The Strait of Gibraltar proves no obstacle for eels

Senior Researcher Kim Aarestrup, from DTU Aqua, is part of the research team behind the article, and he is very excited that it has now clearly been established that even eels in the Mediterranean are important for the future of eel stocks.

"Our results provide evidence that Mediterranean countries also have an important role to play in helping to save the European eel. There has previously been speculation about whether eels in the Mediterranean could even find their way out into the Atlantic; thus the question was whether the Mediterranean eel was important for maintaining the eel population, or whether we might as well just eat them all. Well, obviously, we shouldn't!"

Eight eels equipped with satellite transmitters The research team tagged eight silver eels in southern France with small advanced measuring instruments, pop-up satellite transmitters, which detect light, depth and temperature along the eels' route. The data were sent to scientists via satellite when, six months later, the tags were released and rose to the surface to be collected.

Five of the eels, however, were apparently eaten by predators in the Mediterranean. From the data, researchers can see when an eel is eaten along the way as the data pattern suddenly changes, for example if an eel is monitored near the surface during the day without the tag registering any light data. Then, the eel is probably in the belly of a predator such as other fish or a marine mammal.

From the temperature and depth measurements, Kim Aarestrup estimates that four eels were eaten by marine mammals, i.e. whales or seals, whilst the last one potentially ended up in the stomach of a blue-fin tuna:

"It's not something we can say with certainty, but it is an educated guess, since no mammals have ever been recorded to dive down for as long as we saw for that tag, and no other kinds of fish in the area will have such a high internal temperature. So here we might be seeing an example of one endangered species eating another endangered species," says Aarestrup.

Swimming deep to avoid the Strait of Gibraltar's strong current At the end of the six months that the study lasted, there were three eels left. One of them was still in the Mediterranean Sea, 719 km from where it had been labelled. But the remaining two eels were in the Atlantic Ocean, 2,000 km from the location where they were tagged. And they must have swum through the Strait of Gibraltar to get there.

"It is both satisfying and interesting that for the first time we have been able to show that eels can migrate from the Mediterranean through the Strait of Gibraltar. One of the arguments against this being the case has been the at times strong ingoing current would, but our data show that the eels do manage to get through. They probably don't like the strong current, so they change strategy, as we can see, and break their normal swimming pattern by swimming towards the bottom to avoid it, and staying there while passing through the Strait. Out in the Atlantic they then change back to their normal pattern again," says Kim Aarestrup.

The eel's ability to switch swimming strategies is not the only thing that has surprised the scientists. Data from the satellite tags have also challenged the hypothesis that it is the temperature differences in the water that normally makes eels swim at a depth of 200-400 metres at night and then dive down to a depth of 600-1000 metres during the day.

"We see the same swimming pattern in the Mediterranean as we have seen previously in the Atlantic Ocean. But in the Mediterranean, there is no discernible difference in water temperature, so it means that it is cannot only be temperature differences down through the water that cause the eels to switch between deeper and shallower water during the day," says Kim Aarestrup.