Report of the Seabass (Dicentrarchus labrax) Otolith and Scale Exchange Scheme 2011
For seabass (Dicentrarchus labrax), Otolith and Scale Exchange Scheme 2011 was the first exchange. A total of 155 fish from Eastern English Channel (ICES area : VIId) was sampled on-board French research vessels (Gwen-Drez and Thalassa) during 2 international surveys (Channel Ground Fish Survey and International Bottom Trawl Survey). The length range of the fish was between 17 and 74 cm, with a mean length at 46.99 cm. For each fish, the Sagittae otoliths and few scales were used to compare the age estimation between both calcified pieces. 4 readers were participated from UK England (1 reader) and France (3 readers). Only images were used during this exchange. There were noted by the readers that it was very difficult to obtain an image of the scale with the same quality on its whole surface. On 4 readers, 2 readers used transmitted light for the otoliths and 2 readers used reflected light. There was no preference to the type of light. The analyses did not show a high mean precision of age estimate for individual fish with Coefficient of Variation (CV) of 13.1% and percent agreement to modal age of 54.1%. Among 155 fish, only 2 were read with 100% agreement (1.3%) and thus a CV of 0%. During this exchange, 2 different calcified pieces (otolith and scale) from the same sampling were analysed. The results showed the same precision of age estimation from the otolith (percent agreement = 60.1 ; CV = 12) or the scale (percent agreement = 62.3 ; CV = 12). However, this exchange showed that the age estimation from the otoliths was different than this from the scales.
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