I think the idea is that the migration instinct passed down by wild salmon is an epigenetic heritable trait. This trait is strongly selected for in wild salmon populations, but not in farmed populations. The constant influx of this maladaptive trait into the wild population is leading to an overall decrease in the ability of the wild population to successfully propagate.

They’ve bred more docile salmon, according to the article. Getting that DNA into wild populations would be problematic I’d imagine.