Perfectly serious food question

 The British Isles, being islands, are pretty much surrounded by ocean.  In the past, and the not so distant past, parts of those islands have suffered famine of various degrees of severity.  Yet there are marine food sources that are/were as far as I know virtually untapped even in periods of dire poverty.  Mussels are an obvious one.  The British and Irish coasts are covered in the things yet they aren't (or at least weren't) much eaten and resources like jellyfish have, as far as I know, never been harvested.  I'm sure there are lots of other examples; kelp,whatever.  Any insights?