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.