Growth and formation of annual zones in whole otoliths of Greenland halibut, a slow-growing deep-water fish

There is currently no generally agreed and validated method for age estimation of Greenland halibut (Reinhardtius hippoglossoides) from otoliths, and ageing of intermediate sized individuals of this deep-water flatfish is considered particularly uncertain. To estimate otolith growth and annual zone formation in this size range, a large oxytetracycline tagging experiment was undertaken in 2005-08 in the nursery grounds north of the Svalbard Archipelago. By January 2015, 89 of the recaptured fish had both been at large for between 1 and 6 years and had reliable length measurements at release and recapture; 29 of the recaptures were returned with whole otoliths and with a chemical time stamp that allowed identification of otolith growth patterns during time at large. Four age readers interpreted the otoliths without knowing the position of the time stamps. The expected number of zones during time at large was between three and six for 79% of recaptures, and the mean reader bias was only 0.04-0.56 years. Juvenile growth was quantified and the data were analysed in relation to sex and recapture area. This study contributes to a fully validated ageing method for use in stock assessments of this commercially important species.