Identifying and prioritizing barriers to Atlantic salmon habitat connectivity in Napu’saqnuk (St Mary’s River), Mi’kma’ki (Nova Scotia, Canada)

Abstract

Atlantic salmon (Salmo salar) are an anadromous fish species that migrate from headwaters of their home rivers out to the open ocean and back; this ecologically, economically, and culturally significant species requires extensive and well-connected freshwater habitat to successfully reproduce and maintain viable populations. However, the installation of road culverts can fragment aquatic habitat and impede Atlantic salmon from reaching their spawning grounds. In mainland Nova Scotia, Napu’saqnuk, known as the St. Mary’s River, represents important habitat for a population of Atlantic salmon that has been assessed as ‘Endangered.’ Removing and remediating culverts that cause habitat fragmentation in the area is therefore of utmost importance, however it can be time and resource intensive. Through field assessments in the West Branch of the river over the summer of 2023 I collected data on 75 culverts’ abilities to successfully pass Atlantic salmon and general information on their structure and state of function. I have combined this information with remotely collected landscape scale characteristics of each site to further investigate the relationship between culvert characteristics and passability. Logistic regression modelling identified the characteristics of shape, diameter, change in elevation, and position in the watershed to be the best predictors of a culvert being able to successfully pass salmon. The results of field assessments indicated that the river continues to experience severe fragmentation, with 77% of culverts assessed to be a partial or full barrier to fish passage. Culverts previously recommended for remediation by the St Mary’s River Association in 2009 were revisited to find some had been removed or modified; other sites continue to be legacy barriers potentially fragmenting the watershed habitat for up to several decades. Using collected data on fish passage and estimated upstream habitat, I prioritized additional sites in the West Branch for further assessment and remediation. My research provides a framework for future culvert assessments and remediation in this important watershed by leveraging data collected through a combination of fieldwork and remote sensing.