Eel murder mystery

By: Lewis Winks Biology Department

On my office floor in a cardboard box is nearly half a metre of fresh eel, found dead near to our fish pond. The mystery of our guests’ presence here has caused quite a stir today- both staff and visitors peering down in curiosity at the leathery corpse. Its dull grey appearance catching the light is anything but ordinary as we dwell on how it would twist its way through the cold water, occasionally moving over land in wet weather- preferring to migrate on moonless stormy nights.

Eels had factored very little on my biodiversity radar until today when I decided it would be worth doing some research into their biology and habitat. It didn’t take long until I had pinned this particular individual to a species – ‘Anguilla anguilla’ or the European common eel. Don’t be fooled; this fellow is anything but common, especially gnawed in half and discarded next to a lake in mid-Wales. In fact these eels are now in steady decline and appear as critically endangered on the IUCN Red List, the reasons for which remain unclear- although it is speculated that damage to habitat, overfishing and global warming might be contributing factors.

In truth, I had my suspicions about the culprit long before. Mutterings abound on site of strange fish deaths- a head here, a tail there. We have suffered at the paws of an otter before, but not for a while. Their taste for fish occasionally brings them up to our pond where they enjoy a voracious feast. The eel however adds a twist. I never imagined there to be eels in the Dulas River, which flows in the valley below CAT. But the national biodiversity network website does indeed show eel populations in this part of Wales. The otter must have caught it down in the Dulas and brought it up to the pond to eat, discarding what it couldn’t finish.

All eels start their lives in the same location; the Sargasso Sea. From these warm waters the newly spawned larvae form part of the phytoplankton community, drifting with the current for the next 3 years. The larvae, called leptocephali are tiny and able to direct themselves to some extent within the water. However, it is the currents which ultimately dictate where they end up. Some drift towards the Gulf of Mexico and end up in the estuaries of Central America, while others are taken further afield by the Labrador Current. Be it the warm tropical waters of the Americas or the freezing arctic waters of southern Greenland, eels adapt to their surroundings and are widely distributed along Atlantic coasts. It is in part, changes to ocean currents due to global warming which is threatening the future of the European eel.

Leptocephali then metamorphose into tiny ‘glass eels’ as they approach the coastal waters of their destination. Glass eels have more control over their direction and the whims of the great ocean currents, so make their way into the continental estuary waters. Once in the muddy estuary they will change again, gaining pigmentation and becoming known as ‘elvers’. These are visibly closer to typical eels and are the stage most commonly sighted in the estuaries of the UK.  The elvers continue their journey landwards and use the ebb tides to forge upstream, anchoring themselves into the substrate during flow tides. Gaining strength until they are able to swim against currents, the fourth stage of life commences as elvers soon become ‘Yellow Eels’.

From here the story becomes a little more complex, for the fate of the yellow eel will depend on its geographic location in adulthood. Some remain in the estuary waters for long periods while others have an urge to continue as far up stream as they can, often surging onwards when favorable conditions prevail. It is mostly the females that move upstream while the males are more likely to remain in the brackish intertidal waters below. What’s more, all eels return to their spawning ground in the Sargasso Sea at the same time- European eels setting off on their migration slightly earlier than their American counterparts. When reproduction calls them back, the eel changes one last time; the body becomes fatter and darker in colour, and the eyes enlarge. She is now known as a ‘Sliver eel’ and begins the last great journey of her life, to the place of her birth. European eels can live for up to 85 years, and once she has spawned many thousands of new Leptocephali larvae, the adult silver eel will die.

Very little research has been completed on eel migration and lifecycles, their spawning habits are almost totally unknown. No one really knows how exactly they migrate such huge distances, it is speculated that they use the moon and stars to navigate. Nor do we know how they behave when reunited at the end of their lives in the Saragossa. More money and effort is now being put into eel research but conservation is difficult. Because the eel has such a huge geographic domain and elvers do not return to the rivers their parents lived in, monitoring of global populations is almost impossible. Landings from fishing boats provide some indication, but efforts to maintain populations locally can be clouded by the larger concern of changes happening in ocean currents, on which the global distribution of eels is dependant.

Barriers to migration upstream and habitat loss are important local considerations for eel conservation. Placing of dams and altering water courses can prevent eels from reaching the upper course of a river. Eels need more habitat, effort on a global scale to manage and restore river catchments with increased sensitively might give them a better chance of survival.

We are certainly lucky to host an eel, albeit a dead one here in the Dyfi Valley. And despite the blood-lust of other guests we are privileged to welcome such a discerning predator too. Indeed, if the eel is threatened by climate change and habitat loss, so too is the otter and any other animal which preys on the mysterious Anguilla anguilla’.

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  • All eels come from the same place? who would have thought?

    Nice blog!

  • Are eels edible to humans?
    What are their common predators?
    How many children can a female eel produce in …?
    For an animal that lives longer than most men, wouldn’t it be a mistake to overbreed them?

    JR Alila

  • Arthur

    Great blog. Keep it eel Lewis!

  • Betunague

    now I’ll stay tuned..