Depending on where you source your omegas from, potentially zero impact!
To be clear my preference would be to source n3s from algal supplements and, once food safety testing for humans is complete, n3s from GM rapeseed.
In time I hope we end up with lab meat/plant-based meat alternatives that use these n3s so we can get the benefits of fish without the environmental and ethical concerns of getting n3s from fish.
If from menhaden, there's a raging debate on the one hand about trout, the Chesapeake Bay (Maryland and Virginia), the ecology and environment more broadly, and the other hand a Canadian company based in a small rural county in Virginia (Omega Protein, which, BTW, does not provide year-round benefits to all of its employees which creates a drain on already super limited services and supports. Omega Protein is not alone in this.).
I don't know enough about any of this to have an informed opinion, but I do understand that menhaden put Reedville, VA on the map.
What would it mean for salmon stocks and increased environmental damage?