Red mullet (Mullus surmuletus) and striped red mullet (M. barbatus) otolith and scale exchange 2011
There were an exchange and a workshop in 2009 of Red mullet Mullus barbatus and Striped red mullet Mullus surmuletus (WKACM). The Otolith Exchange Scheme 2011 was the second exercise for the Red mullet Mullus barbatus and Striped red mullet Mullus surmuletus. 5 readers participated in this exchange. The images collection (n=540) came from the Bay of Biscay, the Balearic Islands, the Southern Spain and the Southern Adriatic sea. 175 Mullus surmuletus and 202 Mullus barbatus have been examined to all Institutes. Among 540 images, there were 377 otoliths and 163 scales. 2 images sets have been realised for Mullus surmuletus in Balearic Islands and for Mullus barbatus in the Southern Spain to compare the age estimation between both calcified pieces. Results among sets of calcified pieces showed a large difference (Tab. 4 and 5). A set of Mullus surmuletus otoliths from the Bay of Biscay presented better percentage of agreement (82% ; Tab. 5). On 75 otoliths, 34 were read with 100% agreement (45%) and thus a CV of 0%. The modal age of these fishes was comprised between 0 and 3 years. The second set with good percent agreement was composed by otoliths from Mullus barbatus from the Southern Adriatic (68.6% ; Tab. 5). On 102 otoliths, 20 were read with 100% percentage agreement (19.6%) and thus a CV of 0%. The other sets of this exchange, for both species, showed a very lower precision under 50% percentage agreement. The reason of these results was not species while it occurs in both species. In Balearic Islands and southern Spain, we compared readings from otoliths and scales but the difference observed between calcified pieces did not explain these results. In Balearic Islands and southern Spain, the results only from otoliths presented respectively coefficient of 37.6% and 48.5% and percentage agreement of 47.3% and 52.8%. Difference of precision could be largely due to the sampling area, indeed (it is easier to estimate age in the Atlantic than in the Mediterranean sea). This is due to the environmental characteristics (e.g. higher seasonal differences in the temperature)generally in the otoliths of Atlantic stocks results in the formation of more clear and distinguishable annuli than their Mediterranean counterparts. Moreover difference of precision could be due also to the and the composition of the samples (all Mullus barbatus from the Southern Adriatic presenting 100% percentage agreement, had the modal age at 0 year. Inversely, modal age of the fish from the Balearic Islands and the southern Spain began to 1 year). Indeed one of the critical point for the ageing in Mullus barbatus is to distinguish the first hyaline ring from(ICES, 2009). It should be noted that there were certainty of bias among readings from otoliths and from scales and modal age. Moreover, there is certainty of bias between the readings from different calcified pieces of the same fish. The images of Mullus surmuletus in the Bay of Biscay were almost all classified into AQ2 (91.6%). For the others sets, Quality level 2 decreased for Quality level 3 (Difficult to age with acceptable precision). In this context where the lack of validated age data is a major source of uncertainty for the application of analytical stock assessments models, as well as to the reliability of the assessment results, the age validation and calcified structures formation studies should be of high priority.
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