Eel stocks are currently undergoing a steep decline, although no one knows exactly why. This is why researchers from a number of institutions including DTU Aqua are in the process of examining the European eel's (Anguilla anguilla) behaviour in order to understand what happens during its reproductive migration from Europe to the Sargasso Sea.
In collaboration with other researchers from DTU Aqua and the Norwegian Institute for Nature Research, Senior Researcher at DTU Aqua Kim Aarestrup has just published the article Survival and progression rates of the large European silver eel Anguilla anguilla in late freshwater and early marine phases in Aquatic Biology.
"We are investigating whether the decline in European eels can be attributed to problems they encounter during their long migration to spawning grounds in the Sargasso Sea," says Kim Aarestrup. "Eels begin to leave their home rivers in autumn, and the first phase of their migration to the coast is very risky for them. We investigated the mortality of eels in this first phase by following 50 migratory silver eels as they swam through the lower parts of the River Gudenaa and in the first phase of the marine migration in Randers Fjord," he continues. "We looked at both the behaviour and the survival rate during this first part of this migration
The results show that while mortality was low in the lower parts of the river, fully 60% of the eels disappeared in Randers Fjord. This confirms the hypothesis that the mortality of silver eels is high in the early marine phase of migration.
Silver eel mortality during migration in higher parts of the river may also be high. Previous studies both in the central part of the River Gudenaa and in other European rivers have shown low survival rates. Overall, it appears that the final survival rate into the Kattegat for silver eels from the River Gudenaa may be less than 10%. On top of this can be added the mortality rate which occurs during the rest of the eel's approx. 6500 km journey to the Sargasso Sea.
Why do 60% of the eels disappear in Randers Fjord?
Researchers examined the migration of the 50 large, female silver eels by implanting an acoustic transmitter. Using a number of automatic listening stations, researchers could then follow the eels as they migrated from the lower parts of the river and into Randers Fjord on the first phase of their marine migration.
21% of the eels in the fjord were reported caught by fishermen (including both anglers and commercial fishermen), but fishing was probably responsible for catching an even greater number of the missing eels in the fjord. Based on interviews with fishermen, it was confirmed that several tagged eels (the exact number is not known) had been caught in the fishery without being reported. Moreover, because the tags are implanted internally, it is not immediately obvious that an eel has been tagged. In consequence, eels that are sold and exported alive may include tagged eels.
No transmitters have been detected in the fjord through manual tracking. This rules out the alternative explanations that some of the eels were eaten by other fish in the fjord. One other possibility is that some eels may have been taken by birds such as cormorants, which are capable of carrying an eel (and the transmitter) out of the water. However, this is probably not the case with the largest eels in the study -- they were simply too big for a cormorant to be able to fly with them. All in all, therefore, the most plausible explanation for the results is that the eels in this study disappeared as a result of fishing, leading to the conclusion that fishing may be a major cause of mortality among silver eels.
Eels migrate too slowly
Aside from the valuable information on mortality rates, the study also showed that eels migrated relatively slowly out of the river and the fjord. At present, it is thought that eels reach the Sargasso Sea the spring after they have left their home river. But even swimming at the fastest speed measured in eels in the outer part of the fjord, only very few would have managed to reach the Sargasso Sea the following spring.
"If eels dont swim faster than what we observed in the outer part of Randers Fjord, there will only be a few eels which are able to migrate fast enough to reach the Sargasso Sea during the spawning season the following spring," says Kim Aarestrup. "so either they increase migration speed after leaving the fjord or some eels may quite simply take longer to reach it."
More eels need to reach their spawning areas
Based on the available evidence, Kim Aarestrup concludes, a considerable increase in eel survival in the river-fjord system will be needed in order to fulfil the goals in the European Union recovery plan concerning the number of eels that must succeed in reaching the Sargasso Sea. It is therefore vital to increase our knowledge about the survival of migrating silver eels in the different migration phases in order to ensure that enough live eels reach the Sargasso Sea to spawn.
Eels are victims of circumstance
It's not easy being an eel. "The eel is a victim of circumstance," says Kim Aarestrup. "In other words, as well as fishing, a number of other factors -- barriers in the form of hydro-electric power plants, loss of habitat, parasites, disease, pollution and potential changes in ocean currents -- are thought to contribute to the eel's decline."
There are numerous hydro-electric power plants throughout Europe, where fishing for eels with nets and traps is also equally prevalent. It is therefore likely that there is a high mortality rate among migratory silver eels in many other rivers and estuaries throughout Europe.
In addition, eels are caught in strategic locations along the coast (e.g. in the Baltic Sea) and this contributes to further reducing the eel's chances of survival.
For Kim Aarestrup, it's hard to ignore the fact that the greatest influence on the decline of eel has been human interference.
"If you look at the decline in available eel habitats, mortality during migration, including getting past hydro-electric power plants and fisheries, pollution and disease, then yes, it's hard to say that human influence has nothing to do with the decline of eels," he says. "However, because we know so little about eels, it is difficult to prove that preventative measures such as restrictions on fishing or the removal of hydro-electric power plants will make a difference," he explains.
"But the question is of course whether we shouldn't be doing more if we actually do want to save eel stocks," he explains.
The studies are part of the EU financed project EELIAD.
- K Aarestrup, EB Thorstad, A Koed, JC Svendsen, N Jepsen, MI Pedersen, F Ψkland. Survival and progression rates of large European silver eel Anguilla anguilla in late freshwater and early marine phases. Aquatic Biology, 2010; 9 (3): 263 DOI: 10.3354/ab00260
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