Research Document 2017/070
Update to Estimation Methods for Geoduck (Panopea generosa) Stock Index
By Bureau, D.
The stock index in the British Columbia (BC) Geoduck fishery is estimated on a by-Geoduck-bed basis and is defined as the ratio of current biomass (Bc) to unfished exploitable biomass (B’). The limit reference point (LRP) for the BC Geoduck fishery was defined as current biomass being equal to 40% of B’. When biomass is estimated for a bed, the stock index is also estimated and beds for which the stock index is below 0.4 are closed to fishing. To date, B’ has been back-calculated as the sum of current biomass and fishery landings on the bed. This method assumes no surplus production on a bed after fishing begins.
The methods currently used to estimate B’ on surveyed and un-surveyed beds were reviewed. Simulations were performed to illustrate how surplus production affects estimates of B’ and stock index over time using the current method. If surplus production occurs, the method currently used to estimate B’ produces biased estimates of B’ and stock index. Density dive survey data, for beds surveyed more than once, showed that surplus production may be taking place on harvested Geoduck beds in BC and that therefore the assumption of no surplus production is likely not met.
Alternative options of estimating B’ on surveyed and un-surveyed Geoduck beds were proposed and evaluated. Data requirements, assumptions, applicability, advantages and disadvantages of each proposed option were reviewed. The performance of each option for surveyed beds was evaluated for beds where early estimates of B’ were available. Estimating B’ as biomass from the first survey plus the landings before 1989 was recommended because it has few assumptions, the assumptions are believed to be reasonable, it is applicable to all surveyed beds and is simple to implement.
Few alternative B’ estimation options were available for un-surveyed beds because less data is available for those beds. For un-surveyed beds, the recommendation was to use estimates of unfished exploitable density from surveyed beds to extrapolate unfished exploitable biomass on un-surveyed beds.
Methods for estimating the stock index at the by-sub-bed spatial scale were presented along with advantages and disadvantages of this approach. An evaluation of the possible impact of changing the spatial scale at which the stock index is calculated was presented. A recommendation was made to implement calculation of stock index at the by-sub-bed spatial scale.
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