Incorporating natural variability in biological reference points and population dynamics into management of Atlantic salmon (Salmo salar L.) stocks returning to home waters

Abstract

Following advice from the International Council for the Exploration of the Seas and North Atlantic Salmon Conservation Organization, Irish salmon stocks have been managed on a river-by-river basis since 2007 with biological reference points (BRPs) based on maximum sustainable yield (MSY). A method for estimating BRPs at the river scale and the associated variability arising from observed variability in population structures and fecundities is presented here. Calculations of BRPs (referred to as conservation limits, CLs) were updated and their natural variability was included. Angling logbooks provided new river-specific weight data to give sea age and fecundity ranges, and improved estimates of river-wetted areas, to account for available nursery habitat for juveniles and river-specific carrying capacities, were introduced. To transport BRPs, Bayesian stock-recruitment analysis was re-run with an updated list of monitored rivers and smolt ages. Results were converted to salmon numbers per river in Monte Carlo simulations incorporating the variability in sea ages and fecundities. Minimum sample size rules were implemented to reduce sampling error effects. Results showed that average total CL increased by 7%, average one sea-winter (1SW) CL decreased by 5% and average multi-sea-winter (MSW) CL increased by 157%. Differences were attributed to increases in wetted areas, MSW proportions, and changes in both 1SW and MSW fecundities. While some changes were large, we believe that these updated CLs provide more accurate estimates and with associated confidence limits they are more robust, river-specific, and readily incorporated into stock assessments. As a significant improvement on their predecessors, they represent a major development for the conservation and management of salmon stocks. Additionally, the approach described is portable across stocks and has the potential to be implemented in other jurisdictions to improve the management of Atlantic salmon. Finally, this method of incorporating variation has application for the development of BRPs and management of other species.