Terms of Reference

Update to estimation methods for Geoduck (Panopea generosa) stock index

Regional Peer Review Process – Pacific Region

March 15-16, 2017
Nanaimo, British Columbia

Chairperson: Julia Bradshaw

Context

In the British Columbia Geoduck fishery, the Limit Reference Point (LRP) has been set at 40% of estimated virgin biomass (Zhang and Hand 2007) and has been applied on a by-geoduck-bed basis. The stock index, defined as the ratio of current biomass to virgin biomass on a bed, is calculated for each bed and compared to the LRP (Bureau et al. 2012). Virgin biomass must be estimated to calculate the stock index for each Geoduck bed. Few Geoduck beds have been surveyed before being harvested and therefore, virgin biomass has been back-calculated as the sum of estimated current biomass on a bed and removals from the bed, with the assumption that recruitment and natural mortality are equal (Bureau et al. 2012).

The current method of estimating virgin biomass in the Geoduck stock assessment framework (Bureau et al. 2012) is likely to lead to all harvested Geoduck beds reaching the LRP. That is, if surplus production occurs between surveys, the current methodology results in a continual increase in estimated virgin biomass, which in turn decreases the stock index. There is concern that the increase in estimated virgin biomass (for surveyed beds) is an artefact of the method currently used to estimate virgin biomass and may not be reflective of true stock status. This could lead to premature closure of some Geoduck beds and translate into loss of fishing opportunity for the industry. Additionally, stock dynamics are unknown on un-surveyed beds which may also lead to their premature closure.

The Geoduck stock index has historically been calculated at the finest spatial scale possible based on available data, i.e., at the by-bed spatial scale. Since 2006, the Geoduck fishery has been managed at the by-sub-bed spatial scale, thereby increasing the spatial accuracy of landings data. Increased spatial accuracy of harvest data may provide improved accuracy in the stock index calculations.

DFO Fisheries and Aquaculture Management Branch has requested that Science Branch provide a review of the method currently used to estimate Geoduck virgin biomass and provide alternative Geoduck virgin biomass estimation methods.

This paper is not meant to review, or provide alternatives to, the LRP currently used in the Geoduck fishery. The goal of the paper is to provide alternative methods to estimate virgin biomass and stock index only. The paper will thus provide updates to portions of the current Geoduck assessment framework (Bureau et al. 2012) and is not meant to be a completely new assessment framework. Further, the paper is not meant to provide Geoduck harvest strategies in areas occupied by Sea Otters as this issue has more general implications across species at the regional level.

This assessment, and the advice arising from this Canadian Science Advisory Secretariat (CSAS) Regional Peer Review (RPR), will be used to inform the management of the British Columbia Geoduck fishery. More specifically, advice will be used to update Geoduck virgin biomass and stock index estimation methods and to update some sections of the previous Geoduck fishery assessment framework (Bureau et al. 2012).

Objectives

The following working paper will be reviewed and provide the basis for discussion and advice on the specific objectives outlined below.

• Bureau, D.  Update to Estimation Methods for Geoduck (Panopea generosa) stock index. CSAP Working Paper 2015-INV-03

The specific objectives of this review are to:

1. Provide a review of the method currently used to estimate Geoduck virgin biomass, for surveyed and un-surveyed Geoduck beds.
2. Describe alternativemethods for estimating Geoduck virgin biomass for both surveyed and un-surveyed Geoduck beds, including evaluation of the relative advantages and disadvantages of each method.
3. Provide methods for calculating Geoduck stock index on a by-Geoduck-sub-bed basis. Describe the advantages and disadvantages associated with this approach.
4. Identify and discuss uncertainties and knowledge gaps in the available data and proposed estimation methods.

Expected Publications

• Science Advisory Report
• Proceedings
• Research Document

Participation

• Fisheries and Oceans Canada (DFO) (Ecosystems and Oceans Science, and Ecosystems and Fisheries Management sectors)
• Industry (fishing industry: Underwater Harvesters Association)

References

Bureau, D., C.M. Hand and W. Hajas. 2012. Stock Assessment Framework for the British Columbia Geoduck Fishery, 2008. DFO Can. Sci. Advis. Sec. Res. Doc. 2011/121. viii + 79p.

DFO. 2012. Status update of wild British Columbia geoduck stocks, 2011. DFO Can. Sci. Advis. Sec. Sci. Advis. Rep. 2011/081.

Zhang, Z. and Hand, C. 2007. Determination of geoduck harvest rates using age-structured projection modeling. DFO Can. Sci. Advis. Sec. Res. Doc. 2007/064. 49p.