International researchers track eels further and longer in their journey to the Sargasso Sea
05 October 2016
Research published today reveals the results of their investigation into the deepest secrets of the oceanic migrations and behaviour of the European eel, one of the world’s least understood fish. The team, led by Cefas’ Dr David Righton, under the auspices of the ‘eeliad’ project, tagged more than 700 eels at their autumn departure from European rivers, and tracked them as they journeyed to spawning areas in the Sargasso Sea. Over 200 tags were recovered, allowing the scientists to map more than 5000km of the migration route from Europe to the Sargasso Sea.
Scientists also showed the risks that eels face against a diverse group of predators, and revealed that, although some eels make a quick journey across the Atlantic Ocean to spawn within 6 months of their departure from the continent, the spawning journey for many eels may take up to a year or more. These results, the most comprehensive and detailed study of eel migration undertaken, opens up a new frontier in our knowledge of these enigmatic creatures.
Dr Righton and colleagues amassed a database of thousands of days of detailed migratory and behavioural information, with many eels tracked for periods of 6 months or more. Previous research efforts typically only gathered data on individual eels for a few days.
The data show that eels across Europe travel across the ocean at up to 47km per day, and an average speed of 20km per day. This range of speeds is slower than previously assumed, and suggests that while some eels are able to reach the Sargasso Sea to spawn in the spring after they leave Europe, many will take up to a year to reach the spawning area and spawn the following season.
The routes of eels from the Baltic, central Europe, Mediterranean and the western continental margin converge on the Azores region after the eels navigate regionally important marine features such as the Norwegian Deep, the English Channel and the Gibraltar Straits.
During their migration, eels are at risk of predation from sharks, dolphins and whales, seals and large predatory fish. The threat is greatest close to the coast, but a number of eels were eaten in the deep (>600m) ocean by deep diving whales.
Many of the datasets were recovered by eels tagged with ‘flotsam tags’, which detach from the eels and drift on ocean currents. Recovery depends on the tags reaching land, and beachcombers finding and returning them. In this way, citizens from the UK to Finland can proudly proclaim their role in uncovering the mysteries of eels.
During their migration, eels move to deep water (up to 800 m) during each day and into shallower water (~350 m) at night-time. This is the same type of behaviour as reported in other studies of eel migration, but it is not clear why eels do this; it might be to regulate temperature, to avoid predators, or for navigation.
The longest migration recorded by the team extended more than 300 days, during which time the eel reach the Azores region. Each day the eel made a vertical migration of more than 300 m and back.
The team also used historic data collected over the last 100 years from expeditions to the Sargasso Sea, and from rivers across Europe in the last 30y, to assess the timings of key events in the eel migratory calendar. A combined analysis of the new tagging data with the old survey data revealed that spawning begins earlier than expected (in December) and peaks in February (14th, yes really!). Coupled with an analysis of the timings of when eels leave continental Europe, the eeliad team modelled the likely arrival times of migrating silver eels at the spawning area. They concluded that, while some eels might make it to the Sargasso Sea in time to spawn within a few months after their departure (as is assumed for all eels), many would need much longer to complete the long journey, taking up to a year or more. This discovery challenges more than a century of assumption about eel migration behaviour, and suggests that eel migratory strategy is much more flexible and risk-averse than previously thought.
Better understanding of eel biology will help us to manage their populations across Europe and beyond more effectively. Eels are an important food fish in Europe, often cooked in regional speciality dishes, such as the famous eel and mash shops in London, or as smoked eel in the Netherlands, Bouillinade in the south of France.
Dr David Righton, Senior Scientist, Cefas said:
“Although they spend most of their lives in freshwater, eels are born into and spawn in the remote ocean, making these aspects of their life very hard to study. Our results are a triumph of technology, logistics and high quality eels. Scientists have dreamed for over 100 years about collecting data of the kind we present today. We tracked eels further than anyone ever before, and for longer periods than anyone before, revealing the roadmap to the Sargasso Sea for eels departing from all regions of Europe.”
“Our results are not just about migration distance. We have shown the risks that eels face from diverse predators, a factor that has not been considered before. By combining our new data with historic data collected in the Sargasso Sea and from rivers across Europe, we have provided a compelling new insight into the oceanic life and migratory strategy of European eels. This is a great achievement for the eeliad team, and our findings offer significant new insights into this mysterious creature. “
Matt Gollock, senior scientist and leader of the IUCN Anguillid Eel Specialist Group based at the Zoological Society of London said:
Studies such as this are essential for our increased understanding of the life history of these incredible fish. Further, new science is essential to ensure the effectiveness of conservation initiatives to safeguard the future of the European eel carried out by ZSL, the IUCN Anguillid Eel Specialist Group and their collaborators.
Notes to Editors
1.The effort was led by David Righton of the Centre for Environment, Fisheries and Aquaculture Science (Cefas), in collaboration with researchers from the University of Agricultural Sciences (Sweden), the Technical University of Denmark (Denmark), the French Natural History Museum (France), the University of Perpignan (France), the Norwegian Institute for Nature Research (Norway), Inland Fisheries Ireland (Ireland), National Museum of Natural Science (Spain), the Institute of Inland Fisheries (Germany) and the University of Porto (Portugal).
2.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.
3.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.
4.The migration routes of eels converged on the Azores (see Figure 1). 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.
5.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.
6.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 start to be ‘born’ in the Sargasso Sea as early as December, with peak abundance occurring between mid-February and early March, considerably earlier than previous estimates.
7. 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.
8.The European eel is an endangered species; the population of European eel has become seriously depleted over the last three decades. Protection of eel populations in countries in Europe is governed by the EU Eel Recovery Plan, and aims to restore the stock of eels to levels last seen 30 years ago.
9.Main image: 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.
10.You can also watch “Tracking the untraceable: following European eels to the Sargasso Sea.” https://www.youtube.com/watch?v=jomcDSnbBtg