Éperlan arc-en-ciel, population d'individus de grande taille du lac Utopia : mise à jour de l'abondance et analyses génétiques

Abstract

Lake Utopia is part of the Magaguadavic River watershed in southwestern New Brunswick. The Rainbow Smelt (Osmerus mordax) living in Lake Utopia (LURS) represent one of the only three confirmed occurrences in Canada of co-existing genetically divergent smelt populations. Two populations of smelt co-exist in the lake: the small-bodied population (SbP) and the large-bodied population (LbP). The SbP has been protected under the Species at Risk Act (SARA) since 2003. The LbP was designated as Threatened by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC 2008) because it is one of a sympatric species pair, it is endemic to a single lake with a small index of area of occupancy (6 km2), it spawns in only three small streams, and it could quickly become extinct through degradation of spawning streams from increasing development around the lake shore and impacts of the dip-net fishery. Potential threats identified by COSEWIC (2008) were the introduction of exotic species and increasing eutrophication. Fisheries and Oceans Canada (DFO) undertook a Recovery Potential Assessment (RPA) for both LbP and SbP populations in 2010 (DFO 2011). Both populations were re-assessed as endangered by COSEWIC in 2018. The LbP was listed as endangered under SARA in 2019. Sampling of the 2014 and 2017 LbP spawning runs were used to estimate population abundance and provide advice on an appropriate population size objective, as well as the likelihood that the population objective could be achieved under current levels of mortality (DFO 2018). However, the abundance estimates for the 2014 and 2017 spawning runs were uncertain as genetic analyses of the 2014 spawning run indicated a mismatch between expected genotype and the minimum fork length used to differentiate LbP from SbP spawners. No genetic analyses were available for the 2017 spawning run. Recent genetic analyses of the 2017 LbP spawning run, as well as tissue sampling of SbP spawners in 2017 and 2018, provide an opportunity to review the minimum fork length that can be used to differentiate phenotypically between the LbP and SbP populations. The objectives of this Science Response Process were to: Provide updated abundance estimates for LURS-LbP in 2014 and 2017 based on genetic analysis of these samples undertaken in 2019. Review the recommended minimum fork length criterion for distinguishing LURS-LbP from LURS-SbP. Discuss how the genetic analyses and updated abundance estimates affect the likelihood that the updated abundance objective for LURS-LbP can be achieved under current levels of mortality (actual mortality levels are not known). This Science Response Report results from the Regional Science Response Process of November 19, 2019, and January 20, 2020, on Lake Utopia Rainbow Smelt, Large-bodied Population (LURS-LbP) Population Abundance and Allowable Harm Estimate.