Apparently spider crab numbers have grown in southern English and Welsh waters, they don't know why that is...
Is it me but don't spider crabs look like the type of crab the Japanese love to eat, I think they're called snow crabs? I lived in Japan for a year and had a couple of legs at a restaurant, it was delicious. These snow crabs (if that's what they're called?) can be absolutely massive, like something from a sci-fi film!
You said about not having a market for spider crabs...in the UK as well as not really eating spider crabs we don't eat green shore crabs either. I think people are put off by them because they're small and also they might be confused about whether or not you can eat them. We have a large crab called an "edible crab" and maybe this has led to people here thinking that it's the only crab you can eat in British waters. Tons of green shore crabs are caught and exported to France, Portugal and Spain where the people there tuck into them!
I often wonder if it would be better if we could expand our culinary tastes to include more of our domestic species of fish and shellfish? Perhaps this might take pressure off the most popular stocks? As an example limpets don't look very appealing and they're rather rubbery and tasteless but if you chop them up very finely they can make a passable chowder!
On a side note...I have a Wenger SAK as my EDC. I take it fishing with me because the scissors have little serrations and are quite good for cutting the braided line that I use for lure fishing. Anyway, I've been very impressed at how it has resisted rusting when I've waded across estuaries and had it soaked. It's even been put to use scaling and gutting sea bass, very good value for money IMO!