Research Document 2018/005
Eastern Scotian Shelf Shrimp (Pandalus borealis) Framework
By Hardie, D., Covey, M., and Cook, A.
Abstract
The eastern Scotia Shelf Shrimp fishery has been ongoing since the early 1980s, although the fishery’s contemporary history began with the introduction of the Nordmøre grate in 1991, which allowed the fishery to overcome bycatch limitations. Salient aspects of the species’ and stock’s biology and ecology are reviewed to support a detailed description of the diverse data streams that have been used in the provision of science following a multiple indicator Traffic Light Analysis for nearly a quarter-century. The details of the collaborative Fisheries and Oceans Canada (DFO)-Industry survey, which is entering its twentieth in 2015, are provided. This includes survey history and design, comparative fishing experiments, trawl mensuration methods for the estimation of total biomass and catch composition, which provides the data that are the basis of the many fishery-independent Traffic Light indicators. The details of the collection and analysis of commercial catch rate and catch composition data are also provided. The holistic manner in which these primary indicators of stock abundance and composition, as well as supporting ecosystem indicators, are analysed annually to provide science advice for adaptive Total Allowable Catch (TAC) adjustments in the new biennial assessment schedule are described. Despite its long track record of the provision of science advice for the highly successful co-management of this stock, the Traffic Light method has been subject to two important criticisms since its inception: the lack of defined quantitative outcomes based on the indicator values, and the lack of quantitative projections. These limitations are explored in detail and options are discussed to address them. The deterministic harvest control rule model discussed herein to provide a range of TAC advice yields relatively conservative quota advice that is generally consistent with the successful and conservative management history of this stock, except at high biomass, where it allows for higher exploitation than this conservative fishery has historically opted for. Its principle benefit is that it provides clear and conservative management guidance at low to moderate biomass levels (i.e. Critical and Cautious zones), where this fishery had very little contemporary experience. In addition to the higher (than historical) exploitation rates at high abundance resulting from this deterministic linkage of harvest advice to biomass indices, this method would risk losing the inclusivity and flexibility that has resulted in effective collaboration between science, management and stakeholders to guide this fishery towards a profitable and sustainable exploitation strategy for so long. Indicator weighting is necessary for deterministic harvest control rules and continues to be a serious and perhaps insurmountable problem with this approach. The results of a Bayesian state space biomass dynamic model is reviewed as a means to provide quantitative stock projections based on the relationship between variations in biomass as a function of previous biomass and other population specific parameters. All three biomass indicator series yield implausible biomass and parameters estimates, particularly those derived from a very low modelled carrying capacity (less than half of the historical survey biomass index values). The model assumptions, which this and most other shrimp stocks tend to seriously violate, are discussed. There has been no detectable influence of fishing mortality on biomass variation in the very conservatively exploited Scotian Shelf Shrimp stock. It is proposed that science advice for the eastern Scotian Shelf Shrimp fishery should continue to be provided by the holistic combination of qualitative and quantitative interpretations of data in the Traffic Light Analysis that has proven so successful. Quantitative analysis of biomass indicators, corroborated by a suite of other shrimp stock and ecosystem data, coupled with qualitative projections based tracking year classes in survey and commercial length frequencies now has a long track record of transparent and inclusive co-management of this stock, and there is no reason to change this. Future research efforts should focus on continued validation and refinement of the Traffic Light indicators rather than the identification of deterministic harvest control rules or quantitative projection methods.
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