It's more a derivative of what they 'normally' obtain in the wild, so that in effect, they have evolved making use of certain foods. Fish often come across insects, insect larvae, algae, other fish, and depending on the species, many more odd things like small birds, clams, shrimp, small monkeys, nuts, and fruit, to name a few.
Bloodworms are a marine species, and one may ask if fresh water fish (which descended from marine fish) have retained some ability to use bloodworms as a proper food source.
However, for most fish, mammal flesh is inappropriate on many levels, including fat content, digestability and probably specific proteins (i.e., amino acids.) There are simply many alternatives.
