Research Document - 2009/034
Georges Bank ‘a’ Scallop (Placopecten magellanicus) Framework Assessment: Data Inputs and Population Models
By I.D. Jonsen, A. Glass, B. Hubley and J. Sameoto
Abstract
This research document presents a summary of relevant data inputs and an evaluation of a proposed population model for the assessment of sea scallops (Placopecten magellanicus) on the Canadian portion of Georges Bank, zone 'a'. This document was presented as a working paper as part of the advisory process for developing a new assessment framework for Georges Bank 'a' scallops.
Until 2007, the status of the Georges Bank scallop stock has been assessed using a cohort analysis tuned to a commercial catch rate-stratified survey index and a commercial catch per unit effort index. This model suffered both from strong retrospective effects, which resulted in inconsistent estimates of biomass from year to year, and from a lack of recent, reliable aging data, which were used to construct the catch at age matrix. Georges Bank scallops are difficult to age reliably due primarily to the high prevalence of shock marks that are difficult to distinguish from annual growth rings. Consequently, an alternate population model that placed less reliance on aging data was thought to be a more appropriate candidate for assessment of this stock.
A delay-difference population model fit to both the survey and commercial catch rate indices was proposed. This model was implemented in a Bayesian state-space framework, accounting for observation error and process variability separately. Model parameters were estimated using Metropolis-within-Gibbs sampling, a type of Markov chain Monte Carlo sampling method. Scallop growth rates were parameterized from previously published von Bertallanfy growth parameters for the Georges Bank scallop stock. Biomass estimates from this model were compared with estimates from both the cohort analysis and from a surplus production model. These comparisons indicated that the delay-difference model produced biomass estimates that were more consistent with the pattern suggested by both the survey and commercial catch rate indices than did the cohort analysis. Although the biomass trajectories estimated by the delaydifference and surplus production models were similar, the survey catchability estimates from the delay-difference model were more consistent with estimates from the US scallop dredge survey.
Development of a biological reference point was explored in the context of scallop growth rates, natural mortality and exploitation. We show that exploitation rates required for no change in stock biomass can be considered a function of scallop growth rate, discounted for natural mortality. The reference exploitation should then change as the growth rate changes, primarily through changes in average stock size. This approach can be considered precautionary as scallop recruitment to the fishery, which is currently modeled as a log-normal random process, in most years would reduce the realized exploitation level. Posterior distributions of projected biomass under different harvest scenarios, and the associated probabilities of a decrease in biomass, were used to evaluate in-season increases to the interim total allowable catch limit.
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