Pas yisroel and yoshon can be optional, but isn't big checking necessary?
Checking for the worms in fish is a machlokes. They are pretty large and gross. But since they grow in the flesh of the skin of the fish, it is debatable if they are halachicly prohibited.
Some poskim hold they are o.k. to eat, and not everyone checks for them even when they know they are present.
They are very often found in white fish, and in wild salmon. The farmed salmon never had these worms. (Costco sells both types of salmon frozen. The farmed is worm free.
Personally, even if they are kosher, I would never touch fish with them.
When they are present, the expert knows how to locate them in the flesh and remove them. They are very hard to find and remove. They are alive and can blend in to the fish.
In this picture, the expert removed some of them from the white fish.