Marine and freshwater challenges to recovery efforts in an endangered Atlantic salmon population

Anadromous Atlantic salmon populations have experienced significant reductions in their global range. In the US, the endangered Gulf of Maine Distinct Population Segment is limited to eight populations. Successful recovery requires a robust, quantitative understanding of survival dynamics across life history stages. We studied the Narraguagus River population in Maine, utilizing smolt and adult trapping, as well as redd counts (1997 to 2021). This research helped separate and evaluate freshwater and marine survival rates and assess population dynamics. Concurrent stocking, habitat restoration efforts, and river ecology studies added additional context to our findings. We found natural spawning and conservation stocking produced an average of 1,700 smolts annually, which is significantly below recovery targets. Smolt timing shifted earlier in spring, with primarily age-2 (85%) and age-3 (14%) emigrants. Natural returns averaged 25 adults, with most returning after 2 sea-winters (2SW; 77%) or 1SW (16%). Naturally reared smolt-to-adult return rates were low (0.96%) for 2SW fish, though this rate is 20 times higher than that of hatchery-reared smolts. Low marine survival rate is a main obstacle to recovery. System-wide freshwater production is below conservation targets, despite stocking efforts and restoration of access to historic habitat. Currently, management practices are maintaining a hatchery-dependent population with limited natural spawning. This population is essential to the future of Narraguagus and other US Atlantic salmon populations as part of a living gene bank that utilizes river-reared broodstock sourcing. Rebuilding populations will require concerted, innovative strategies to improve hatchery fish performance in the wild, alongside habitat rehabilitation focused on cold-water conservation, thermal refuges, and habitat quality and complexity.