Oh yes they will!
My dad lost an entire pond full of expensive Koi in less than a week while on vacation. Many of the Koi were to big to eat but they killed them trying and many were left floating or drug onto shore.
Koi is a useless trash fish any way. The birds should get a little corn as a reward for destroying an invasive species.
May be the DNR could teach them to make some kind of motorboat noise when they fly close to the water and catch those imported jumping koi/carp aquarium trash lose in the rivers.