Irish researchers from Inland Fisheries Ireland have contributed to an EU funded research which has helped solve the deepest secrets of oceanic migration and behaviour of one of Europe’s most mysterious fish, the European eel. The international research team, led by the Centre for Environment, Fisheries and Aquaculture Science (Cefas) in the UK, tracked more than 700 eels as they made their annual mass migration from Europe to the Sargasso Sea. Over 200 tags were recovered, allowing the scientists to map more than 5,000 kilometres of the migration route.
Ireland was one of four countries alongside Sweden, France and Germany which released eels between 2006 and 2012 for tracking purposes. The study allowed scientists to map migration routes from Europe to the Azores region, approximately half the distance to the area where they spawn in the Sargasso Sea. Forty four eels were successfully tracked from Ireland with pop up satellite tags. This study monitored eels further and longer than any previous research with one of the tags from an Irish eel registering a journey of 6,982 kilometres and 273 days at sea.
The study has overturned previously accepted theories on the European eel which believed that eels made one large journey to the Sargasso Sea, located in the Western Atlantic near the Bahamas, to breed once before they die. This new research shows that their arrival to the Sargasso Sea is more staggered; a finding which will have impact on how this critically endangered species will be managed and conserved in the future.
While some eels do take the quickest route available, travelling from Europe and spawning in early spring within six months of departure, the evidence suggests that the majority of eels undertake a slower paced migration which enables them to arrive in the Sargasso Sea a year later than previously thought. This means that the eels are at sea for longer and exposed to a higher risk of predation and other mortality events.
Dr. Cathal Gallagher, Head of Research and Development at Inland Fisheries Ireland said: “During this programme, Irish eels were released from the Shannon, Corrib, Erne and Burrishoole catchments to make the journey from our freshwaters out into the open ocean to join their European counterparts and cross the Atlantic. While there is still so much we don’t know about this mysterious fish, this research has revealed more than ever before.
The life cycle and migration of the eel continues to puzzle scientists as they are born into and spawn in the remote ocean, making them difficult to study. While previously it was understood that eels travelled to the Sargasso Sea to spawn, we did not understand the duration and the dangers our eels are exposed to during this migration. We now know more also about their behaviour patterns as all eels exhibited diel vertical migrations, swimming though deeper water during the day and moving closer to the surface at night. This understanding of eel biology will help manage and conserve their population across Europe and beyond more effectively.”
The study relied on two different types of tags, ‘pop up’ satellite tags and internal and external data storage tags, to track the eels. Satellite pop up tags are attached to the eel and pop off on a predefined date, automatically transmitting migration data to researchers via a satellite link. The data storage tags, which are bright orange in colour, need to be physically recovered when they float back to land on the tides. Researchers rely on citizens recovering and returning the tag to help them complete their research. To date, 82 data storage tags have been collected and returned by citizens in several countries, including in Ireland.
Dr. Patrick Gargan of Inland Fisheries Ireland was one of the authors on the research article ‘Empirical observations of the spawning migration of European eels: The long and dangerous road to the Sargasso Sea,’ published in science publication Science Advances.
Interesting facts about the European Eel:
- The growth stage, known as the yellow eel, takes place in marine, transitional or freshwater and can last from 2 – 25 years depending on the location.
- In Ireland, male silver eels range in age from 5-15 years old with female silver eels ranging from 10-20 years of age. However, eels of greater than 30 years old have been recorded.
- Eels from the southern region of Europe tend to migrate earlier than eels from the northern region with faster growth recorded in warmer waters.
- The male eel tends to migrate at a length between 30 and 40 cm, with females migrating at greater than 40cm in length and reaching one metre in length.
- The male and female eel undertake different life strategies with the male reaching the minimum condition required to undertake the migration and spawn; and as a result migrate at a younger age and smaller size compared with females who try to maximise condition.
- During their migration, eels move to deep water (up to 800m) during the day and into shallower water (-350m) at night. This may be to regulate water temperature, avoid predators or for navigational reasons.
- The longest migration recorded in this study was more than 300 days. Each day the eel made a vertical migration of more than 300 metres and back.