Cod SD 22 Age reading exchange December 2015

In 2014, concerns were raised by WGBFAS when it was noted that the age structure of catches and survey data of eastern Baltic cod were diverging. These concerns echoed the opinions of expert groups who for many years have struggled with standardising the age interpretation methods used for these stocks due to the obscurities observed in the fundamental otolith structure. In 2014, an otolith exchange for Eastern Baltic Cod was initiated by the Thünen Institute of Baltic Sea Fisheries (OF), Rostock, Germany and coordinated and analysed by DTU Aqua National Institute of Aquatic Resources, Denmark. For SD 22, the average percentage agreement was poor at just 70% with a high coefficient of variation of 23.4%. The main issue being that the two German readers estimated the same age while the Danish reader estimated the fish as one year older due to the misidentification of the first winter ring. However, the result confirmed that the otolith structure observed in cod from SD22 complies with the general requirements for age estimation. Presently, it is the only area within the Baltic from which it is still possible to read cod otoliths by traditional age reading methods. WGBFAS 2015 agreed that a yearly otolith exchange will take place for cod from SD22. The results of the current exchange indicate that the problems with the mis-identification of the first winter ring are almost resolved. The data will be used by WBGFAS 2016 to assess the error in the age information and to implement an age error matrix in the assessment following the general recommendations outlined by WKSABCAL 2014.

The current age calibration exercise was coordinated and analysed by Julie Coad Davies of DTU Aqua National Institute of Aquatic Resources, Denmark. Samples were selected by the Thünen Institute of Baltic Sea Fisheries (OF), Rostock, Germany.