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

Abstract

Abstract. 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.

Living apart together: Long-term coexistence of Baltic cod stocks associated with depth-specific habitat use

“…Due to former age reading and archiving procedures, up to 50% of the left and right otoliths from the sampling period between 1977 and 2006 were broken. This occurred particularly often for EBC otoliths, as the otolith annuli of this stock are more difficult to interpret [ 15 ]. To avoid a bias, i.e.…”

“…The cod stocks also differ in several other parameters, e.g. the WBC stock has a lower stock size [ 11 ] but cod in the western Baltic Sea have a higher growth rate and reach larger sizes [ 12 , 13 ], their livers are less infested with anisakid larvae [ 14 ] and they have more easily readable otoliths [ 15 ].…”

Is analysis of otolith microstructure a valid method for investigating early life history of Western Baltic cod?

“…However, comparison of estimated ages of juvenile WBC from micro-structural analysis of otoliths (Rehberg-Haas et al 2012) with results of an age validation study (McQueen et al 2019) indicates that micro-structural analysis of WBC otoliths can under-estimate the age of individuals, despite the constant formation of micro-increments reported by Rehberg-Haas et al (2012). Furthermore, the appearance of the macroincrements of WBC and EBC otolith differ markedly (Stötera and Krumme 2016), with EBC otoliths lacking well-defined zones, likely due to a life in a deeper, permanently stratified environment (Hüssy et al 2003a) with the unusual overlap of temperature and food availability cycles . Given the differences in environmental experience and otolith appearance between the two cod stocks, we cannot assume that the periodicity of WBC micro-increments will necessarily be similar to those of EBC.…”

Fish otoliths as indicators in ecosystem based management: results of the 5th International Otolith Symposium (IOS2014)

“…In a similar manner to elemental or isotopic geotags, fish collected from different locations may also have distinct growth patterns, which visually mark the otoliths. Stötera and Krumme (2016) found significant variability in visual quality of growth increments in otoliths of Baltic cod that led to quality problems in age readings. Instead of using this variability to indicate less reliable readings, they used the quantity and distribution of the 'poor' readings as an index or metric for spatial segregation of sub-populations.…”