Photo Henrik Baktoft

Eels take longer than expected to reach the Sargasso Sea

Wednesday 05 Oct 16


Kim Aarestrup
Senior Researcher
DTU Aqua
+45 35 88 31 42

Mapping oceanic migration with tags

The group used electronic tagging techniques to map the oceanic migration from eels released from four regions in Europe. Of 707 eels tagged, the group received 206 data sets. The route extended from western mainland Europe to the Azores region, more than 5000 km toward the Sargasso Sea. All eels exhibited diel vertical migrations, moving from deeper water during the day into shallower water at night. The range of migration speeds was 3 to 47 km per day. Many migrations ended soon after release because of predation events, but the group was able to reconstruct in detail the migration routes of more than 80 eels.

A new study published this week in Science Advances challenges a century-long–held assumption that eels spawn in the spring following their escapement from European shores

Using fishery data from 20 rivers across Europe in combination with larval surveys from the Sargasso Sea, senior researcher Kim Aarestrup, DTU Aqua and international colleagues suggest that escapement and spawning may not be as tightly coupled as previously assumed. The data and analyses, “Empirical observations of the spawning migration of European eels: The long and dangerous road to the Sargasso Sea”, are published this week in Science Advances.

"This discovery challenges more than a century of assumption about eel migration behavior"
Kim Aarestrup, DTU Aqua

Additionally using available fisheries data they show that most adult eels begin their migration between August and December throughout Europe. Using data from larval surveys in the Sargasso Sea, they show that spawning likely begins in December and peaks in February. This leaves Eels departing from the shores of Europe only 3-4 months to reach the spawning area 4000-5000 km away- or, according to the current migratory paradigm, fail to spawn. These limitations coupled with the routes and speeds of migration calculated from an extensive tagging program on adult eels, show many eels migrate too slow to arrive at the first coming spawning event.

"Even if migration were perfect, eels would have to migrate at speeds well in excess of those observed in our study, and all others, including laboratory studies of swimming speed, to arrive in the period of peak spawning. This discovery challenges more than a century of assumption about eel migration behavior, and suggests that eel migratory strategy is much more flexible and risk-averse than previously thought,“ Kim Aarestrup, DTU Aqua explains.

Migration patterns of European eel.

Migration paths of European eels to the Sargasso Sea (arrows).  Release locations are shown as green circles.  Predators are shown in silhouette.  The locations of trawl surveys for larval eels that have demonstrated the location of spawning are shown by the fishing vessel.

Suggest mixed migratory strategy

Based on the data and analyses Kim Aarestrup, DTU Aqua and colleagues suggest that European Eels do not all arrive at the same time, but rather use a mixed migratory strategy, with some individuals able to achieve a rapid migration to the Sargasso in time for the spawning season immediately after escapement, whereas others arrive only in time for the following spawning season one year later:

“To have a flexible spawning ecology might even minimize the risk of reproductive failure by ensuring arrival at the spawning area ahead of time and enabling the development of full sexual maturity before the spawning season begins in earnest,” Kim Aarestrup argues.

Such a rewriting of the spawning paradigm of the European Eels could have ramifications for population models used in stock assessment, the authors argue. For example, the predation risk and reproductive potential of eels migrating at different speeds may differ, with consequences for estimates of reproductive potential within the current European eel recovery plan.

Righton et al: Empirical observations of the spawning migration of European eels: the long and dangerous road to the Sargasso Sea

Science Advances 05 Oct 2016
Vol. 2, no. 10, e1501694,
DOI: 10.1126/sciadv.1501694

CEFAS has made a short film about the project:

Photo above by Henrik Baktoft, DTU Aqua.

Background on the findings

Eeliad was a 4 year, €4m research project funded jointly by the EU and member states, with the goal of increasing understanding the marine phase of the eel life cycle.  The project involved 12 laboratories in seven countries, and was multidisciplinary in nature.  Scientists used techniques ranging from physical examination of individual eels to high tech genetic and mathematical modelling. The work-package on ocean migration was led by Dr Kim Aarestrup of the Technical University of Denmark. The remaining four technical work-packages involved population structure, eel quality, methods development and larval processes. The overall project was led by Dr. David Righton, Cefas, UK.

The team attached satellite trackers and data loggers to 707 eels captured in rivers in Sweden, Ireland, France, Germany and Spain.  Several months after release, the satellite trackers resurface and communicate their final position via satellite, in addition to depth and temperature data collected during the migration. The buoyant data loggers (flotsam tags) were recovered from beaches by members of the public (see figure 2 and 3) after becoming detached from eels and drifting to shore. The data from the tags’ memory are then used to map the migration paths of each individual eel in detail.

The migration routes of eels converged on the Azores. Migration speed was ~20km per day (varied between 3 km and 47 km per day).  Eels undertook large vertical migrations each day, staying in deep water in the daytime and moving into shallower water at night.  Some of the eels experienced temperatures close to 0°C as they migrated through the deep waters of the Norwegian Trench.  The mysterious vertical migrations are a common feature of eel behaviour.  They might be related to temperature regulation, predator avoidance, or navigation.  Scientists are continuing to search for an answer.

Approximately half of the eels tracked suffered predation. Tag data suggested that diverse predators were responsible, ranging from deep living benthic fish to deep diving whales and endothermic sharks. Most predation occurred near the coast but ten predation events occurred in oceanic waters (water depth >200m), demonstrating a continued risk throughout migration.

In addition to the data collected by the electronic tags, the team analysed data on the abundance of eel larvae in the Sargasso Sea, collected between 1862 and 2007.  The growth rates and timings of captures of larval eels were used to back-calculate the time of spawning for more than 14,500 larval eels.  The analysis showed that larval eels hatch in the Sargasso Sea as early as December, with peak abundance occurring between mid-February and early March, considerably earlier than previous estimates.

The team also used data from eel fisheries across Europe to determine the timing of the autumn departure of eels from the continent.  The analysis showed that, although there is regional variation, the average time of departure was late October.