Use of otolith quality flags to assess distributional dynamics in Baltic cod stocks

In the Baltic Sea, cod spawn in several basins separated by shallower sills. The mixing dynamics between two cod stocks and their components remain largely unclear, yet such mixing has gained attention in recent years because signs of recovery in the eastern Baltic cod population suggested spillover into the western basin. In the present study, we assessed whether quality flags (QF) of cod otoliths (QF categories: readable, uncertain or unreadable) can be used to evaluate spillover. Analysis of ~80 000 otoliths taken between 2007 and 2013 showed that the Darß and Drogden sills consistently separated large numbers of readable otoliths in the shallower western area (subdivision (SD) 21–SD23) from large proportions of uncertain and unreadable otoliths in the deeper eastern basins (SD25–SD29). SD24 was a mixing area: the western statistical rectangles resembled SD22 and SD23, whereas the eastern rectangles resembled SD25, in close association with basin topography. QF proportions did not differ on the various spatial and temporal scales examined, regardless of grouping by sex or length class. This suggests that increased spillover from the east has not occurred since 2007. However, the large proportion of uncertain otoliths in SD24 and inconsistencies in QF determination may mask the detection of trends in mixing.