Report of the North Sea cod (Gadus morhua L.) otolith exchange 2009 - 2010

The last North Sea cod otolith exchange took place in 2005/2006 (Woods 2006). The results were presented at the WKARNSC Workshop (ICES CM 2008/ACOM: 39). The overall result of the exchange was that there were significant variations in North Sea cod age estimates between readers. Both low precision, and relative biases between readers were found and overestimation of fish ages seemed to be the problem. The Planning Group on Commercial Catch, Discards and Biological Sampling (PGCCDBS), recommended a small-scale exchange of North Sea cod otoliths in 2009 and 2010. The Institute of Marine Research (IMR), Bergen, Norway, agreed to coordinate the otolith exchange and evaluate the results.

Otoliths from two regions were included in this exchange:

- ICES Division IVa, collected in Q2, Q3 and Q4

- ICES Division IVb, collected in Q2 and Q3

Institutes participating use basically two different preparation and reading methodologies. Some institutions break the otoliths manually and read the broken surface with reflected or partially transmitted light (light form the side of the otoliths with the broken surface shadowed). The other institutions prepare otoliths sections by sawing, and mount the sections onto glass slides. Both methodologies are covered in this exchange. Both sagittae otoliths of the selected fish are used, one have been broken manually, the other have been sectioned using a saw. Direct comparison of fish age using the different methodologies is thus possible.

The objectives of the exchange were to:

- investigate the levels of agreement on age readings

- analyse the relative differences between reader ages