Science Advisory Report 2017/037

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

Summary

• For each Geoduck sub-bed, virgin biomass (B0) is now defined as the theoretical long-term equilibrium exploitable biomass in the absence of fishing. Because B0 cannot be estimated using the current methods when harvest has occurred before the first survey, estimated unfished exploitable biomass (B’)will be used as a proxy for B0.
• Current methods for estimating B’ on surveyed and un-surveyed beds were reviewed.
• Simulations showed that the current model produces biased estimates of B’ and stock index if the assumption of “no surplus production” in the existing model is not met.
• Based on available data, literature review and fisheries stock assessment and population growth theory, the assumption of “no surplus production” in the current model is unlikely to be met, i.e., surplus production is likely occurring on BC Geoduck beds.
• Alternative options for estimating B’ for surveyed and un-surveyed beds were presented, along with their assumptions, advantages and disadvantages.
• For surveyed Geoduck beds, “Option 3B – Unfished biomass equals biomass from first survey plus landings before 1989” was recommended for estimating unfished biomass (B’).
• For un-surveyed Geoduck beds, “Option 2 – Use regional estimates of unfished density to estimate unfished biomass (B’)” was recommended.
• It is recommended that unfished biomass (B’) is re-estimated when estimates of Geoduck current biomass are updated, so that the most up to date data inputs (bed area, mean Geoduck weight and density) are used.
• Methods were presented for estimating stock index on a by-Geoduck-sub-bed spatial scale, for surveyed and un-surveyed Geoduck beds. These methods take advantage of increased spatial accuracy of landings data that has been available since 2006. It was recommended to calculate stock index on a by-sub-bed spatial scale, instead of by-bed.
• It was recommended to explore the use of probabilistic methods, which could incorporate uncertainties in the estimated biomass, rather than the current deterministic methods, to determine the probability that the stock index is above 0.4.
• Because of the influence of Geoduck mean weight on the estimation of B’, further analysis of Geoduck mean weight data and how mean weight is estimated for use in B’ estimation was recommended.
• Further research is recommended in anticipation of future modifications to the Geoduck stock assessment framework that may be necessary due to expansion in the range of Sea Otters on the BC coast.

This Science Advisory Report is from the March 15-16, 2017 regional peer review on the Update to estimation methods for Geoduck (Panopea generosa) stock index. Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

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