Revised Report of the Horse Mackerel Otolith Workshop
Objectives of this Workshop:
A validation of the ageing reading method is essential and a short review on how this was achieved should be given.
At the beginning of the Workshop the terms accuracy, precision, bias, average age, modal age and agreement, etc. should be explained to the otolith readers. The results of the 1996 Horse Mackerel Otolith Exchange should be discussed and explanations should be given to the otolith readers on how to improve their age reading techniques.
The processing methods for the otoliths have to be described and discussed for the different fishery institutes, because these methods determine what quality of age readings can be achieved.
A synopsis of the horse mackerel biology should be presented to indicate how the horse mackerel (Trachurus trachurus L.) is distributed in the northeast Atlantic area, how the three stocks are distributed, on what basis these stocks are separated, at what time adult fish migrate from overwintering area to spawning area, to feeding area and back to overwintering area, when the periods of pre-spawning I spawning and of feeding I non-feeding are, how the juvenile fish are distributed over time in these areas, etc. Compiled information on the monthly deposition of translucent and opaque material on the otolith edge should be available by age group to all otolith readers, because the time of the opaque and translucent material deposition is dependent on the age of the fish and the time of the year.
This biological background of the horse mackerel should be known for correct interpretation of the ring structures and otolith edges. Once this biology is understood by the otolith readers, training in otolith reading can be started by using only 'known' age otoliths with only translucent edges (exclusively caught in the first half of the year). Then the age readings are not dependent on the otolith edge interpretations and therefore differences in age readings can only be related to the interpretations of the rings (annuli) within the otoliths. The results can be discussed immediately after reading in order to correct the ageing techniques of the individual otolith readers.
Once the interpretation of the inner rings does not cause problems in the interpretation a start can be made with the training in otolith reading by using only 'known' age otoliths with a mixture of translucent and opaque edges (exclusively caught in the second half of the year). Then the age readings become dependent on both the otolith edge interpretations and the interpretations of the rings within the otoliths. The results can be discussed immediately after reading in order to correct the ageing techniques of all individual otolith readers.
This training is very important to remove the bias in the age readings of especially the older fish (improving the accuracy). However, in addition special attention will be paid to the improvement of the precision (reducing the variation in the age readings). During the otolith exchange it became apparent that some readers were able to achieve a high precision. It is therefore important that they try to explain to other readers how they are able to achieve this.
A manual on horse mackerel age reading will be prepared, which will provide the guide-lines to experienced and inexperienced otolith readers on how to read and interpret the rings and the edges of the horse mackerel otoliths in order to achieve a high accuracy and a high precision. Recommendations will be given on how the quality of the age readings can be improved.
At the end of the Workshop it will be tested how well all readers can carry out age readings on horse mackerel otoliths. The Workshop will provide information on the bias, accuracy and precision of the age readings by age group for each individual otolith reader and for the whole group of otolith readers combined. Furthermore the changes. in bias, accuracy and precision compared to the beginning of the workshop will be given.
Errors in the age readings affect the stock assessments of horse mackerel. It should be determined to what extent these assessments are affected.
Published under the auspices of the following ICES Steering Group or Committee
- Historical content