Just a theory here but maybe they write "may contain - __________" as a catch all statment so they are able to source a variety of proteins and use the cheapest ones. This month/quarter fish protein happened to be cheaper so your gainer includes more of it.
Also, found this on line:
"Fish Protein Powder is a new source of protein to replace overused soy and other protein powders. The powder can be added to a variety of food products to provide a healthy source of easily digestible protein. Bakery and pasta products are particularly amenable. The fish protein can be added to weight loss and protein supplements and can be used as powder capsule filler."
Apparently some Norwegian company has come on the market offering it at a "good" price.
No idea what that would be compared to bulk milk, soy, egg proteins.