Fish Radio

Crab ages revealed for first time

February 26, 2016

Age bands on eye stalks of crustaceans Credit:

Age bands on eye stalks of crustaceans

This is Fish Radio. I’m Laine Welch – How old is that crab?  The mystery could be solved…I’ll tell you more after this —

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Knowing the age compositions of marine stocks is crucial to sound management. Fish can be aged easily by examining their ear bones or scales. Not so with crabs, because they molt.

For years it’s been assumed that crab that don’t retain their hard parts throughout their lifetime due to growth by molting at which they lose their lose their exoskeleton and it was always assumed everything went with that. 

Joel Webb is with the AK Dept. of Fish and Game’s Age Determination Unit in Juneau. About three years ago, he says researchers in Australia and Eastern Canada produced evidence to the contrary.

Parts of the stomach- the crab and shrimp stomach and the eye stalks are retained through the molt and my be retained through the lifetime. And if you process those structures into very thin sections and look at them under a microscope and shine light through them, there are band patterns present in those structures similar to rings in a tree, or similar to otoliths or scales used to age fish. 

Webb says researchers are always trying to determine how many crabs are dying of natural causes, like old age, because that death rate is factored in to fishing quotas.

It’s a key parameter – plus when you know how big an organism is and what age it is, you know fast it grows. So those two things – the growth rates and mortality rates are key pieces of information for fisheries management and stock assessments. 

Fish and Game has funded a study to apply the aging technique with red king crab, Tanner crab and spot shrimp from Southeast Alaska. Preliminary evidence is showing promising results. it might be three to five years before the aging process transfers to the fisheries, but Webb says it will be transformative.

It’s a phenomenal thing because the availability of age information is transformative for what we know about how these organisms grow and survive. Those are two key pieces of uncertainty as to how we currently manage and assess these populations and set our harvest rates.  The availability of accurate information would shift the paradigm in what we know. 

Researchers estimate it takes king and Tanner crabs five to six years before they are big enough for harvest. Soon, they’ll know for sure.  Find more crab stories at

Fish Radio is also brought to you by Ocean Beauty Seafoods.  Ocean Beauty has contributed over 10 million meals to the U.S. Food Bank network, and is committed to ending hunger in America.   In Kodiak, I’m Laine Welch.