Håkan Westerberg: Eel Migration
Channel: Fishbase Sweden
Date: 2019-11-08

Håkan Westerberg, Institute of Freshwater Research, Sweden, gave a lecture titled "Eel Migration" at FishBase Symposium 2019 in Stockholm, Sweden, 21 October 2019.

ABSTRACT: The life cycle of the eel was finally mapped by the Dane Johannes Schmidt in the 1920’s. It proved to be a 20 000 km roundtrip, between the breeding area in the Sargasso Sea and the continental, freshwater growth area, which spans from Iceland to north Africa. Most of this movement takes place in the open ocean, and studies have been few since the time of Schmidt. The last 3-4 decades has seen a dramatic decline in the recruitment of the eel. The cause is unclear, which has initiated a new interest in what is happening in the ocean – both regarding the adult migration and the larval transport back to Europe. New technology has given opportunities to track migrating eels for long periods and distances and more refined ocean circulation models make it possible to follow the larval transport. This has led to new insights as well as new questions.
 
Regarding the adult migration it appears to be slower and possibly more dangerous than what was expected. Typically, the migration starts from Europe in the autumn to early winter. The breeding activity – as seen from the presence of very small larvae – starts in February-March. With the observed migration speed the majority of the adult eels will be unable to reach the Sargasso during the nearest breeding season. It was also found that a large fraction of the eels became predated. In the western Mediterranean 70% of the eels were eaten by toothed whales before reaching the Strait of Gibraltar.

An analysis of historic surveys of larvae from Schmidt’s to the present indicates that the abundance of larvae in the breeding area follows the size of the total catch of adult eels in the continental distribution area – which can be seen as a measure of the size of the spawning stock. The arrival of glass eels on the European shores has declined much faster after 1980, however. This indicates an increased mortality during the drift between Sargasso and Europe, or possibly a shift in where the glass eels arrive.