Aingi's bladders
2024/12/01 Etxebeste Aduriz, Egoitz - Elhuyar Zientzia Iturria: Elhuyar aldizkaria
The eel has always been surrounded by many mysteries. Some have been clarified over the years of research. But today there is another mystery that scientists will hardly clarify: although it is an animal in serious danger of extinction, they appear scientifically on their plates. What's that like?
The advice is clear: “Zero capture.” The Scientific Council of the Anguilla of the International Council for the Exploration of the Sea (ICES) annually analyses the situation of eel and provides scientific advice on this matter. Estibaliz Diaz Silvestre, AZTI researcher, is part of the ICES/ICES eel team: “2000. This advice has been made since 2000, so far it has been recommended that the catches should be as close as possible to zero and now, directly, they should be zero”.
The decline of the European eel began in the 1980s, and the number of angles reaching the coast has fallen by about 90%. In recent years it is at a very low level, below safe biological levels, according to ICES/ICES. And the International Organization for the Conservation of Nature (IUCN) included it in the Red List in 2008, in the category of “seriously endangered”. The next category is to be gone.
According to the latest ICES/ICES evaluation, in 2024 7 of the 100 angled streams reached before the 1980s (7.2%) in southern Europe and one in the North Sea (1.1%). “The situation is serious,” Díaz clearly says.
The researchers of the AZTI research center perform very relevant measurements in the Oria River for this annual evaluation. In fact, the Oria is a pilot basin for monitoring the evolution of the species. All phases of eel are measured in order to calculate the approximate biomass. In fact, eel has a very special life cycle, and they don't know how much they reproduce. “In the case of anchovy or many other fish species, the breeding biomass is measured, and that is the main criterion for measuring the state of the population; but in this case we cannot do it, as the breeders are in the Sargasso,” explains Díaz.
The sampling of Oria is of great importance because it is the only scientific sampling carried out throughout the Iberian Peninsula. The rest of the data corresponds to the catches taken. “There we have a problem,” says Díaz, “most of the data is from the north. In southern Europe we have very little information, but most of the recruiting (the amount of angle reaching the streams) takes place in the south, specifically in the Gulf of Bizkaia. That’s why the Oria series has a double importance, because it’s from the south and it’s scientific.”
Scientific fishing
The angle sampling is carried out at the entrance of the estuary, under the highway bridge. They do it with boat, with the same instruments as angled fishermen, but with flow meters to the nets. “We start when the tide begins to rise, because the angle enters with the tide, and we continue until the tide completely rises,” Díaz explains. “We measure the number of angles entering for a volume of water and thus calculate the density of the angles.”
The second sampling is carried out in the Orbeldi dam at Usurbil. “The tide limit is there. Therefore, we measure down the recruitment of the estuary and the Orbeldin River, that is, how many enter the river”.
Finally, sampling of eels is carried out through electric fishing. “We measure different parameters to the captured eels in order to know, in addition to the number of eels, how many are willing to migrate that year,” explains Díaz. “When they are willing to reproduce, the eye begins to atrophy, to develop the fin, to change color…” It is called a silver eel which is at this stage and which is going to take the path to the sea. And the yellow eel is the one in the previous phase, which grows and gets fatter in the river.
“Autumn is the right time to distinguish silver and yellow eels and that’s when we sampling,” explains Josu Elso Huarte, Environmental Management Biologist of Navarra, GAN-NIK. They track the eel in the Bidasoa River and have a tracking station in Bera-Lesaka. “Here, above all, we keep track of salmon and trout. The eels enter the trap, but they also come out, so it does not serve us for it.” For eel samplings, electric fisheries are also carried out. “We have eleven points all over the Bidasoa and every year we look at the same points to see how many eels there are, whether they are males or females and whether or not they are plated.”
An automatic system for counting and capturing fish images will soon be installed at the monitoring station. “In principle, it is not intended for eels, but all the fish will have to go through a tunnel, and there are the cameras; I think it will also serve us for eel,” says Elso.
In the Bidasoa they also see it clear that the eel is not right. “The situation is serious,” Elso also says; year after year we are seeing how it is falling.”
There is no threat to it. “In many other species, fishing is the main threat,” says Díaz, but the eel has many more: the threats of the sea, the streams, the estuaries…”. In fact, it goes through so many places and phases ... It's a fascinating animal. “It’s awesome,” says Díaz.
Illuminating mysteries
Many centuries and researchers have needed to uncover the secrets of eels. Aristotle was astonished at the lack of reproductive organs; Sigmund Freud went crazy trying to find the testes of the eels; Yves Delag discovered that the strange Leptocephalus brevirostris fishes caught in the sea became angled; and Johannes Schmidt for 18 years sought the formation of the leptozefallos (larvae). In 1923, Schmidt discovered that the eel laying area is located in the Sargasso Sea, where he found the smallest larvae.
Another century has passed and eggs and breeders have not yet been found. Every three years, a German expedition is going to fetch eggs, but at the moment, the only thing you've seen is that there are fewer and fewer larvae. Therefore, it seems that fewer and fewer players reach the Sargasso Sea.
