Embryonic development is already disturbed at concentrations 10 times lower than the norm for human consumption.
Dioxin-like contaminants such as PCB's are probably playing a key role in the extinction of the eel. Embronyic development in this species is halted at dioxin concentrations 10 times below the levels for human consumption. This discovery was made by a team of biologists, led by Guido van den Thillart, who are conducting research in Leiden into the sexual maturity and reproduction of the eel. To date, the search for the causes of the reduction in the numbers of eels have concentrated mainly on quantitative factors, such as over-fishing. The researchers published their findings on 1st March in the journal ' Naturwissenschaften'.
The eel is on the point of extinction. Since the fifties, the number of eels has declined sharply. The influx of elvers, young eels which swim from the ocean up river, in recent years is less than 1% of the level in 1950. It was in the fifties that dioxins started to appear in the environment.
Dioxin-like contaminants are not just toxic, they also have a disruptive effect on hormone regulation and gene transcription mechanisms. It has been known for some time that this caused a considerable reduction in the fertility and reproduction of a large number of species of animals and it now appears that the eel is one of the species affected. Van den Thillart: 'Only, this species seems to be more vulnerable'.
To study the consequences of dioxins on eel embryos, the researchers dug up eels in the Grevelingen lake and in the Loire, which had just started their 5500 km journey, probably to the Sargasso Sea where they would eventually breed. Eels only become fully mature during the course of this journey. The researchers brought the eels to maturity in the lab, using hormone injections, and fertilized their eggs artificially. They measured the dioxin levels in the fat of the eels and followed the development of the embryos. Their conclusion was that there is a very strong negative correlation between embryonic development and the levels of dioxin-like substances. Even at dioxin concentrations 10 times lower than the norms for human consumption, embryonic development ceased; embryos suffered serious deformities and died.
Eels live on the beds of all freshwater systems from Norway to Morocco and Egypt. It is these seabeds in particular where there are high levels of dioxin contaminants. Within Europe, only Sardinia and the west of Ireland have lower concentrations than those measured in the biologists' experiments. It can therefore be assumed that dioxins play a key role in the disappearance of the eel.
Dioxins are stored in fat. The eel accumulates fat, which makes up about 20% of its body weight. This fat constitutes the eel's reserves for the long journey to the Sargasso Sea where biologists believe reproduction takes place. Fat is in the first instance the fuel for swimming, but in the course of the journey, a great deal of fat finds its way into the egg cells of the females, explains Arjan Palstra. 'In these egg cells, first fat is accumulated, and then proteins. The fat and proteins then become the substance of the yolk sac of the larva.'
Palstra: 'To date there was only evidence of quantitative causes of the decline in the eel population. Over-fishing is one example. But we are studying the quality of the silver eel, eels which are not yet mature. Silver eels leave the freshwater sites and swim across the ocean. We gradually learned that the quality of this species is declining. A particular virus seems to make silver eels anaemic once they have completed a considerable part of the journey and have expended a lot of energy. They also suffer from a parasite which damages their air bladder. And then you also have the high concentration of PCB's in their fat, which drastically reduces their fertility.'