Age determination of Northeast Arctic cod otoliths through 50 years of history

Abstract

Norwegian and Russian Marine Research Institutes have investigated the possibility that biases in age determination from otoliths may have contributed to the long-term trends observed in the Northeast Arctic cod stock productivity: changes in growth, maturation rate, fecundity, etc. To determine the effects of changes in age reading protocols between contemporary and historical practices, randomly chosen material from each decade for the period 1940–1980s was re-read by experts. The quality of old otoliths stored in paper bags for more than 50 years was also assessed. Although some year-specific differences in age determination were seen between historical and contemporary readers, there was no significant effect on length at age for the historical time period. A small systematic bias in the number spawning zones detection was observed, demonstrating that the age at first maturation in the historic material as determined by the contemporary readers is younger than that determined by historical readers. The difference was the largest in the first sampled years constituting being approximately –0.6 years in 1947 and 1957. Then it decreased with time and was found to be within the range of –0.28–0 years in the 1970–1980s. The study shows that cod otoliths could be used for age and growth studies even after long storage.

Keywords:

• age determination
• bias
• history
• Northeast Arctic cod
• otoliths

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark

Acknowledgements

The authors are indebted to all those who at different stages helped in the preparation of our manuscript. We greatly appreciate the helpful comments and suggestions provided by Dr Steven Campana and Dr Vladimir E. Kostylev that led to improvement of an earlier version of the present paper. Additionally, we wish to thank the two anonymous referees for their comments and suggestions, which have contributed to improve the paper substantially.

Notes

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark