Could be either Henneguya spp. or Kudoa spp. Both are myxozoan parasites.

Henneguya forms yellow or white cysts about the size of a grain of rice. Usually don't affect the flavor, texture, or safety of the meat but they don't look to attractive.

Kudoa forms white cysts, too, that can be large patches or streaks in the meat. After the death of the fish, the parasites release an enzyme that breaks down the muscle tissue and causes the flesh to become soft. It is fairly heat stable and may continue to breakdown tissue even if the meat is smoked.