Atlantic Salmon (Salmo salar ) are one of the most heavily cultured fish in North America, as this practice presents an alternative to supply the growing global demand for marine food resources. However, this process has resulted in highly domesticated salmon, with notable genetic and phenotypic differences relative to their wild counterparts. Due to this divergence, interbreeding of farmed Atlantic salmon with wild salmon may result in the expression of maladaptive traits for life in the wild,
affecting local adaptation. Therefore , it is essential to understand how genetic interactions with farm escapees affect wild populations at local scales. Yet, the few studies that have quantified survival and phenotypic differences among wild, farmed, and hybrid individuals in nature
have been done in Europe or at limited scales. In this study, survival, growth, and the genetic impacts of farmed Atlantic Salmon introgression on a wild population in a natural environment in southern Newfoundland were measured.
A release experiment was conducted in three sites in the Garnish River system, located near an area of intensive aquaculture in the Burin Peninsula. Four cross types were produced in 2017 and released in July of 2018 (pure wild, pure farmed, farm-mother hybrids [F?hyb], and wild-mother hybrids [W?hyb]). Recapture occurred in the fall of 2018, 2019, and 2020, where samples were weighed, fork length measured , and caudal fin tissue collected for later use in parentage analysis. A panel of 25 microsatellite loci with a total of 277 alleles was used to assign individuals back to their cross type and family. With this, the proportion of wild, farmed, and hybrid individuals at recapture over two years was quantified and their differences in survival and growth were analyzed. Furthermore, the differences in survival over time among families of each cross type were studied.
Th is builds on the previous survival and growth analyses done the first summer after release , adding two further years of dat a, as well as the family composition during this time.
Survival, measured as odds of recapture, differed significantly among cross types in certain sites, with farmed and
W?hyb having consistently higher odds both years. However, the sites where this difference was evident differed over time. Size at recapture differed significantly among cross types in sites 1 and 3 one year after release (mean length: p < 0.001 ; mean weight: p < 0.001 ). In both sites, farmed and W?hyb were significantly larger than wild
and F?hyb, with no notable differences between one another. The family specific analyses demonstrated that the composition of families remained relatively consistent from 0+ to 1+ and then to 2+.
These results indicate that direct genetic interactions between farmed and wild Atlantic Salmon can impact the survival and fitness of wild populations, contributing to their decline over the last decades.