Research Document 2017/011
Guidance to Derive and Update Fishing-Related Incidental Mortality Rates for Pacific Salmon
By Patterson, D.A., Robinson, K.A., Raby, G.D., Bass, A.L., Houtman, R., Hinch, S.G., and Cooke, S.J.
Abstract
This paper provides guidance towards deriving and updating estimates of fishing-related incidental mortality (FRIM) for Pacific salmon (Oncorhynchus spp.) captured in salmon-directed fisheries. We recommend condensing the multiple mortality components of FRIM into drop-off (i.e., avoidance, escape, depredation, and drop-out mortality), capture (i.e., on-board mortality) and post-release (i.e., short-term and delayed mortality) mortality to assist in the practical information needs of fisheries management and stock assessment. However, for the purposes of assessing the risk of mortality, capture mortality and post-release mortality are combined into a single release mortality risk value. A risk assessment approach was designed to provide relative values of mortality risk across all major salmon-directed fisheries (i.e., various species, sectors, gears, and locations). An objective process to characterize salmon fisheries in a manner that reflects their potential to cause FRIM is proposed. A procedure for generating the overall mortality risk values for both drop-off and release mortality combine the separate mortality risks associated with different levels of impact for key risk factors that drive FRIM, namely capture, handling, injury, water temperature, and predators. The cumulative impact of multiple risk factors for a given fishery is presented as a range of mortality risk values using multiplicative, dominance, and synergistic interactions among these factors. The risk assessment tool was validated with a set of experimental telemetry projects for which we had detailed information on the risk factors and estimates of release mortality. Next, we provide advice on anchoring the relative mortality risk values to a range of mortality estimates from FRIM studies whose purpose was to directly assess components of FRIM in a real fishery. Recommendations on sourcing and selecting the most appropriate studies to inform the risk assessment and anchoring process are provided. In addition, the major considerations in interpreting the reliability and relevance of previous FRIM research are highlighted to emphasize the potential problems inherent in selecting only a few studies. Major limitations of most FRIM research include the lack of true controls, the study realism (i.e., resemblance to the real fishery), and the mortality response time (e.g., immediate versus delayed). The guidance provided herein is designed to be repeatable, transparent, and scientifically-defensible. Areas with important knowledge gaps include sub-lethal effects, cumulative impacts, disease, and scoring of the relative mortality risk associated with different risk factors. Recommendations include the use of alternative survival analyses and the incorporation of the risk assessment process as part of a larger risk analysis plan.
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