Mikrochemische Analysen an Otolithen von Ostseefischen: Möglichkeiten und Grenzen von Otolithen-Elementanalysen zur Beschreibung individuellen Wanderverhaltens und Bestandsstrukturen von Fischen in der Ostsee

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.In dieser Arbeit wurden anhand mikrochemischer Otolithenanalysen Erkenntnisse über die individuelle Lebensweise und die Bestandsstruktur von drei Fischarten aus der Ostsee gewonnen – dem Europäischen Aal Anguilla anguilla, dem Atlantischen Kabeljau Gadus morhua und der Dicklippigen Meeräsche Chelon labrosus. Die hydrographischen Besonderheiten der Ostsee als weltgrößtes Brackwassersystem bieten vielversprechende Voraussetzungen für die Anwendung von Otolithen-Elementanalysen zur Untersuchung des individuellen Wanderverhaltens von Fischen und deren Bestandsstrukturen. Hier wurden sie genutzt, um managementrelevante Fragestellungen von Fischarten zu untersuchen, die sich hinsichtlich ihrer Biologie, Ökologie und Bestandsstruktur sehr unterscheiden. Es wurde untersucht, ob das kontinentale Wanderverhalten Europäischer Aale deren Gesundheit und Laicherqualität beeinflusst. Dazu wurden anhand der Otolithen-Strontiumverteilung charakteristische Wandermuster ermittelt. Die Ergebnisse zeigen, dass der Muskelfettgehalt von Blankaalen mit strikt katadromen Wanderverhalten signifikant geringer ist als der von Blankaalen, die niemals ins Süßwasser eingewandert sind. Darüber hinaus sind Prävalenz und Befallsintensität mit dem Schwimmblasen-Nematoden Anguillicoloides crassus bei katadromen Blankaalen stark erhöht. Sowohl eine verringerte Einlagerung von Fettreserven als auch der intensive Befall mit A. crassus stehen im Verdacht, den Reproduktionserfolg von A. anguilla zu vermindern. Die vorliegenden Ergebnisse heben die Bedeutung von Brackwasserhabitaten für die Produktion von gesunden Laichtieren hervor und stellen den Nutzen von Besatzmaßnahmen in Binnengewässer in Frage. Um den Einfluss von Wassertemperatur und Futterverhalten auf die Elementeinlagerung in A. anguilla-Otolithen zu prüfen, wurden zwei experimentelle Studien durchgeführt. Die Ergebnisse zeigen, dass die Einlagerung von Elementen in die Otolithen zwar von der Wassertemperatur beeinflusst wird. Die Unterschiede zwischen den Futtergruppen waren aber so gering, dass eine Rekonstruktion erlebter Temperaturänderungen in situ schwierig erscheint. Im Futterexperiment konnte kein Effekt auf die Elementeinlagerung detektiert werden Die Ergebnisse zeigen, dass weder Temperaturschwankungen noch unterschiedliches Futterverhalten die Verwendung von Sr zur Rekonstruktion diadromen Verhaltens einschränken. Damit wird die Aussagekraft solcher Migrationsstudien deutlich erhöht. Des Weiteren wurde untersucht, ob Otolithen-Multielementanalysen dazu geeignet sind, G. morhua-Individuen ihrer Herkunft zuzuordnen. Dazu wurde die Multielementzusammensetzung von Otolithen adulter Dorsch von Laichplätzen aus der Nordsee, der westlichen und der östlichen Ostsee untersucht. Des Weiteren wurde die Zusammensetzung der Kernbereiche von Otolithen juveniler Dorsche aus der westlichen und der östlichen Ostsee analysiert. Multivariate statistische Analysen konnten sowohl die adulten Bestände erfolgreich trennen, als auch die Juvenilen aus der westlichen und der östlichen Ostsee unterscheiden. Signifikante Unterschiede zwischen den drei östlichen Laichgebieten konnten aber nicht detektiert werden. Die Ergebnisse zeigen das Potential von Otolithen-Multielementanalysen zur Untersuchung der Struktur und Konnektivität von G. morhua-Beständen. In einer weiteren Feldstudie wurde das Wanderverhalten der Dicklippigen Meeräsche C. labrosus erstmals an Individuen aus der Ostsee untersucht. Ziel der Studie war es, mittels der Analyse von Sr-Konzentrationen in C. labrosus-Otolithen grundsätzliche Informationen über Aufenthaltsorte und Habitatspräferenzen der Ostsee-Meeräschen zu erlangen. Die Ergebnisse bestätigen die Präferenz von Brackwasserhabitaten und lassen eine hohe Plastizität des Wanderverhaltens vermuten. Sie stützen die Vermutung, dass ein erheblicher Teil der in die Ostsee einwanderenden Individuen regelmäßig in die Nordsee zurückkehrt. Die Ergebnisse dieser Arbeit bestätigen die gute Eignung mikrochemischer Otolithenanalysen zur Untersuchung unterschiedlichster Fragestellungen an Fischarten aus der Ostsee. Das Wissen über die untersuchten Arten wurde erweitert und neue Wege für zukünftige Studien wurden aufgezeigt

