The European Eel: To Migrate Or Not To Migrate In Freshwaters?

Over-fishing, pollution, river development, global climate changes, etc. are threatening a large number of migratory fish. Among the endangered species, one can cite the European eel whose numbers have been divided by 10 over the two last decades. To save the species, the priority is better management of the populations subjected to the pressures of the environment. However, this first assumes better knowledge of the species’ biology and ecology.

The exercise is complex, because the European eel’s life cycle is still not fully understood. The species has long been considered a migratory fish that reproduces at sea and grows in rivers. Yet studies conducted over the last 10 years have shown that certain individuals do not spend their growth period in freshwaters. Migratory divergences may exist at the elver stage. At Bordeaux and Saint-Pée-sur-Nivelle, in doctoral work co-supervised by Cemagref and the INRA, Sarah Bureau du Colombier has been studying the source of these different migratory patterns in European eel elvers.

Sorting migrant fish and sedentary fish

What morphological characteristics differentiate a sedentary elver from a migratory elver in the natural environment? Today the response is “none”. To sort individuals by their propensity to migrate, the young doctoral student chose to concentrate on behavioral studies. To swim up the estuaries, the migratory elvers use the current from the ebb tide. Since light inhibits migration, they prefer to move in darkness, therefore especially at night. By reproducing the two main stimuli of migration in experimental conditions, it was possible to distinguish highly mobile individuals (with a strong propensity to migrate) from more passive individuals, buried in the gravel (with a weak propensity to migrate). The hundreds of elvers tracked during this study were captured upon entering the estuary or in the middle of the Adour estuary.

A question of an individual energy threshold

When they swim up the Adour estuary, the young elvers stop feeding more or less totally. This is a true obstacle course which requires good energy reserves upon entering the mouth of the estuary and then rapidly resuming feeding once the river has been reached. To verify these hypotheses, the young scientist compared the energy state of the two categories of elver as well as their capacity to resume feeding. In the estuary environment, elvers with a strong propensity to migrate seemed better equipped, in terms of energy stores, to conquer the freshwater environment and pursue their growth. Upon entering the estuary, the differences are not significant. This result points towards the existence of an individual energy threshold determining whether the individual will pursue its migration. In addition, the results have also underscored the greater energy expenditures and slower and/or lower resumption of feeding in sedentary elvers.

All of these data were then used to feed an estuarial migration model that will eventually be used to simulate the migratory behaviour of elvers according to different parameters, some of which are related to global climate changes.

A complex life cycle

The European eel reproduces near the North American coast in the Sargasso Sea. The young larvae, called leptocephalus, cross the Atlantic Ocean on the ocean currents. Near the European and North African coasts, they metamorphose into elvers (young yellow eels). These individuals then settle in coastal zones or in estuaries, or swim up rivers. After they have metamorphosed into silver eels, the adults embark on their migration and reproduction in the Sargasso Sea.