The progress in clarifying this journey has been made in the last yearos.En 2016 published a paper in the journal Science, for five years followed by telemetry the eels from all over Europe, and saw that they all reached the archipelago of the Azores. In another study published in 2022, transmitters were placed to 21 silver eels, which were released in the Azores. They managed to continue to the Five Sargasso Sea, and one of them went precisely to where it is believed to be the eels laying area. “That was a great achievement, a great milestone,” Diaz stressed, as the first direct evidence of the journey of the eels is that the European eel followed until the Sargasso for the first time.”
Larvae born in the Sargasso, following marine currents, travel 5,000 km to North Africa or Europe. This journey can last between ten months and three years. Once on the coast, they become angles that can live in fresh, salty water. As they enter rivers or wetlands, they become yellow eels, which will feed on invertebrates and small fish for 5-20 years. Once the time has come, and when they have accumulated enough reserves (as they will not be fed during the trip), the eels become silver and return to the sea of the Sargasso to reproduce. In 2013 it was demonstrated that eels have a magnetic compass, that it is the larvae that record the journey made, and that because of this they find the way back.
Many threats
A fascinating cycle in which, as has been said, there are many threats. In addition to fishing, river barriers can be one of the most important. In European rivers, 1.2 million obstacles are inventoried, almost one obstacle per kilometre. “Our data clearly shows the influence of dams,” says Elso. “When we do electric fisheries in the areas below and above a dam, we see that the population is falling to less than half.”
In fact, the dams remaining in the Bidasoa have footsteps for fish, but are not suitable for eels. “They’re proper ladders for trout and salmon, but eels move differently, move at the bottom of the river and don’t jump. They need another kind of catwalks, and here we are still far from putting them.”
They're also affected by pollution. They are able to live in polluted waters, but that can have negative consequences. In addition, they accumulate a lot of fat to be able to travel to Sargasso without eating, where they accumulate many heavy metals and persistent organic pollutants (PCB, HAP, PBDE). Normally they do not die, but these pollutants hinder their migration and, if they reproduce, can cause malformations in eggs and larvae.
They also have a parasite that comes from outside and makes things very complicated for infected eels: Anguillicola crassus nematode. As the Asian eels enter the rivers of Europe, they entered with them and are now located in most of the rivers of Europe and North Africa.
Moreover, climate change can lead to changes in marine currents. “This is probably already happening and the larvae may go somewhere else,” says Díaz. “The problem is that at the moment we cannot quantify it, we suspect, but we cannot quantify it. And the same is true of streams; the increase in temperature will not affect them so much, as it is a species of tropical origin, but some areas may be dried up, or the rains that the plateads use to descend will be modified, and that could influence.”
“Fishing mortality can be measured fairly easily, but the rest can’t,” added Diaz. “That’s why it’s very difficult to say what the main threat is. There are many factors. And the advice we give from the ICES/ICES is that, given the seriousness of the situation, we must have an impact on all the factors, that is, the advice is to stop fishing and all those other anthropogenic factors that cause mortality. And to do so, it is very important to take measures to improve habitat, for example.”
Comprehensive and data-driven management
In addition, Díaz considers global management indispensable: “In the end, the eels coming out of here will come together in the Sargasso with the eels coming from other places; and if there are few, the problem will not be solved. Improving the situation of eel will depend on what is done in the whole range of eel.”
Elso also believes that habitat recovery is fundamental. And on the other hand, it's very clear. “Fishing should be banned and illegal fishing should be combated.” However, it is not forbidden from Europe, and the Basque Government has professionalised the fishing of the angles, which until now was leisure. This year 150 licences have been distributed and 554 kilos can be fished. For Elso, that is “today’s bread, tomorrow’s hunger.” “If we continue like this, the eel will disappear and the whole business that has been organized here will disappear.”
Elso recalled the case of anchovy to demand data-driven management: “Anchovy data began to be taken at AZTI many years ago and decisions have been made accordingly. Thanks to this, we now have anchovies at sea and on the plate.”
“After all, we all want the same thing. When I talk to the salmon fishermen, I tell them that, being everyone on the same boat, we all want salmon to be on the river. If the situation is serious, fishing must be closed and when it improves we can re-fish. The same thing happens with eel, and it may be more serious.”
Culture and tradition are the main reasons for maintaining angled fishing. “We must also bear in mind how traditions start,” says Elso. “They started fishing ananges because there was a lot, simply. The old people tell us that the poor ate them. Now it's the food of the rich. In any case, a means is needed to maintain tradition.” That is, if the eel disappears, the tradition will also disappear.
Meanwhile, an endangered animal is on the plates. “On an angle plate there are hundreds of eels (about 300 per serving),” Elso stressed. “But people don’t know; and that’s another challenge we have, the socialization of these issues.”
Everything will be done. In fact, in the case of eel, the distance between fish craving and cheese making is very small.
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