Microchemical analyses of otoliths in Baltic Sea fish - Possibilities and limitations of otolith elemental analysis to describe individual life history and stock characteristics of fish in the Baltic Sea

In this thesis otolith microchemistry analyses were used to gain insights into the individual life history and stock characteristics of three fish species from the Baltic Sea - the European eel Anguilla anguilla, the Atlantic cod Gadus morhua and the thicklip grey mullet Chelon labrosus. The special hydrographic environment of the world’s largest brackish water system provides promising conditions for the use of otolith elemental analysis to investigate individual migration patterns and stock structures of fish. Here, it was used to gain information with relevance for stock management of fish species that differ widely in their biology, ecology and stock structure. It was investigated whether individual continental migratory history affects health and spawner quality of the European eel. Otolith strontium (Sr) composition was used to identify characteristic migration patterns. Results show that the muscle fat contents of silver eels with strictly catadromous life cycles are significantly reduced compared to silver eels that never entered freshwaters. Furthermore, prevalence and infection intensities of the swimbladder nematode Anguillicoloides crassus are highly increased in catadromous silver eels. Both, a reduced accumulation of fat reserves and intense A. crassus infestations are assumed to impair the successful reproduction of A. anguilla. These results highlight the importance of brackish water habitats for the production of high quality spawners and question the benefit of restocking measures into inland waters. In order to investigate the influence of water temperature and feeding behaviour on the element incorporation into A. anguilla otoliths, two experimental studies were conducted. Results showed that otolith element incorporation is influenced by water temperature, but detected differences between treatments were low to allow an in situ reconstruction of temperature history. In the food experiment no effect on otolith microchemistry was detected between feeding groups. Results indicate that neither temperature changes nor individual dietary behaviour impair the use of otolith Sr concentrations as a tracer of diadromy. Thus the reliability of such migration studies is fundamentally increased. It was further investigated whether multi-element otolith analysis can be used to discriminate Atlantic cod individuals according to their origin. Otolith multi-element concentrations from adult individuals caught at spawning grounds in the North Sea, the western Baltic Sea and the eastern Baltic Sea were analysed. Furthermore, the multi-element composition of the core region of juvenile cod otoliths from western and eastern Baltic areas was examined. Multivariate statistical analyses successfully discriminated between adults from different stocks as well as between western and eastern Baltic Sea juveniles. Significant differences between the eastern Baltic spawning grounds, however, were not detected. These results demonstrate the potential of otolith microchemistry analysis to investigate the structure and connectivity of G. morhua stocks in the Baltic Sea. In a further field study the migratory behaviour of thicklip grey mullet was examined for the first time on individuals caught in the Baltic Sea. By detecting C. labrosus otolith Sr concentrations, this study aimed to gain first insights into preferences and whereabouts of individuals in the Baltic Sea. Results confirmed a preference of brackish habitats for all analysed specimens and suggest a high plasticity of C. labrosus migratory behaviour. The results support the assumption that a considerable fraction of individuals entering the western Baltic Sea regularly migrate to the North Sea. The results presented in this thesis confirm the feasibility of otolith microchemistry analyses for the investigation of a broad range of questions on Baltic Sea fish. Knowledge about investigated species was expanded and opportunities for future studies were indicated.