The phenology and ecology of the critically endangered European eel (Anguilla anguilla) during their marine to freshwater transition

Conference: Bournemouth University, Faculty of Science and Technology

Abstract:

Environmental changes, including climate change and habitat fragmentation, are negatively impacting populations of migratory species at global scales. Species with particularly complex life cycles involving large-scale movements across major ecosystem boundaries are particularly vulnerable to population declines. A strong example of such a species is the European eel (Anguilla anguilla), a catadromous fish that utilises habitats in both marine and freshwater environments during multiple life stages. In the last 30 years, there has been a collapse in eel recruitment in Europe to the extent they are now classed as ‘Critically endangered’. Although there is a focus on the management and recovery of eel, there remains high uncertainty in their ecology and phenology during earlier life-stages, in particular the timing of their arrival into freshwater, age structure, their transition from non-pigmented (glass) to yellow eel and their foraging behaviour in estuarine habitats during both upstream and downstream migration. The aim of this research was to thus derive new understandings on the migration phenology of the critically endangered European eel with a focus on the ecology of their marine-freshwater transition.

Advances in the ecological use of natural chemical tags, such as stable isotopes (SI) and the development of geostatistical models of SI landscapes (‘isoscapes’) provide an alternative technique for studying the movement and ecology of eel throughout different life stages. Stable isotope signatures of marine, estuarine and fresh waters are highly distinct, enabling their application to determine the extent of the freshwater residency for non-pigmented and pigmented eel as they switch from using marine to freshwater dietary resources. Significant individual variability was observed in the migration patterns of eels into the lower reaches of four rivers in England. In the heavily engineered River Parrett, all eels were enriched in 13C, indicating recent arrival from the marine environment. However, in areas with some habitat complexity, migrating eels exhibited a mixture of newly arrived individuals, those in transition, and those already established in freshwater (depleted 13C values). These results suggest that the upstream migration of eels is not uniform, emphasising the importance to provide eels with a range of settlement habitats. The development of a dual isoscape model enabled the prediction of foraging locations of silver eels during their downstream migration. Eels were predicted to forage in freshwater environments, highlighting the importance of these habitats for eel during both their upstream and downstream migration.

Environmental factors were then modelled against migration timings to determine whether abiotic factors influenced the migration timings of eel across three UK rivers. Migration generally occurred from April to September, with peak movements in June, and with few eels sampled after September. The onset and peak of migration was linked to environmental factors, with eels arriving earlier when river temperatures were warmer, while cooler sea surface temperatures delayed migration. These results suggest that, despite annual fluctuations in the timing of eel migration initiation and peak entry into freshwater, such variations can be predicted based on differences in environmental conditions which may be altered by a changing climate. Otolith microstructure analysis was then applied to estimate the age of non- pigmented and pigmented eels as they enter freshwater environments. Otoliths were aged by counting daily increments. It was estimated that migration from spawning grounds in the Sargasso Sea takes 7 to 11 months, with individuals arriving earlier at higher latitudes. Overall, this research enhances our understanding of the complex life history, migration, and foraging behaviour of eels, underlining the importance of prioritising freshwater habitat protection and restoration.

Source: Manual