrats ascribed to a sampling space
      • Sampling density variations over time and assess potential bias (presence and magnitude)
      • Extended sampling within a season
    2. Alternate sampling designs (random stratified by depth, other)
    3. Advantages and constraints of sampling design

  2. Estimation models
    1. Geostatistics approach
      • Ordinary kriging or kriging with external drift
      • External drift variable: depth or temperature or other
      • Treatment of edge effects: pseudo-zeroes versus external drift
      • Choice of variogram (annual or 3-year averaged, or other)
      • Adjustment of variogram (using model or adjusted manually)
    2. Alternate estimation models (for ex. GLM, mixed effects models, other)
    3. Description of the analytical functions in Mathlab and R, verification of coding, and comparison of results among platforms

2)  Definition of the polygon for which biomass will be estimated.

Objective: to estimate total biomass of crab within the biological unit and available to the fishery

  1. Constraints based on coverage
  2. Auxiliary data to inform on the appropriate polygon
    1. Depth or temperature
    2. Species distribution based on September multi-species survey

3)  Reconstructing the time series: is it possible to reconstruct a homogeneous time series for 1989 to present?

  1. Review of sampling methods and changes over time
    1. Crab life-stage categorization
    2. Gear changes, vessel changes, technology changes
    3. Effects of adjustments to start and end points of trawl touchdown
    4. Proposed revision of area swept for 1989 to 1998
    5. Determine appropriate approach for estimating area swept
  2. Review of differences associated with the adjustments back in time to reconstruct the time series
    1. Review analytical method used to produce original estimates (before 1998) – old 29,254 km² assessment polygon, ordinary kriging, pseudo-zeroes, single year variogram, old area swept estimates. Calculate biomass, confirm that the estimates match the previously reported values (before adjustment)
    2. Update swept area method, calculate biomass as in (i), report difference relative to (i).
    3. Use three year variogram, calculate biomass, report difference relative to (i), (ii).
    4. Remove pseudo-zeroes, report difference relative to iii.
    5. Change from ordinary kriging to kriging with external drift, report differences.
    6. Do this for all years (1989 to 2010) using the small polygon used prior to 1998.
  3. Alternate approaches to estimate bias with change of polygon
    1. Examine bias associated with expanding polygon without sampling in extended zone using data from 2006 to 2010. Bias is estimated based on expanded biomass versus survey covered biomass.
    2. Using September groundfish survey, adjust small polygon biomass to southern Gulf biomass using the relative abundance of snow crab in surveyed polygon to southern Gulf polygon based on groundfish survey.
    3. Can the time series be retained (with estimated bias and uncertainty) or is the earlier time period not usable for entire southern Gulf?

4)  Under the current management, the proportion of the biomass contained within each of the four management areas is required. What is the best approach to estimate and include uncertainty in the biomass within management areas from a survey designed to estimate biomass for the entire southern Gulf?


Participation to CSAS peer review meetings is by invitation only.

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