Total mercury (Hg) levels were determined in the tissues of ten taxa of pelagic fishes, with a special emphasis on apex predators (large vertebrates). Highest Hg levels were observed in blue marlin (Makaira nigricans), carcharhinid sharks (Genus Carcharhinus) and little tunny (Euthynnus alletteratus), ranging from 1.08 to 10.52 ppm. Moderate to low concentrations (<1.0 ppm) were observed in blackfin tuna (Thunnus atlanticus), cobia (Rachycentron canadum), dolphinfish (Coryphaena hippurus), greater amberjack (Seriola dumerili), king mackerel (Scomberomorus cavalla), wahoo (Acanthocybium solandri) and yellowfin tuna (Thunnus albacares). For the majority of species examined, Hg level did not vary significantly between locations (Texas and Louisiana) and years (2002 and 2003). The relationship between Hg level and fish size/weight was also explored and six taxa (blackfin tuna, carcharhinid sharks, dolphinfish, king mackerel, wahoo, yellowfin tuna) showed significant positive relationships between Hg level and body size and/or weight. Natural dietary tracers, stable isotopes (15N, 13C) and fatty acids were used to evaluate the relationship between Hg and trophic position and the relationship between Hg and dietary history. Stable nitrogen isotope analysis showed that Hg levels in fish tissues were positively associated with trophic position. Based on the 13C and 15N values of pelagic consumers examined in this study, three natural groups were identified with cluster analysis, and the same groupings were detected based on fatty acid profiles. This not only confirmed the existence of these natural groupings, but also indicated that the distinguishing factors for the grouping was likely connected with the dietary history of these fishes. The classification tree based on the fatty acid profiles of pelagic fishes readily separated fishes from different regions, suggesting that diets of pelagic taxa within the same region are similar or these consumers share a common source of organic matter in their food web. Findings from this study complement other Hg investigations conducted in the Gulf and also furthered our understanding of the link between feeding ecology and Hg accumulation. Moreover, the combined use of stable isotope and fatty acid techniques provided new insights on the dietary history of pelagic fishes in the Gulf of Mexico.

Stable isotope analysis (SIA) is a valuable tool in ecotoxicology because δ13C and δ15N may provide insights into the trophic transfer of contaminants in a food web. The relationship between a species’ trophic position (TP, determined from δ15N) and internal concentration of biomagnifying contaminants can be established and used for regulatory purposes. However, the exposure of organisms to xenobiotics incurs physiological costs, and the stable isotope signature of a consumer reflects not only diet but also a physiological state. The latter raises questions regarding the interpretation of stable isotope signatures in contaminated areas. Therefore, the aim of this Thesis was to evaluate the behaviour of consumers’ stable isotope signatures in stressed systems, with a primary focus on the effects of environmental contaminants. In paper I, the physiological costs of chemical exposure were found to alter incorporation rates of dietary nitrogen and carbon in a consumer by influencing both growth and metabolic turnover, with resulting changes in isotope signatures relative to a control system. In paper II, the diet-consumer discrimination factors for 15N and 13C were confirmed to increase under chemical exposure mediated via increased metabolic costs. However, the physiological response was low and translated into only minor shifts in the δ13C and δ15N. The predictability of exposure effects on the stable isotope signature was demonstrated in paper III, in which animals exposed to a chemical with a known mode of action presented expected effects on elemental composition, body size, biomarkers of oxidative stress and the stable isotope signatures. Moreover, consumers’ oxidative balance was found to be related to their δ15N values, thus providing evidence of the kinetic isotope effect on the oxidative status. However, despite the alterations in stable isotope signatures observed in laboratory settings (papers I-III), the effect of xenobiotics on the TP estimates was nil or minor in the field-collected animals. Moreover, the TP values were not significantly different between the animals in the contaminated and the reference habitats because of the high overall uncertainties in the TP estimates (paper IV). Also, the TP estimates based on δ15N in bulk material were more similar between the contaminated and the reference systems than TP estimates based on δ15N values in amino acids. Therefore, the latter method appears more sensitive towards xenobiotics (and, possibly, other environmental stressors) and thus less suitable for TP assessment in contaminated areas. This Thesis improved the overall understanding of the applicability of SIA in stressed systems by establishing relationships between various exposure regimes, physiological responses and the stable isotope signatures in consumers. In model species at low trophic levels, the exposure to xenobiotics was found to significantly affect δ13C and δ15N values, which can be expected whenever physiological responses are detected. However, because of the overall high uncertainty in TP estimates, no significant differences between contaminated and control systems were detected, although the estimated TP were consistently higher in the contaminated systems. Future research should focus on higher trophic levels, in which effects of a greater magnitude can be expected. Moreover, the effects in entire food webs should be addressed rather than single prey–consumer relationships as well as other environmental variables that may contribute to the stable isotope variability in and among systems under various environmental pressures. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript.</p>

