European eels are critically endangered. Their larvae in particular are exposed to great danger as they migrate from their birthplace in the Atlantic east of Florida to the Mediterranean. Researchers have now discovered how the fish larvae, which are only a few centimeters long, overcome the Strait of Gibraltar. They therefore take advantage of nighttime currents and “surf” on the perfect wave from the Atlantic to the Mediterranean.
The European eel (Anguilla anguilla) has one of the most extraordinary life cycles in the animal kingdom. Its larvae hatch in the Sargasso Sea east of Florida and from there cross the Atlantic to reach the Mediterranean. For most of them, however, this is far from the end of the journey. The young eels migrate further into the rivers and spend most of their lives there. As soon as the eels reach sexual maturity between the ages of 15 and 30, one last big journey awaits them: they migrate back to their birthplace in the Sargasso Sea, give birth to the next generation of eels and then die.
“Border control” in front of Gibraltar
In recent decades, European eel populations have declined sharply, so that the extraordinary fish is now considered critically endangered. Eels are most vulnerable in their early stages of life. In order to take appropriate protective measures, it is essential to find out more about this phase of life and the dangers associated with it. Researchers led by Marko Freese from the Thünen Institute for Fishery Ecology have now investigated how the larvae manage to get from the Atlantic to the Mediterranean and thereby cross the Strait of Gibraltar, which is only around 13 kilometers wide.
To do this, the team took samples with a trawl net at different times of the day and evaluated how many larvae were caught in each one. In this way, Freese and his colleagues wanted to find out whether the eels prefer to swim through the strong currents at certain times of the day or at certain depths. And in fact: the researchers caught most of the eel larvae at night. They were at a maximum depth of 100 meters.
Surfing the perfect wave
The team concludes from this that the larvae, which are only a few centimeters long, specifically use currents that are strongest at precisely these times of day and in the upper water layers. So you basically “surf” on the perfect wave into the Mediterranean. “The direction and speed of currents in the Strait of Gibraltar are largely influenced by the Atlantic tidal system, which results in a constant movement of oceanic water from the Atlantic to the Mediterranean and back,” Freese and his colleagues explain. During the day, deep water from the Mediterranean flows westward towards the Atlantic. This prevents the larvae from crossing the strait during the day. At night, however, the currents are more favorable for them.
The new findings are now helping to better understand the complex life cycles of the endangered species and to better protect the eels in the future. But there is more to learn: “Future studies should also take into account larval arrival times, environmental factors such as weather conditions, different tidal phases, and changing environmental conditions due to climate change,” write Freese and his team.
Source: Johann Heinrich von Thünen Institute, Federal Research Institute for Rural Areas, Forests and Fisheries; Specialist articles: Scientific Reports, doi: 10.1038/s41598-024-82929-z