Report of the Workshop on Age Reading of North Sea Cod (WKARNSC)
The overall result of the workshop exercises is that there are 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 seems to be the problem.
The image analysis exercise clarified that the lack of agreement can be referred to the perception of the first age structure (O1) and the very widespread appearance of some age structures resulting in several translucent bands within one year (split rings).
Exploring the application of image analysis, the group agreed that applying such tools in the routine age estimation of North Sea cod may prove very valuable. It gives the opportunity to use metrics to rule out doubt when defining the age structures to count and also gives a very useful exchange tool for the individual readers both within and between laboratories.
The workshop achieved quite a lot in terms of ironing out, through discussion and calibration, some of the major problems in ageing otoliths of North Sea cod. The group reached agreement on a definition of an ageing protocol/guidelines mentioned in the present report and the aim is to employ these guidelines to eliminate some of the problems with e.g. split rings in the otolith structures.
A collection of Agreed Age cod otoliths were started at the workshop. The reference collection will have to be expanded considerably through exchange of otoliths and images. Additionally, the collection of agreed age otoliths should not stand alone, but be a part of a larger compilation of data on ‘typical’ otoliths for the species and area, in which measurements of O1, typical distances between age‐structures, edge development over season, and general growth curves for the sub‐stocks of cod in the North Sea.
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