There is a lot of exo skeleton & bone waste in most seafood. I think a decent size midden could accumulate in only a few months... (imagine the pile of lobster shells at any seafood restaurant in one night.)

Coastal may be difficult though; during the last ice age wouldn't the coast be 200 feet underwater now?

As to the question, did they eat shrimp... I think hunter/gatherers would eat ANYTHING they could catch. Now you just need some evidence they could catch it.

Jammer