The nutrients are captured shortly after leaving the pens. The reservoirs are nutrient deserts. As to bugs, they have to find a receptive host. Many of the bugs are rather species or stock specific.

For those with long memories, back in the late 80s VHS virus showed up in WA. Folks went bat**it crazy about this because in Europe VHSV was really deadly to Pacific salmon. So, WDF, the Feds, and Tribes sterilized a couple facilities and were even looking at nuking a whole watershed in order to ensure the bug was killed. The Atlantic Salmon net pens were loudly blamed for bringing it in.

Studies of the local bug found out that (1) it was actually endemic to the PNW, (2) it mostly attacked cod and herring, (3) it was not a problem for native salmonids, and (4) the Atlantics had nothing to do with it.

I worked for a number of years with the "big hunters" and basically we don't know what is out there because we don't actively look. We deal with local sites, local infections, and work to keep a facility healthy.