Massive Crab die-off at Mukilteo? Actually not, but rather an interesting natural phenomenon highlighting a key stage in crabs’ lifecycle. . .
By Don Velasquez, WDFW shellfish biologist
A number of people have called WDFW to report dead crabs on the beach at Mukilteo Lighthouse Park, some as early as last Sunday.
This is a case where currents and winds have concentrated the molted shells of Dungeness crab that have grown to the next size.
The majority of these are not dead crab as some may have assumed.
Unlike us, crabs and shrimps must shed their old shells in order to grow to the next size.
In this amazing process, a crab’s shell splits and it literally backs out (legs and all) with it’s soft new shell that was hidden underneath. The old shell is cast off, is empty of meat, and only has some brown gill linings inside. The new shell swells to a larger size and during the next few weeks this crab is vulnerable as the shell hardens. For example, the new shell of a male Dungeness crab grows about an inch in width when it molts to legal size.
Similar events have happened at this Mukilteo beach before. In 2015, I noted a number of similar events all through the Whidbey/Camano waters before crab seasons began. As it turned out, that was a sign of good things to come and it was a record year for total harvest of Dungeness crab by the end of that season. There are some common features of these molting events:
· Almost all the shells are collected at the wrack line as in this picture, indicating these are lightweight (empty molted shells) and likely without meat inside.
· Birds are not obviously tearing at these, also suggesting they have no meat inside. Birds appear to be able to discriminate between a molt shell and a dead crab.
· Most shells are of the similar size and same sex of crab. Crabs of the same sex and size molt often around the same time. (Shells observed from Sunday were predominantly male Dungeness crab shells 4.9–5.7 inches wide.)
· When systematically counted, there may be a few dead crabs mixed in but the vast majority are molted shells. Molting is a stressful growth event and a few crab do not survive it. This was the case at the beach at Mukilteo Lighthouse Park.
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We want to thank those who alerted us to this phenomenon recently. These calls indicate real care and concern for our environment and we really value the effort and time people put in to report this to us. Thank you!