In Numbers 11:22 it states, "If sheep and cattle be slaughtered for them, will there be enough? If all the fish of the sea will be gathered for them, will there be enough?" From this quote we can understand that sheep and cattle require slaughtering, however, fish require only "gathering" and do not need to be slaughtered. They may be killed in any fashion.
The reason for this is given in the Talmud (Chulin 27b).Animals and birds are considered a higher level of creation than fish.
A reliably certified product marked DE can be eaten after, but not with meat. If served at a meat meal, it should be eaten with disposable utensils.