Research Document - 2009/033
Georges Bank ‘a’ Scallop (Placopecten magellanicus) Annual Stock Assessment: Survey Design
By B. Hubley, S.J. Smith, I.D. Jonsen, and J. Sameoto
Abstract
This Research Document presents an evaluation of the current and alternative designs for the annual August survey of sea scallops (Placopecten magellanicus) on the Canadian portion of Georges Bank. This document was presented as a working paper as part of the advisory process for developing a new assessment framework for Georges Bank scallops so that decisions could be made on the design of future surveys.
The original design with stratification based on annual commercial catch rates had several disadvantages including strata boundaries that changed each year and inconsistent coverage that was tied to fishing patterns. This design also performed poorly in terms of precision when evaluated against a simple random sampling design.
Two alternative designs with fixed strata boundaries were proposed: stratification based on commercial catch rates from 1981-2007 and stratification based on the survey index from 1981-2007. Relative efficiency was used to evaluate designs by comparing the variance of the estimated number of scallops when a given design is imposed. A portion of the relative efficiency results from the allocation of tows and because this could not be controlled in this analysis, potential relative efficiency that would result if allocation was optimal was also examined. Both of these designs were found to be improvements over the original annual catch rate design and simple random sampling in terms of gains in realized and potential efficiency. However, the historical survey index (HSI) performed slightly better than the historical catch rate (HCR) in both realized (overall mean: 53% vs. 44%) and potential (overall mean: 69% vs. 64%) efficiency.
Other variables that could potentially be used to create strata boundaries were also evaluated including depth, scope for growth, and disturbance. These variable were found to be less predictive of scallop abundance than historical measures of abundance. Substrate type was not evaluated.
The recommendation from the framework meeting was to adopt the HSI design with a division between the north and south portion of the bank. The final design to be used in future will be the HSI design with the top 3 strata divided by the Northwest Atlantic Fisheries Organization (NAFO) line between areas 5Zej and 5Zem and the lowest strata common over the whole bank. This design provided further improvements in relative efficiency over the HSI design. Since strata boundaries remain the same for the northern and southern portions of the bank, any subsequent analysis could be conducted with or without a north/south division.
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