Unravelling the eel’s slippery story

IN 1950s Limerick, we turned over stones at low tide in the bed of the Shannon to catch eel-fry wriggling underneath.

The little creatures, a few centimetres long, were plentiful back then, but not any more; the European eel is in trouble.

Numbers have fallen by 95% since the mid 1980s; the species is now ‘critically endangered’. That the cause of the problem is unknown is hardly surprising; we know so little about this elusive creature.

Indeed, a critical part of the lifecycle remains a total mystery. Hopefully, that will soon change; the EU-funded Eeliad Project has made some interesting discoveries.

Eels begin life in the ocean. The larvae, feeding on plankton, drift on currents for about a year. Arriving off the European coast, they change into what are called ‘silver’ eels and enter rivers. Then they become ‘elvers’, tiny versions of the adult, and battle upstream surmounting all obstacles; waterfalls, weirs and dams. The term ‘elver’, short for ‘eel fare’, refers to this rite of passage. Once at its destination an eel, now brown in colour, will remain in its pond, irrigation ditch or stream for years or decades. One has lived in a Swedish well since 1859. When last checked in 2008, it was 150 years old.

Sexually mature silvery eels must eventually say goodbye to their freshwater homes and return to the ocean. They will be at least five years old by then and some will be more than 20. All are believed to travel to an area 3,000km long and 1,000km wide between Bermuda and Cuba, called after a sea-weed known in Portuguese as ‘sargasso’. Mating has not been observed, nor have adult eels or their eggs been found in the Sargasso Sea. However, newly-hatched larvae drifting on ocean currents seem to come from there. In this remote place, 5,000km from Ireland, millions of eels spawn. Then, their great task accomplished, they die.

Four years ago, Eeliad scientists began studying eel migration. They fitted data storage units and radio tags to 500 eels in rivers throughout Europe. Irish ones were tagged on the Shannon, Corrib and Erne. The units were designed to detach from the eel in due course and transmit their information to a satellite. Migration, up to distances of 3,000km, has been mapped over periods of six months or more. Surprisingly, the eels did not swim directly towards the Sargasso Sea. Instead, they headed for the Azores, presumably to avail of ocean currents to carry them towards their destination. The tags allowed only part of the migration route to be tracked. The final phase of the journey has yet to be studied.

But the scientists have other technologies to help them unlock the elusive eel’s secrets.

When the genetic make-up of larvae caught in the Sargasso Sea was compared with that of eels found in European rivers, an interesting pattern emerged. Wild creatures tend to mate with partners from their own or nearby communities and this local bias can be detected by DNA analysis.

However, no such biases were found among the eels; they don’t team up with the girl or boy next door. Their unions, it seems, are much more cosmopolitan. Individuals from widely differing locations, therefore, must meet in a communal ‘ballroom of romance’ to breed and this is likely to be in the Sargasso Sea.

Otoliths, bony structures in the head which enable back-boned animals to detect gravity and acceleration, are especially useful to fish scientists. Micro-chemical analysis of otoliths, taken throughout their range, confirms that eels from widely different locations mate randomly, an unusual result for such a widely distributed species. According to Thomas Dammals, research scientist at the Technical University of Denmark, it suggests that “mating takes place at the Sargasso Sea and nowhere else”.

But why have eel numbers collapsed? Fish scientists have tended to blame rising sea temperatures and changes in ocean currents brought about by global warming. However, according to Eeliad, “climatic and oceanographic changes do not appear to be responsible for the sharp decline in eel recruitment observed over the last 30 years”.