Results of the anchovy (Engraulis encrasicholus, L.) otolith exchange programme
The 1995 Working Group on the Assessment of Mackerel, Horse Mackerel, Sardine and Anchovy agreed to carry out an anchovy otolith exchange programme in order to compare otolith age readings made by the scientists of the different countries, which fish anchovies.
In total, 650 pairs of otoliths were exchanged, of which 200 were from ICES Division VIIIab, 250 were from ICES Division VIIIc and 200 were from ICES Sub-division IXa North, and three readers from the two countries (France and Spain) took part in the exchange. The comparison of otolith readings of the three areas was carried out separately.
Taking into account the few ages read, due to the anchovy being a species with a short life-span, the average general agreement between readers was unsatisfactory in Division VIIIab (71%) and VIIIc (80%). In Division IXa North agreement was greater than in the other two areas (96%) owing to the sample of otoliths consisting of specimens of one single age. Agreement between readers varied between 59% and 88% for the Division VIIIab, between 77% and 82% for Division VIIIc and between 94% and 97% for Sub-division IXa North.
Standard deviations by age groups increase from age 2 for the Division VIIIab and VIIIc area sample. In Division VIIIc the standard deviations also increase in age 0.
The age bias plots of each reader against the modal age show variability in the samples from Divisions VIIIab and VIIIc. The age bias plots for all readers combined show that bias increases with age, and that those from age 2 tend to be underestimated in Division VIIIab and that those of age 0 tend to be overestimated in Division VIIIc. In Division IXa North the level of bias is very low.
Considering the importance of age readings in assessment, new investigations are needed to increase agreement, and to standardize ageings.
Published under the auspices of the following ICES Steering Group or Committee
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