Building bridges in Marine Science Education

The brochure “Building Bridges in Marine Science Education” introduces the stunning spectrum of doctoral research in marine sciences in Kiel and highlights the outstanding features of the ISOS doctoral programme. A central element of the brochure are 10 doctoral candidate profiles that show the “face of the science” by giving doctoral researchers from a broad spectrum of disciplines the opportunity to introduce their work in their own words

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

Operationalizing Ocean Health: Toward Integrated Research on Ocean Health and Recovery to Achieve Ocean Sustainability

Protecting the ocean has become a major goal of international policy as human activities increasingly endanger the integrity of the ocean ecosystem, often summarized as “ocean health.” By and large, efforts to protect the ocean have failed because, among other things, (1) the underlying socio-ecological pathways have not been properly considered, and (2) the concept of ocean health has been ill defined. Collectively, this prevents an adequate societal response as to how ocean ecosystems and their vital functions for human societies can be protected and restored. We review the confusion surrounding the term “ocean health” and suggest an operational ocean-health framework in line with the concept of strong sustainability. Given the accelerating degeneration of marine ecosystems, the restoration of regional ocean health will be of increasing importance. Our advocated transdisciplinary and multi-actor framework can help to advance the implementation of more active measures to restore ocean health and safeguard human health and well-being

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

Factors challenging the European eel (Anguilla anguilla) Stock recovery in continental waters

In the present thesis, it was for the first time possible to directly assess the entire silver eel escapement from a medium freshwater system in Northern Germany (Schwentine river system) over a 3-year period (Chapter I). The rare possibility to document the complete escapement of silver eels out of the system and the complete recruitment (via stocking) to the system, allowed for the characterization of factors acting on the amount of emigrating spawners. Thereby, it was possible to document the potential of the Schwentine river system to produce (healthy) silver eels and compare the results with the escapement requirements of the EU Regulation. It became obvious that current estimates of the silver eel escapement from a freshwater system in Northern Germany are overstated. Such an overestimation will have severe consequences for the future development of the European eel stock, because it might lead to a false-positive evaluation of the management success. As it is usually not possible to directly assess the entire silver eel escapement from a freshwater system, scientists across Europe rely on population dynamics models to describe the spawner output. To evaluate the accuracy of such model-approaches, it is of high importance to assess how close to reality the model output actually is. Comparing modeled escapement numbers with the directly assessed in situ silver eel output provides possibly the best evaluation of the model potential. In Chapter II, the German Eel Model II (GEM II), applied in the German EMP, was used to estimate the potential silver eel output of the Schwentine river system. To do so, it was first necessary to adjust the GEM II for the system-specific population characteristics of the Schwentine river eel population. Estimated escapement numbers were in the same range of the actually documented escapement in Chapter I. Results, thereby, document the potential of the GEM II in providing the necessary base for sound management decisions. At the same time, however, Chapter II illustrates the absolute necessity to minimize the generalization of input parameters. Only by a fine adjustment of input parameters to system-specific population characteristics, it is achievable to ensure the highest model accuracy possible. Up to date, management of the European eel is primarily restricted to the inland waters of the European Member States. In order to fulfill the required minimal escapement numbers, many Member States decided to extend already existing stocking programs. Today, these programs already require huge numbers of glass eels, caught at the coast of France and Spain and then allocated all across Europe. This procedure is subject of a lot of controversy, because it is still not entirely clear, whether the translocation of glass eels from the Atlantic coast to Northern Europe interferes with their ability to find the spawning location once they become silver eels and leave continental inland waters. Therefore, in Chapter III, formerly stocked silver eels from the Schwentine river system were tagged with T-Bar Anchor Tags to track their migration route through the Baltic Sea and evaluate their migratory success. Recapture reports accumulated in the Danish Belt Sea and results indicate the capability of stocked eels to successfully find the passage from the Baltic Sea into the North Sea without being dependent on an imprinting. In addition, however, documented migration speeds had to be considered to be too slow to reach the spawning ground in time for reproduction. The lack of imprinting seems to force the eels to search for the right route and it is highly questionable whether these eels are still part of the effective population size and thus can contribute to the stock recovery.In der vorliegenden Arbeit konnte erstmals die gesamte Blankaalabwanderung eines mittelgroßen Binnengewässers (Schwentine Flusssystem) in Norddeutschland über einen Zeitraum von 3 Jahren direkt erfasst werden (Kapitel I). Die seltene Möglichkeit die komplette Abwanderung von adulten Aalen, sowie die Rekrutierung von Jungtieren (durch Besatz) direkt zu erfassen und nicht abschätzen zu müssen, sowie die Quantifizierung verschiedenster Sterblichkeitsfaktoren für Aale im untersuchten Binnengewässer, ermöglichte die Dokumentation der Einflussfaktoren auf die Anzahl der abwandernden Blankaale. Hierdurch war es möglich, die Blankaalproduktion des Schwentinesystems zu beurteilen und die erfasste Abwanderung mit den Vorgaben der EG-Verordnung, bzw. der Aalbewirtschaftungspläne, zu vergleichen. Dabei wurde deutlich, dass bisherige Abschätzungen der Blankaalproduktion des Schwentine Systems die tatsächlich stattfindende Abwanderung offensichtlich überschätzen. Dies kann weitreichende Folgen für die weitere Entwicklung des Aalbestandes haben, führt die Überschätzung der Abwanderung doch zur fehlerhaften Beurteilung bereits etablierter Managementmaßnahmen. Da die Möglichkeit die Blankaalabwanderung direkt zu erfassen in der Regel nicht gegeben ist, sind Wissenschaftler in ganz Europa darauf angewiesen, die tatsächliche Abwanderung mit Hilfe von Populationsmodel zu schätzen. Um die Genauigkeit solcher Modellierungen zu beurteilen, ist es dringend nötig die Realitätsnähe der Modelle zu evaluieren. Die Modelergebnisse mit der tatsächlich dokumentierten Abwanderung vergleichen zu können, schafft hier für nahezu ideale Bedingungen. In Kapitel II wurde das in den deutschen Aalbewirtschaftungsplänen verwendete German Eel Model II (GEM II) genutzt um die potentielle Blankaalabwanderung der Schwentine zu modellieren. Hierfür war es zunächst nötig, dass Modell an die systemspezifischen Charakteristika der Aalpopulation der Schwentine anzupassen. Die mit Hilfe des angepassten GEM II geschätzte potentielle Blankaalabwanderung entsprach in etwa der tatsächlich dokumentierten Abwanderung (Kapitel I). Die Ergebnisse dokumentieren das enorme Potential des GEM II. Gleichzeitig verdeutlichen sie aber auch die Notwendigkeit der Erfassung systemspezifischer Populationscharakteristika um die höchstmögliche Realitätsnähe des Modells zu gewährleisten. Bisher beschränken sich die Maßnahmen zu Wiederauffüllung des Aalbestandes in erster Linie auf die europäischen Binnengewässer. Um die geforderten Mindestabwanderungen zu sichern, konzentrieren sich viele EU Mitgliedstaaten unter anderem darauf, bestehende Besatzprogramme auszubauen. Dafür werden schon jetzt große Mengen Glassaale an den Küsten von Frankreich und Spanien gefangen und auf Binnengewässer in ganz Europa verteilt. Diese Praxis ist nicht unumstritten, konnte doch bisher nicht endgültig geklärt werden, inwiefern sich der Transport von Glassaalen aus dem Atlantik nach Nordeuropa auf deren Fähigkeit die richtigen Wanderouten zum Laichgebiet zu finden auswirkt. Deshalb wurden für Kapitel III Blankaale aus dem Schwentine System mit externen „T-Bar Anchor Tags“ markiert und dann erneut in die Schwentine gesetzt um eine ungehinderte Abwanderung zu ermöglichen. Wiederfangmeldungen konzentrierten sich in der dänischen Beltsee. Die Ergebnisse lassen darauf schließen, dass besetzte Aale trotz des Mangels eines imprintings der Wanderroute in der Lage sind, den Weg aus der Ostsee zu finden. Gleichzeitig konnte jedoch auch gezeigt werden, dass die Wandergeschwindigkeiten der besetzten Aale deutlich zu langsam sind, um rechtzeitig im Laichgebiet anzukommen. Es scheint, als würden die Aale nach der richtigen Route suchen müssen und wären somit nur bedingt in der Lage ohne imprinting zielgerichtet abzuwandern. Es muss deshalb in Frage gestellt werden, ob diese besetzten Aale noch Teil der effektiven Population sind und somit zur Arterhaltung beitragen können

