The farming of fish such as salmon at aquaculture facilities poses a threat to marine ecosystems because

The farmed fish can escape and out compete wild fish for food and territory

Explanation:

At aquaculture facilities the salmon have been known to escape into the wild. Then the salmon that have always been in the waters find themselves fighting for food and their territory.

The farmed salmon poses many risks to wild salmon and possibly other species of fish. The main risk involved is the farmed salmon spreading diseases and parasites to the wild salmon. This can hurt the offspring and cause complications in the wild salmon for years to come.