hi mate, here is a tip. Go to Ringstead beach with a mask and snorkel, swim ten ft out, pick up said spider crabs take back to the beach and cook the claws on the barbie. You will then see the little buggers in a new light
and sadly they are here all summer
For what it's worth spider crabs, (Maja squinado), undertake an autumn/winter migration from coastal nursery areas to offshore sites where they overwinter in depths of more than 50m. Adults return to the coastal nursery areas during the spring. I can understand you thinking them a pain in the thingy, but try landing the next one that grabs your bait - they're absolutely delicious to eat.