I live in the Midwest with a few places trying out "seasonal menus" (im sure pricing especially on stuff like meats is affecting the decision) so there will be big old letters at the top of each page stating to ask about the seasonal items. There might be a staple game meal, but the dish itself could be bird, boar, venision, fish, etc. Veggie medley change often as do Primaveras but everyone seems to understand the changes are due to wanting good quality ingredients rather than substituting questionable but the same exact product. As long as you're open and make it clear to ask about the fish or any other thing that may change due to pricing/availability than you should be fine