I grew up in the SW and cockling / musselling / seine(sp?)-netting for whitebait was a popular childhood past time. I think the idea of 11-15 year olds swimming around in a big semi-circle dragging a net behind them would give the HS Exec a heart attack!
Don’t clean them in fresh water. It kills them and they quickly go rotten.
Fresh sea water is best or if not, salted water to the same ratio as the sea (can’t remember that).
We always put a handful of oats in. No idea why. Probably nonsense but my grandfather had been doing so his whole life too.
It’s worth de-bearding them and scraping the barnacles off. It’s the extra crap that can give the sauce a bitter taste. If you’re killing animals you should put the extra effort in to maximise their deliciousness!
Biggest isn’t always better as they can be a bit tougher. On the other hand, it means they’ve had chance to breed ie. keeping the population sustainable.
You identify the dead ones post-cooking. If they haven’t opened, chuck them.
Cockles are best pickled for accompanying fish and chips.