Allow me to explain. In vanilla Civ, I recall seafood resources only spawning in tiles that could concievably be worked by cities. The map generator made some errors, as the only city spot that could work them might be inside a mountain, but for the most part it worked.
The fact that the Kuriotates can work tiles further away from their cities is causing the map generator to label tiles that only they can work as "workable." This means that seafood is being placed further out to sea, which means that one of two things is true.
1) There are more seafood resources globally spawning. This means that the yields for the average coastal (not Kuriotate) city are the same, but it also means that there are more seafood resources to trade. This is a balance issue, as these resources shouldn't be this common.
-or-
2) The existing number of seafood resources are being spread further around. There are around 50% more viable tiles that seafood can spawn in, so the average coastal city has less seafood resources in range of it. The number of seafood resources that can be traded are the same, but the average coastal city is still worth less. Again, a balance issue.
Do you see?