Progress on an old question for stock assessment of the edible crab Cancer pagurus

Abstract

Stock assessments for the edible crab Cancer pagurus have been limited by an inability to determine its age. We applied neurolipofuscin-based ageing techniques to this problem for the first time. We constructed a neurolipofuscin concentration frequency distribution for a sample of crabs from the western English Channel and found clearly resolved modes, which we interpreted as annual cohorts. These enable the calibration of neurolipofuscin concentration to chronological age under natural conditions and in the absence of known-age microtags. The resulting crab age determina- tions offer an unprecedented demographic perspective of the most important English regional crab fishery and substantially reduce uncertainty in previous growth estimates. A cubic von Bertalanffy model is the best fit to the new size-at-age data, reflecting progressive growth in early juveniles. On average, recruitment to legal size in both sexes is at 4+ yr, but individual growth is highly variable so that size recruitment is protracted across ~5 year-classes. C. pagurus lives to ~9 yr in the western English Channel; natural mortality rates of ~0.48 are higher than previous estimates. Age composi- tions indicate past year-class strength fluctuation, with a particularly strong settlement in 1995 — a relatively warm year. Reinterpretation of size compositions in light of the neurolipofuscin results suggests that it may be possible to detect some cohorts in stratified adult size frequency data. These findings will strengthen the basis for sound management and sustainability of this important bio- resource and will provide guidance for wider application of the neurolipofuscin ageing method.