Perhaps someone can explain the reason for the hundreds of dead crabs discovered recently (01.08.2011) at Aberffraw beach on the North Wales coast. I can't say for certain just how long they had been there but long enough, presumably, for some decay judging by the smell and the amount of destruction caused by scavengers. The attached images may help shed some light:
The crabs are Spider crabs which live below low water mark (I occasionally find a live juvenile at extreme low water of a spring tide). In winter they are in deep water of down to 100metres or more. They move into shallow water in summer. Numbers fluctuate from year to year and sometimes it seems like an invasion. The hot year of 2003 was such a year in Cardigan Bay, and, from your photos, it looks as if that is the situation this year. Sometimes they form living mounds, off shore, of 100s or 1000s of individuals.
I think the numbers of dead are just an indication of great numbers off shore nearby- maybe they wander into the shallows, get stranded as the tide recedes and die rapidly in the sun because they are not adapted for shore life. It might be tempting to suggest they have died because of recent high water temperatures, but they live well to the south in warmer waters, so I don't think that is the case. If it were a pollution incident I'd expect other species to have suffered too.
These are the biggest spider crabs in our waters, Maja squinado. I would agree with Ian on this one. I have encountered these mass congregations myself whilst diving the south coast. If these huge numbers of crabs get so caught up in "a breeding frenzy" that they get stranded on an outgoing tide, I can well understand how successive tides might scatter them in huge numbers around the local beach, as in the photographs.
On a similar but different tack, on a dive from a harbour years ago I found the sea bed absolutely littered with dead edible crabs, Cancer pagurus. Dead crabs everywhere. I could not understand it. But after examining a few, the reason was obvious.
These were a fisherman's "by-catch". Every single crab I looked at had had its front claws wrenched from its body - not as a crab would detach its own leg or claw, but this was a wrenching-off carnage caused by a fisherman only after crab claws. The rest of the animal was simply by-catch and thrown back. With injuries such as this they will have, and did die. What an obscene trade.
Never buy crab meat unless there is brown meat in there as well.