Precision of the Northeast Arctic cod age determination under variable environmental and information conditions

For 16 years (1992-2007) comparative age-reading studies for cod were carried out by the two national laboratories: Polar Research Institute of Marine Fisheries and Oceanography (PINRO), Murmansk, Russia, and Institute of Marine Research (IMR), Bergen, Norway. Pairs (6786) of Northeast Arctic cod otoliths were read and compared to assess the precision of age estimation between the two laboratories. Cod otoliths which provide basic information for stock assessment were re-analysed to determine if there were any discrepancies since additional information on fish length may or may not have been available. Two groups of otoliths which were sampled during 1992 and 1994 were analysed in parallel: 285 otoliths with known length data and 661 otoliths without length data available. Knowing the length of the cod appeared to increase the discrepancies between the two laboratories. Analysis of long-term discrepancies in cod age determinations between two laboratories showed that variability of ageing is influenced by environmental variability (temperature and food supply) observed when the otoliths were formed.