In every store I have ever worked in, the morning seafood clerk sets the case, trays fish, runs frozen fish, and basically "manages" the seafood department. The evening seafood person waits on the service case mostly, but is also responsible for helping out a lot more with the meat department side of things. You're really a closing meat clerk, not a seafood clerk in the evening in other words.
The morning is a lot more responsibility, but also in my opinion a more satisfying position that can lead to advancement. The evening clerk, like all evening clerks, get the **** end of the stick.
If you open, you'll be responsible for making sure everything is filled. If you do the closing, you'll be responsible making sure everything is cleaned up and put away.
6 to 2 is a great shift to me. Less customers to deal with. Lots to do to keep busy and make the time fly. I like getting off early so you still have some day left to do something. People in other departments wait years to get a day shift.
I'd take the day shift too. You'll learn more about how to set up the seafood counter and basically how to run the department. Do you have a seafood lead where you are? It might be a good opportunity to become a lead, and (don't quote me on this i'm not sure how meat/seafood works) possibly open doors into becoming an Assistant Meat Manager someday.
Always take an opening shift if you're offered one. If you work closing shift there's a higher chance of you encountering a customer. And if you don't tell that customer your life story you will get written up.
From what I've understood, there's a seafood lead here too. Leaning towards wanting to open heavily even if the hours just don't support it for me. Just have to make sure I'm given the job.