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Blue Catfish Ictalurus furcatus are native to tributaries of the Mississippi River but are now invasive in several Atlantic slope drainages. This includes subestuaries of the Chesapeake Bay, where their feeding ecology and potential impact on native species was largely unknown. We collected stomach contents from 16,110 Blue Catfish at 698 sites in three large subestuaries of the Chesapeake Bay (James, York, Rappahannock rivers). Cumulative prey curves revealed that sample size was sufficient for diet description, though 1,000 – 1500 stomachs were needed per river. Blue Catfish are opportunistic generalists that feed on a broad array of plant and animal material. Logistic regression models reveal that Blue Catfish undergo significant ontogenetic diet shifts to piscivory at larger sizes (P<0.01) though the lengths at which these shifts occur varies by river system (500 – 900 mm total length; TL). Over 60% of Blue Catfish stomachs contained other invasive species, primarily Hydrilla verticillata and Asian clams Corbicula fluminea. Canonical Correspondence Analysis (CCA) revealed that salinity and season explained the most variation in Blue Catfish diet, while Generalized Additive Models (GAMs) demonstrated that there is considerable spatiotemporal and length-based variation in predation of species of concern. Species of concern include American Shad, American Eel, and river herring, which are imperiled, and blue crab, which support valuable fisheries in Chesapeake Bay. Predation of American Shad, American Eel, and river herring was rare (max predicted occurrence in Blue Catfish diets = 8%), while blue crab was much more common in the diet (max predicted occurrence =28%). Predation of American Shad and river herring peaks in freshwater areas in April, while predation of blue crab peaks in brackish areas in October. Predation of all species of concern is highest for large catfish (500 – 1000 mm TL). Field and laboratory-based estimates of consumption rate revealed that Blue Catfish feed at similar rates as Channel Catfish Ictalurus punctatus, and daily ration is estimated to be 2-5% bodyweight per day during warm temperatures, while peak feeding (maximum daily ration) can approach 10% bodyweight per day. While consumption of imperiled species is rare, Blue Catfish could still have negative impacts on these species due to dense catfish populations. / Ph. D.

Amphibien sind global von einem Artenrückgang betroffen, wobei Ausbeutung bestimmter Froscharten einer der Gründe hierfür ist. Die vorliegende Arbeit gibt erstmals Einblick in Form, Umfang und Auswirkungen der extensiven Nutzung von Fröschen in Westafrika. Interviews mit Konsumenten und in den Froschhandel Involvierten ermöglichten die Nutzung und den Handel von Fröschen in Burkina Faso, Benin und Nigeria zu quantifizieren und zu bewerten. Während der Froschhandel in Burkina Faso eher auf lokaler Ebene abläuft, ließ sich im Norden Benins und in Nigeria ein intensiver grenzüberschreitender Handel nachweisen. Um einen möglichen anthropogenen Einfluss auf natürliche Amphibien Gemeinschaften zu ermitteln wurden natürliche Gewässer untersucht. Auf Kaulquappenebene wurden Artenverluste und die sich daraus ergebenen Konsequenzen studiert. Temporäre Savannengewässer wurden in zwei Untersuchungsgebieten in Burkina Faso untersucht, wobei die Gewässer jeweils in Gebieten mit unterschiedlichem Störungsgrad lagen: in Dörfern mit Froschfang, gegenüber in geschützten Gebieten mit Froschfangverbot. Generell konnte in den anthropogen gestörten Gebieten ein niedrigerer Artenreichtum mit gleichzeitig veränderter Zusammensetzung der Artengemeinschaften konstatiert werden. Für diese Unterschiede waren primär anthropogen veränderte Habitatfaktoren verantwortlich. Aber auch das Froschfangen kann Grund für veränderte Kaulquappengemeinschaften sein. Um Voraussagen für mögliche resultierende Konsequenzen treffen zu können ist es wichtig die ökologische Rolle betroffener Arten im System zu kennen. In künstlichen Gewässern wurde die trophische Position von vier Kaulquappenarten mit jeweils unterschiedlicher Nahrungsstrategie und die Konsequenzen ihres Verlustes für die restlichen Kaulquappen und bestimmter Ökosystemfaktoren, wie Moskitolarven, ermittelt. Die vier Kaulquappenarten unterschieden sich in ihrer trophischen Position. Zudem konnten komplexe Interaktionen zwischen den vier Fokusarten aufgedeckt werden, wobei die trophische Position je nach Gemeinschaft wechseln konnte. So konnte ich mit meiner Arbeit zeigen, dass der Verlust nur einer Art essentielle Konsequenzen für Amphibien Gemeinschaften und ökosystemische Prozesse haben kann. Chapter 1 Summary II I / Amphibian populations are declining world wide, with overexploitation being one of the main reasons. In Africa frog exploitation has not been in the focus of scientific research yet. Herein I report first investigations of the frogs use from West Africa. Interviews with consumers and participants in the frog trade facilitated to quantify and evaluate the use and trade of frogs in Burkina Faso, Benin and Nigeria. In Burkina Faso the frog trade mainly took place on a local scale, whereas in northern Benin and Nigeria, I detected an intensive cross-border trade. In an empiric approach I examined if a possible anthropogenic impact could be found in natural tadpole communities. In two study regions in Burkina Faso I compared natural freshwater ponds situated of different disturbance regimes: ponds in and around villages, where frogs were collected for consumption, and in protected areas, where frog harvest was prohibited. Species richness was lower and species composition differed in anthropogenic disturbed areas. Anthropogenic altered habitat factors were mainly responsible for these differences. However, harvesting of adult amphibians could have affected tadpole composition and occurrences of particular species as well. To evaluate the effect of the potential loss of a species a better understanding of its ecological role is important. Therefore, I conducted an experiment to investigate the trophic position of four tadpole species and the consequences of species exclusion for the remaining species and on certain ecosystem parameters such as mosquito larvae. Isotopic analyses revealed that tadpole species differed in their trophic position. Various complex interactions between species occurred including shifts in trophic positions due to tadpole species composition. Thus the loss of just one species can have crucial consequences for the remaining amphibian community and furthermore for ecosystem processes. Chapter 1 Summary II I

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