Inactive
Notice ID:NFFS7100-20-01681SRG
BACKGROUND & PURPOSE: Insufficient data on Eastern Bering Sea snow crab longevity and natural mortality estimates continue to drive instability in the stock assessment model used to inform snow crab m...
BACKGROUND & PURPOSE: Insufficient data on Eastern Bering Sea snow crab longevity and natural mortality estimates continue to drive instability in the stock assessment model used to inform snow crab management decisions. Natural mortality can be estimated from empirical relationships with maximum age, although this method is limited by the complicated nature of age determination in crustaceans. Radiometric methods have also been employed to estimate crab age, which assume a known decay rate of radioactive isotopes in snow crab exoskeletons. Radiometric shell age estimation using naturally occurring 228Th/228Ra is based on radium incorporation with calcium into the exoskeleton, which decays to thorium with time. 228Ra and 228Th activities can then be measured with ? or ? spectroscopy counting equipment. The isotope 228Ra (half-life t1/2 = 5.76 ?) displays a ? ray at 13.53 keV, while 228Th (half-life t1/2 = 1.912 ?) ? rays can be found at 84.4, 216.0, 131.6, 166.4 keV. The age of the specimen is determined as the time elapsed since the terminal molt (i.e. final molt to maturity) and calculated by the time (t) in years where ATh = activity of 228Th, and ARa = activity of 228Ra using the following equation: t = -4.12 ln (1 – 0.669 ATh/ ARa)
