Hi all,
Without seeing them hard to say exactly but they sound like nematodes.
Many species of nematodes.
Anisakis sp. is the cod worm ,(also found in all gadoids) , other stages in seagulls and seals. Probably the biggest and known to infect humans , if adults or eggs not killed by cooking. This is a marine species and does not occurr in freshwater.
There are a large number of freshwater nematodes which are found in many freshwater fish species including trout. These have not been found harmful to humans . No evidence yet anyway.
Best advice is to discard fish with them if you are worried. Most are found in the viscera anyway. Some do encyst in fish flesh though.
Nematodes are fascinating in that they are found in every environment known to man , even in high temperature mud geysers where that species seems to survive the cooking ...............
The Japanese , like the Dutch, have high infection rates of many parasitic worms , nematodes and tapeworms, due to their fondness for raw fish , Sushi and raw herring , cases in point.