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

Informations

Funding country

Norway

Acronym

-

URL

-

Start date

1/1/2018

End date

12/31/2022

Budget

785,355 EUR

Fundings

Abstract

The European eel (Anguilla anguilla) is a facultative catadromous species; this means that it spawns in the sea but grows in freshwater (e.g. rivers and lakes). However, certain eels skip the freshwater-phase and remain in the sea, growing in marine coastal habitats instead. Other eels, known as ?interhabitat shifters?, move between fresh- and saltwater habitats during their growth phase. Which strategy is more beneficial for the species is still unknown. The different proportions of eels that remain in the sea or shift habitat are also unknown. A. anguilla is critically endangered. Many of the factors that have threatened A. anguilla over the past 50 years are linked to its life in freshwater, including habitat loss, parasite-associated diseases, and environmental pollution. Therefore, eels living in marine habitats may have a better chance of completing their life cycle successfully. The overall objective of the MAREEL project was to determine the advantages of eels growing in either fresh- or saltwater, or of switching between habitats. Thus, we aimed to understand what drives them to choose any of these life-strategies. To do this, we sampled eels from different habitats from fresh- and saltwater, and retraced their life history. To find out where they spent their growth phase, we analyzed their otoliths, i.e. ear-stones that absorb the chemical elements present in the fish's environment. From these, we also measured the age of eels -- in the same manner as one counts the rings of a tree from its trunk. We sampled 470 eels at locations between Arendal and Smøla in Norway, representing the largest study ever on the salinity history of eels. Marine resident eels dominate in coastal waters of Norway. We found that, on average, 20% of eels shift habitat. In other words, once they arrive from the Atlantic Ocean, some eels settle in freshwater, and others in saltwater, but 20% of them change habitats after one to several years. Shifts may happen at several occasions during their lifetime. Eels in saltwater habitat grew twice as fast (34 mm/year on average in saltwater versus 17 mm/year in freshwater), and they reached the reproductive stage at a younger age (15 vs 21 years on average for marine versus freshwater residents). Eels in saltwater also had a lower prevalence of the swim bladder parasite (Anguillicola crassus) that has been infesting eels since the 1980s. On the other hand, we found that eels growing in freshwater accumulated greater energy stores (higher lipid contents), and were characterized by higher contents of certain essential fatty acids that are key for maturation and reproduction (e.g. arachidonic acid). We also identified key macroparasites (over 34 species), with most sampled eels infected with a freshwater parasite. This indicated that they had been in contact with freshwater (or freshwater prey items) during their lifetime. Parasitic infection probably occurs when eels are juveniles since these are attracted to river run-off when they arrive from the ocean. Some marine sampling locations had limited access to freshwater habitats (Austevoll archipelago or Bømlo), yet these eels had also inhabited freshwater for different periods of time. We also tagged live eels with transmitters to see if they displayed any seasonal migrations between habitats. This was done in the Etne River, in Vestland using antennas which registered the passage of eels as they were migrating up or down the river. However, migratory movements between fresh- and saltwater outside of the spawning migration in the fall never occurred during our 3-year survey. In Sandnesfjorden (Risør), we investigated the behavior of eels in the marine environment and found that eels do not reside in deep areas; they usually remain in areas shallower than 10 m, and warmer than 5°C, unless preparing for their reproductive migration, in which case they display deep dives into the fjord (down to 66 m). During the winter period, most eels become dormant (90%), also in the marine environment, and this occurs as soon as daylength decreases to <10 hours. Eels become active again in the spring when daylength is >13-14 hours. Dormant periods may be interrupted as some eels showed spurts of activity in the middle of the winter. The dormancy period was longer in the marine part of the fjord compared to the inner freshwater part. In conclusion, we found that although there are certain advantages for eels to grow in the marine environment, certain essential fatty acids which are important for maturation and reproduction seem to be accumulated at higher extent in freshwater. This may explain why eels are still attracted to freshwater, at least early in their life cycle. Our study highlights the importance of shallow coastal habitats for A. anguilla, and of preserving free fish passage in transitional waters where eels are most abundant.