The European eel (Anguilla anguilla) is critically underanged after they have suffered a recruitment of 95% since the 1980s. The reason can be the factor of their marine phases in life cycle and the introduction of the eel swimbladder nematode (Anguillicoloides crassus) in the 80s from Taiwan, which enters the eels and causes severe inflammation, which weakens the eels.
The European eels undertake the longest spawning migration of all the anguillid eels, about 5000 to 10.000 km from across the Atlantic Ocean to the Sargasso Sea. Johannes Schmidt has proposed the Sargasso Sea as the breeding place for the eels, but almost 100 years later, it could not be confirmed, because no eggs or adult eels have been sampled there. To get answers to the oceanic migration of the adult eels, their timings of arrival, navigation mechanisms, the taken routes and their spawning locations, 26 eels from rivers in the Azores archipelago got tracked between 40 to 366 days with attached satellite tags.
Johannes Schmidt has surveyed in the beginning of the 20th century the spread of the eel larvae (leptocephali) with concentration of the smallest specimens in the Sargasso Sea, which is far from their freshwater, estuarine and coastal growth habitat which are in Europe and North Africa. His conclusion was that the spawning grounds comprise a restricted area in the western Atlantic, north-east, and north of the West Indies, between 65o and 48o long, for here – only - are the youngest, newly hatched larvae found. Leptocephali have been found by other surveys since then found in a 2000 km wide region of the Sargasso Sea.
Even though after Schmidt’s discovery the data of larvae founds are limited and still surveys failed to locate eggs or adult eels, in the last 70 years, researcher tried to investigate the oceanic migration of the European eel. Only in the last 10 to 15 years, important progress has been made by the use of pop-up satellite transmitting tags.
21 tags of 23 have recorded data before the tags detached after 40 to 366 days, the average has been 187 days. 2 of the tags got detached within a week, the reason could be through attachment failure or predation. The tag pop-up location confirmed that the eels migrate in the direction of the Sargasso Sea, with covering a straight-line distance of 272 to 2275 km, which means that the average is about 1179 km. One tag got located in the breeding area and five others were located within the Sargasso Sea boundaries.
The data is transmitted when it appears at a time that has been previously determined or when the fail-sage facility is triggered. This includes a critical pressure, the temperature range (-4 to 40) will overshadow or the depth measurement range of 0 to 1300. The time of detachment in this study was programmed to 6 months, 7 months or 1 year.
The European eel from the Azores has the shortest route of 2500 km, yet the eels in the study did not make it to the area before the end of the spawning season. The spawning season begins in December, peaks in February/March and ends in May, so they would have 1 to 6 months. They would have to travel more than 12 km per day to be in time for spawning, but these eels traveled an average of 6.5 km per day. This would mean that the eels are making a slower migration to conserve their energy and reduce the risk of death. Although our study did not yet yield definitive evidence of the mechanism(s) of navigational influence such as perhaps ocean currents, olfactory cues, temperature fronts, magnetic fields or seamounts, ours is the first direct evidence of migrating adult European eels reaching the presumed breeding place in the Sargasso Sea.
It is important to research all the aspects of the life cycle, especially since the eel population is critically underanged and to preserve the species.
This article was presented by Carole at the LEM.SCIENCE Journal Club on 19 december 2022.
Wright, R.M., Piper, A.T., Aarestrup, K. et al. First direct evidence of adult European eels migrating to their breeding place in the Sargasso Sea. Sci Rep 12, 15362 (2022). [link]
Copyright © 2020-2023 Lycée Ermesinde Mersch