Escapement success of silver eels from a German river system is low compared to management-based estimates

The European eel (Anguilla anguilla) stock experienced a sharp decline during the last decades and is suffering from massive anthropogenic impacts on inland waters. To evaluate the benefit of management measures and to better understand the contribution of single drainage systems to spawner production, knowledge of the respective silver eel escapement is required. Furthermore, a better understanding of environmental conditions that potentially trigger the onset of spawning migration is needed to reduce anthropogenic mortalities during riverine eel migration. Investigations are also necessary to clarify whether fish protecting devices and bypasses at barriers are functional and truly increase eel survival and escapement rates. In this study, total female silver eel escapement from a northern German drainage system (Schwentine River) was assessed over a period of three consecutive years, and downstream migration patterns were compared to potential environmental triggers. Furthermore, the benefit of two fish bypasses (surface and deep) and a trash rack at the hydropower station for the survival of migrating eels was examined, and the spawner quality of escaping silver eels was determined by analysing lipid content and infection intensities with the swimbladder parasite Anguillicoloides crassus. The results indicate that silver eel escapement from the Schwentine drainage system is far below the estimated values underlying the respective eel management plan, highlighting the necessity of direct migration assessments to validate indirect estimations that include multiple assumptions and uncertainties. Major downstream migration events took place during short time periods in autumn and appear to be influenced by river discharge and water temperatures, suggesting that a precise prediction of escapement events is possible. Regarding spawner quality, fat reserves appear sufficient for escaping silver eels to migrate and spawn. However, high A. crassus prevalence and infection intensities are assumed to further reduce the number of potential spawners. Another matter of concern is the high trash rack mortality at the hydropower station that illustrates the need of fish protecting devices that fulfil eel-specific requirements