The importance of the marine habitat for the critically endangered European eel

Funding agencies: The Research Council of Norway, MARINFORSK Program + The Institute of Marine Research, Coastal Ecosystems Program.
Project period: 2018-2021
Project leader: Caroline Durif
Co-investigators: Howard Browman, Anne Berit Skiftesvik, Even Moland, Eva Thorstad, Francoise Daverat, Michael Arts, Janet Koprivnikar, Michael Power, Leif Asbjørn Vøllestad.

Project summary
     The European eel, (Anguilla anguilla) is semi-catadromous: it spawns in the sea but spends most of its life in freshwater (FW). Some individuals either skip the FW phase or shift habitat throughout the growth phase of their life history. Despite habitat shifting sometimes being a dominant trait at high latitudes, little is known about eels that remain in marine habitats (saltwater (SW) residency). A. anguilla is currently red listed as critically endangered. Causes for the decline are associated mainly with FW residency: dams and power plants, parasites and elevated contaminant levels. Although the proximate and ultimate drivers of FW vs. SW residency are unknown, residing in SW, or shifting habitats, may confer considerable advantages. To test whether eels that have lived in the marine environment are fitter and have a better chance at reproduction than FW eels, we will investigate condition, growth, length and age at maturation of eels caught in different salinity environments along a latitudinal gradient in Norway. The salinity history of eels will be retraced using microchemistry analyses on otoliths and compared with back-calculated growth rates. Their overall condition will be linked to the parasite load, their fatty acid (FA) profile and their long-term dietary patterns obtained by stable isotope analysis (SIA). Once FA and SIA profiles are established, we will use these proxies to infer the salinity histories of a larger sample of eels at different latitudes along the Norwegian coast. We will investigate seasonal movements of eels between SW and FW, using tags and fixed monitoring station. Habitat use in the sea (behavior, depth, effect of the swimbladder parasite) will be investigated using acoustic telemetry. This project will provide unique knowledge on the determinants of diadromy and on the SW and “shifting” life history strategies of eels that will contribute to worldwide efforts to conserve this ancient species. MAREEL will explore the drivers of catadromy vs. marine residency in the Norwegian subpopulation of the European eel.
     MAREEL’s objectives are: 1) To understand the factors that drive European eels to either colonize freshwater systems or remain in saltwater systems or shift habitat by determining their relative ecological advantage in terms of growth, fatty acid profiles, dietary pattern and parasite load; 2) To identify patterns in the different life-history strategies of eels (saltwater, freshwater, or habitat shifting), for example along a latitudinal gradient 3) To determine the proximate drivers (environmental and biological) of migrations between freshwater and saltwater. 4) To characterize habitat use of eel in marine waters and investigate the effect of the swimbladder parasite on their swimming behavior.

Read about the project here:
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In Norwegian