Cooking fish dishes
Why does a fish need a certificate?
Fish is classified as a neutral product ("parve"). However, it is still forbidden to eat dishes prepared from fish and meat in a mixture - not for reasons of kosher, but for health reasons. For the same reason, they avoid cooking and eating fish with milk.
The kosherness of canned fish should be attested by a reliable "ehsher": apart from the kosherness of the fish itself, it matters in what oil and with what additions it is preserved.