Migratory fish in German waters

Background and Objective

Diadromous migratory fish species (i.e., migrating between freshwater and marine habitats during their life cycle) are threatened by the extensive modification of rivers and estuaries (e.g. through hydropower plants and navigation channel engineering). These strong alterations significantly restrict fish migration and have a major impact on river flow dynamics from headwaters to tidal reaches. Together with general problems of water quality in many regions (high sediment and nutrient inputs, pollutant loads, rising temperatures), this has already led to a severe loss of spawning and nursery habitats for many migratory fish species and a reduction in their occurrence.

In Germany, basic information on the ecology, life cycle, current distribution, and key migration routes is lacking for many diadromous fish species, especially those of low economic importance. Against this background, the project aims to significantly improve the data basis for non-commercial diadromous migratory fish species in Germany and to work toward the development of a long-term monitoring framework for effective population management and adaptive conservation of these underrepresented species.

Target Group

EU. ICES, Bundesländer (the German federal states)

Approach

In a first step, existing monitoring data will be compiled in a central database as a basis for distribution modeling of diadromous migratory fish. To meet the requirements of the Water Framework Directive, data on fish occurrence in inland waters are regularly collected in all federal states using standardized methods. Through a comprehensive compilation and analysis of these state-level datasets, as well as the evaluation of historical datasets available in existing databases, an overview of the status of migratory fish species in Germany will be created. This will allow the identification of threats, migration obstacles and barriers, as well as data gaps.

In a second step, existing fish monitoring activities for the focal species—twaite shad (Alosa fallax), smelt (Osmerus eperlanus), river and sea lamprey (Lampetra fluviatilis and Petromyzon marinus, respectively), and burbot (Lota lota)—will be expanded and complemented by more detailed investigations. Environmental DNA (eDNA) analyses will be used to test whether these species can also reliably be detected using non-invasive methods, to establish this method as a complementary tool to conventional monitoring. Beyond site-specific species detection, eDNA analyses will be conducted on a large spatial scale and across multiple waterbody types and compared with capture-based methods. In parallel, focal species will be tagged with different methods (PIT tags, acoustic transmitters, Floy tags), and their migration behaviour and habitat preferences will be exemplarily determined in the Weser River and its tributaries. The resulting movement data will be supplemented with analyses on age, growth, and diet (e.g., via stable isotope analysis).

In a third step, the marine phases of the focal species’ life cycles—which have been largely neglected in the past—will be investigated. Stable isotope and trace element analyses will be conducted on archive tissues such as otoliths and eye lenses, as well as on turnover tissues like muscle, to reconstruct marine residency areas (e.g. overwintering and feeding habitats) and migration pathways using spatially explicit distribution models of these biotracers (so-called isoscapes and chemoscapes). In addition, water samples will be collected with spatial and seasonal resolution and analyzed for their biotracer composition in order to validate and, if necessary, update existing iso- and chemoscapes.

The resulting data on the distribution and ecology of migratory fish will then be integrated into a synthesis module to improve the distribution models developed in the first project phase. Model comparisons will then allow the identification of the most important parameters for future monitoring of these species.

Our Research Questions

• Comprehensive analysis and evaluation of existing data on migratory fish in German waters. Identification of data gaps and strategies for collecting missing data in the future.
• Development of concepts for the expansion of monitoring activities in selected water bodies to improve the data basis for individual migratory fish species. Development of species-specific methods for the detection of migratory fish in different types of water bodies on the basis of eDNA analyses.
• Development of proposals for improved species-specific stock management.