Reaching the steady state: 30 years of Anguillicola crassus infection of European eel, Anguilla anguilla L., in Northern Germany.

A 30 years long data series on the infection dynamics of European eel (Anguilla anguilla L.) with the non-native invasive nematode Anguillicola crassus Kuwahara, Niimi & Hagaki, 1974 is presented. Parasite burden was evaluated for 30 years in inland and coastal waters in Mecklenburg-Western Pomerania from 1991 to 2020. The total prevalence, mean intensity and damage status of the swim bladders were very high during the first decade (1991­2000), and significantly decreased in both marine and freshwater eel populations in the following decades (2001­2010, 2011­2020). The parasite intensity of eels in coastal waters was significantly lower compared with the freshwater systems (61.3% vs 79.5% in the first decade), indicating the vulnerability of the parasites to brackish water conditions and the fact that the life cycle of A. crassus cannot be completed under high saline conditions. Eel caught in the western part of the Baltic Sea (west of Darss sill) had the lowest mean infection (51.8% in first decade) compared to the eastern part with 63.8%. Thus, besides different infection patterns caused by the environmental conditions, a temporal trend towards a reduced parasite intensity and a more balanced parasite­host relationship developed in the 30 years of interaction after the first invasion. Possible reasons and mechanisms for the observed trends in parasite­host interactions are discussed.

Infection with swim bladder nematode Anguillicola crassus in relation to European eel growth, age, and habitat along the German Baltic coast.

One possible reason for the European eel (Anguilla anguilla) population decline is the neozoan eel swim bladder nematode Anguillicola crassus. To investigate whether the prevalence of A. crassus and the associated swim bladder pathology is related to eel habitat, growth rate, and age, 728 yellow eels from 6 habitats differing in salinity and located along the German Baltic coast were examined between 2005 and 2009. The prevalence of A. crassus varied between habitats, ranging from 9 to 57%. Infection prevalence and the percentage of eels with a damaged swim bladder were significantly higher in inner coastal waters compared to more saline open coastal water. In infected eels, 1 to 32 adult and preadult individuals of A. crassus were observed. The mean infection intensity varied between habitats from 2 to 7 nematodes per eel but did not significantly differ between inner and open coastal waters. Infection prevalence and intensity decreased significantly with age when all open coastal waters and all habitats were combined. Both the lower prevalence of A. crassus and the swim bladder damage of older eels and of eels originating from open coastal water habitats suggest that these eels have a higher fitness for spawning migrations than eels from inner coastal waters. The present study underlines the importance of eel screening on a sufficiently small geographical scale for the accurate estimation of eel recruitment and the identification of priority areas that are likely to produce healthy silver eels.

Evaluation of the bioaccumulation potential of alizarin red S in fish muscle tissue using the European eel as a model.

For fish stock management and large-scale stocking programs, the chemical substance alizarin red S (ARS) is an important tool to mark fish permanently. Equally, for the IUCN red list species European eel (Anguilla anguilla), ARS is proven to be the most promising option for mass marking. ARS binds to calcified structures (i.e., bones and otoliths) and can be detected using a fluorescence microscope. Despite the frequent application of ARS, not only for eels but also for fish in general, until today, no study has evaluated its bioaccumulation potential. Therefore, the German Federal Risk Assessment Authority was unable to classify ARS as harmless because of a potential risk to consumers' health. Using the technique of liquid chromatography mass spectrometry, an ARS detection protocol was developed and the bioaccumulation potential of ARS in European eel muscle tissue was estimated. A detection limit of 8.9 µg kg-1 could be reached by optimizing the detection method in fish muscle tissue. In the current study, 250 eels between 6 and 57 cm of total length have been analyzed for ARS between 0 day and 3 years after the marking process. The highest concentration of ARS (6056 µg kg-1) was observed immediately after marking in the smallest length class. Only 1 year after the marking procedure, the ARS concentration was below detection limit. A new method for ARS detection in fish muscle tissue, followed by utilization on marked eels, was able to show that the bioaccumulation of ARS in edible fish muscle was highly unlikely.