Towards Accurate Age Determination of Greenland Halibut

Based on tag-recapture experiments, this paper shows that previous age determinations of Northeast Atlantic Greenland halibut from whole otolith surfaces greatly underestimates the age of older individuals. It also shows that the mean individual annual growth of adults is slightly below one cm per year. Surface methods are much more effective than other more time-intensive methods, which is an important consideration for use in stock assessment. The paper describes a new surface method that is in accordance with growth increments from tag-recaptures. The method relies on improved protocols relating to storing, imaging, choice of reading axis, and definition of annuli. The definitions of the first two annuli were validated by length frequencies of juveniles. The new reading axis and annuli of older otoliths were validated by tagging experiments involving injection of OTC, a chemical tag that incorporates into the otolith as a visual band marking the otolith size at time of release. With the new method, several measures of otolith size were correlated with age after correcting for fish length. This is expected for an accurate age determination method and was not apparent with the traditional method.