Research Document - 2015/074
Considerations for defining reference points for Atlantic Salmon that conform to the Precautionary Approach
By G. Chaput
Abstract
This paper addresses four of the terms of reference in support of a science review of reference points for Atlantic salmon that conform to the Precautionary Approach (PA) framework. The fixed escapement strategy and single reference point approach presently used for the management of Atlantic salmon does not conform to the PA as it does not set a maximum removal rate for the stock nor does it define an Upper Stock Reference (USR) at which the maximum removal rate would apply. The stock dynamics of Atlantic salmon are most often presented as spawner and recruit relationships which differs from the PA framework for which stock status is presented relative to the removal rate of the stock. It is straightforward to reconcile these two views by treating the recruitment on the stock and recruitment framework as the stock status axis in the PA framework. Three candidate limit reference points can be derived from exclusively freshwater dynamic models which respect population conservation considerations, are determined by the environment where the stock and recruitment density dependent dynamic is expressed, and are robust to variations in productivity associated with density independent dynamics (marine survival). The choice of the USR will depend upon the objectives of the fishery and the risk profile of the management strategy. The removal rate reference could be determined once the upper stock reference point is defined. The modelling of stock and recruitment relationships and the development of reference points are challenged by two conflicting considerations; the need for a long time series of contrasting abundance with which to adequately estimate life history parameters versus the risk that there will be systematic and sustained changes in the life history parameters being estimated. The consequences of reduced productivity, manifest in either freshwater or marine environments, are to reduce the adult recruitment per spawning stock which lowers the values of most reference points derived from full life cycle models. Provided the productivity in freshwater is stationary, then the few limit reference points defined on the basis of maintaining freshwater production levels would be robust to variations in marine productivity. The objective should be to maintain freshwater production to take advantage of better marine productivity periods when they occur. Since it is not possible to obtain stock and recruitment data from the over 1,000 rivers with Atlantic Salmon populations in eastern Canada, consideration must be made to transferring reference values from monitored rivers to rivers which lack such information. Hierarchical Bayesian methods are most appropriate in situations where reference points from data rich situations are transported to populations with limited to no information because the uncertainties associated with intra-population stock and recruitment dynamics and inter-population variation of this dynamic within a set of exchangeable units can be quantified. Quantifying uncertainty in the development and use of reference points consists of three components: uncertainty associated with the derivation of the reference point, the choice of the value of the reference point posterior distribution used in management, and uncertainty in the current status of the stock relative to the reference point. Bayesian approaches provide a means of considering these uncertainties in reference point development and application.
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