Well, there actually are seasonal variations among certain species of fish...Just saying, you know.
Yes, but these migrations often do not coincide with the crop harvest season. Cod, for example, migrates in the winter, and medieval Europe traded heavily in dried and salted cod (as well as other stockfish), as it lasts for several years without refrigeration. Overall, I think food production from the sea would be constant over the seasons, as "fishing" represents a variety of seafood collection (including perhaps whaling, I imagine, and I can't even begin to tell you how lucrative and useful that is), rather than a particular species of fish.
