Farmed salmon eat fishmeal, but fresh ocean salmon (also called wild salmon) eat krill and shrimp. Is wild salmon kosher?
Salmon, whether raised by fish-farming or in the wild, are a kosher species of fish.
The fact that salmon, or any kosher species of fish, feed on species that are not kosher does not make them unfit or non-kosher (treif in Hebrew).
Similarly, many kosher animals raised commercially are fed "commercial feed" that contains non-kosher ingredients. Cattle may be fed animal by-products and offal. Ducks eat non-kosher insects and grubs. Chickens will often attack the weaker chickens in the coop and eat them. These kosher species of animals remain kosher for us no matter what they eat.