Neanderthals ate sharks and dolphins

[Sunlei]

3. tide goes out and there's a gap in the ice where shellfish are easy to gather

Sea animals of course swim close to lands and could be easier to trap or beach. I think this ancient Inuit method to gather mussels was much the same for Neanderthals when they gathered shellfish in their iced over lands.

Inuits in northern Canada risk their lives to gather mussels under the frozen sea ice. But it's dangerous - the tide is only out for 45 minutes and the ice could collapse on them at any moment.