Insects that are found on the outside of a fish are prohibited, since they are assumed to have come from the outside.
By the same token, insects that are found in the intestines of a fish are prohibited, since they are assumed to have been swallowed and to have come from the outside 17.
On the other hand, insects that are found within the flesh of a fish, or between the skin and the flesh, are assumed to have been born inside the fish―and are permitted .
Is it not possible that an insect could have come from the outside, been swallowed by a fish and gone into the intestines of a fish―then migrated to the flesh of the fish? No. An insect cannot penetrate the intestines and travel through to the flesh 18.
However, if one purchased (or caught) a whole fish, and, in “cleaning” that fish mistakenly punctured the intestine―there could be an issue, since the worms may have been born outside the fish, entered the intestines, and may have gone into the flesh 19.