Partial migration of a maraena whitefish Coregonus maraena population from the River Elbe, Germany

The maraena whitefish Coregonus maraena is a threatened anadromous species in the North Sea, which in the past was decimated to near extinction. Since the late 1980s, several re-establishment programs have been implemented in rivers draining into the North Sea, but the scientific basis for sustainable conservation measures is often lacking, since little is known about the biology of this species. In this study, otolith microchemistry of fish ranging from 24.6 to 58.4 cm in total length (median 31.3 cm, SD 8.4 cm) was used to characterize the migration behavior of a reintroduced population of maraena whitefish from the River Elbe, Germany. Our analyses revealed the presence of 3 different migration patterns: (1) one-time migration into high-salinity habitat (North Sea) within the first year of life (29.6%), (2) multiple migrations between lowland high-salinity habitats starting in the first year of life (14.8%) and (3) permanent residency within low-salinity habitats, a pattern displayed by the majority (55.6%) of sampled individuals. Not only do these results reveal differential migration behavior, but they also indicate that permanent river residency is common in the River Elbe population of C. maraena. The role of the Elbe as both a feeding and a spawning habitat should thus be considered more explicitly in current conservation measures to support recovery of this species

Recruitment collapse and population structure of the European eel shaped by local ocean current dynamics

Highlights: • We combine high-resolution ocean models with population genetics • Variation in wind-driven ocean currents mediates the collapse of A. anguilla • Female eels are philopatric within the Sargasso Sea, while males maintain gene flow • We present first evidence of the role of ocean currents in shaping species’ evolution Summary: Worldwide, exploited marine fish stocks are under threat of collapse [1]. Although the drivers behind such collapses are diverse, it is becoming evident that failure to consider evolutionary processes in fisheries management can have drastic consequences on a species’ long-term viability [2]. The European eel (Anguilla anguilla; Linnaeus, 1758) is no exception: not only does the steep decline in recruitment observed in the 1980s [ 3 and 4] remain largely unexplained, the punctual detection of genetic structure also raises questions regarding the existence of a single panmictic population [ 5, 6 and 7]. With its extended Transatlantic dispersal, pinpointing the role of ocean dynamics is crucial to understand both the population structure and the widespread decline of this species. Hence, we combined dispersal simulations using a half century of high-resolution ocean model data with population genetics tools. We show that regional atmospherically driven ocean current variations in the Sargasso Sea were the major driver of the onset of the sharp decline in eel recruitment in the beginning of the 1980s. The simulations combined with genotyping of natural coastal eel populations furthermore suggest that unexpected evidence of coastal genetic differentiation is consistent with cryptic female philopatric behavior within the Sargasso Sea. Such results demonstrate the key constraint of the variable oceanic environment on the European eel population

Suitability of otolith microchemistry for stock separation of Baltic cod

Microchemical otolith analyses have been shown to provide valuable information on the life history, dispersal and stock characteristics of teleost fish. In the present study, the suitability of this technique for identifying the origin and distribution of Atlantic cod Gadus morhua L. from the Baltic Sea was examined using laser ablation-ICPMS. The capacity to distinguish individuals from different Baltic Sea stocks and from the adjacent North Sea stock based on incoporation of stock-specific elemental fingerprints along otolith growth axes was investigated. It was further tested if different origins led to spawning-site specific element concentrations in otolith cores. The results indicate that microchemical analyses of Baltic cod otoliths are applicable for differentiating individuals of different stocks. Analyses of similarities including 12 element/calcium ratios resulted in significant differences between individuals from the eastern and the western Baltic Sea and between North Sea and Baltic Sea samples. Sr/Ca, Ba/Ca, Y/Ca, Mg/Ca, Zr/Ca and Mn/Ca ratios had the strongest discriminatory power. A further separation of individuals caught in 3 different spawning grounds of the eastern Baltic, however, was not possible. Elemental compositions from the core regions of otoliths from young of the year cod caught in eastern and western Baltic Sea spawning grounds showed significant differences in Sr/Ca, Ba/Ca and Mg/Ca concentrations. Analyses of similarities again showed significant differences between these areas for juveniles. This study demonstrates the potential of otolith microchemical analyses to provide important information about the stock structure and connectivity of G. morhua in the Baltic Sea

Dietary effects on multi-element composition of European eel (Anguilla anguilla) otoliths

Otolith microchemistry is widely used as a tool to track individual migration pathways of diadromous fish under the assumption that the elemental composition of fish otoliths is directly influenced by the physicochemical properties of the surrounding water. Nevertheless, several endogenous factors are reported to affect element incorporation into fish otoliths and might lead to misinterpretations of migration studies. This study experimentally examined the influence of eight different diets on the microchemical composition of European eel (Anguilla anguilla) otoliths using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Seven natural prey types and one artificial diet were fed during 8 weeks in freshwater circuits. Results show for the first time that food has no significant influence on the incorporation of Na, Sr, Ba, Mg, Mn, Cu and Y into European eel otoliths. This indicates that the incorporation of elements usually chosen for migration studies is not affected by diet and that individual feeding behaviour of A. anguilla will not lead to any misinterpretation of migration pathways