Science Spin January 2010

Where have all the eels gone?

By Christopher Moriarty

This international eel research project, directed by Dr Kim Aarestrup of the Danish Institute for Fisheries Research, began in 2006. The leading Irish participant is Dr Paddy Gargan of the Central Fisheries Board. His task in 2006 was to find 22 eels and see to the tagging - and that is not as easy as it might seem. A further 29 were tagged in the autumn of 2008 in the current phase of the work.

Each tag costs €4,000, looks like a small torch and is about 15 cm long. It contains sensors which measure and record light, temperature and depth every 15 minutes. Programmed to be released from the eel after a specified time, the tag floats to the surface of the ocean and transmits its store of data to the Argos satellite and from there the information makes its way to Kim Aerstrup's computer for analysis.

The idea behind the project is threefold. The primary aim is to track the eels to their breeding place. But the tags are programmed to 'pop-up' at intervals so that the migration route may also be traced. Thirdly, the data on light, depth and temperature will provide important information on the habits of the eel and its preferences as it makes its journey.

SARGASSO SEA

The reason why the most knowledgeable eel specialists have reservations about the Sargasso Sea - which is located in the middle of the Atlantic Ocean - as a breeding place is that no adult European or American eel and no eggs of either species have ever been found there. So the final scientific proof is still lacking, eighty years after the publication of the theory that the Sargasso is where both American and European eels mate and lay their eggs. On the other hand, nobody seriously doubts the theory, because the smallest larval eels have been caught there and nowhere else.

The Sargasso Sea is an immense area of the Atlantic Ocean, its western extremity lies between Bermuda and the West Indies and it extends nearly half way across the ocean towards the east. Its waters are warm and deep and contain an abundance of free-floating seaweed species called sargassum, hence the name.

A number of species of marine eel spend their entire lives there, but the larvae of two, the European and the American, are carried by ocean currents towards their respective continents and metamorphose from larval to adult shape in offshore waters. These small fishes, known as 'glass eel' and 'elver' migrate inshore. Many of them settle down in the open sea or in lagoons, others move into rivers and lakes. Why and how they should leave the Sargasso to cross the ocean remains one of the many mysteries of the species.

One German scientist attempted a statistical calculation and concluded that the chances of catching an eel in that deep, weedy, ocean water were 80,000 to one against. Part of that problem comes from the size of the Sargasso Sea and the known wide distribution of very small eel larvae: they have been found between 23º and 29º north latitude and 48º and 74º west longitude, an area in the order of 150,000 square kilometres.

Only a minuscule portion of this great sea can be reached by a deep-water trawl in the course of the research vessel cruises which take place there nearly every year. Sargasso trips cost tens of thousands of euro and no country's fisheries research institute can afford either the manpower or the money to indulge in many of them. If tagging and satellite tracking could pinpoint a time and a place to trawl, the possibility of catching a breeding eel would be greatly increased.

TAGGING

People have been tagging eels in Ireland for 40 years and some experiments with pressure-recording tags in the ocean were made here by Fred Tesch and his German team in the 1980s. But the pop-up tags are both immeasurably more sophisticated and very much bigger than anything used before. In the earlier studies, it was easy to buy suitable eels from a number of fisheries.

But the big tags need big eels, monsters, up to one metre in length and weighing two kilograms or more. Eels rarely attain such sizes: the majority become mature and migrate to the ocean for their once-in-a-lifetime spawning when they are less than 70 cm long. At the Galway eel weir, it is reckoned that fewer than one in a thousand reach the required 2kg .

So Paddy Gargan and the staff of the Regional Fisheries Boards were faced with a considerable exercise in logistics, sourcing eels from the Corrib in Galway and from the Shannon, the Burrishoole River and Lough Neagh. But they succeeded and, in the autumn of 2006, the target of 22 big eels had been met and the tagging began. The first three were released at the inner end of Galway Bay at the end of October. The remaining nineteen were taken out to sea between Black Head and the Aran Islands and allowed to swim away in deep water, thereby reducing the risk of loss of the tags in the shallows.

The fact that breeding eels migrate to sea in autumn has been known to science since the time of Aristotle, 2,500 years ago. A little more recently, research cruises in the 1970s and '80s showed that there is a peak of abundance of small larval eels in the Sargasso every February. Eels can swim fast enough to reach the Sargasso Sea in the spring following their autumn departure. And that is why the tags on the eels released in October and November 2006 were programmed to float to the surface at intervals up to April 2007.

Fifteen of the 22 tags worked according to plan. The remaining seven disappeared. They might have been snagged or their eels could have been swallowed by large predators. But a great surprise was awaiting the research team. The longest journey made by any of the eels was only 1,500 km, less than half the distance to their destination. Part of the explanation may lie in the discovery that the eels swim to great depths in the daytime and rise to near the surface at night, thereby making their journey very much longer.

Whatever the explanation, the results showed that the time before the tags are allowed to pop up would have to be greatly increased to allow the eels to get all the way to their breeding ground. That is the purpose of the second phase of the experiment which began in 2008 and is still in progress. Once again, the eels were collected from various parts of Ireland, kept in holding tanks in Galway and tagged and released from the outer reaches of Galway Bay in October and November.

At the same time, eels were also tagged and released in the Bay of Biscay and the Baltic by other national partners in the scheme. The date for the last of these tags to pop up will be April 2010 which, it is hoped, will really give the eels time to reach their homeland and finally solve the problem which has occupied scientific minds since the time of Aristotle.

RESULTS

The analysis of the results from the 2006-2007 tagging appeared in the journal Science in September 2009 and have made an important addition to knowledge of the eel and its habits on migration. The current phase of the experiment may very well succeed in pinpointing the ultimate destination of at least some of our eels. And that, in turn, may help with providing the explanation as to why the numbers of eels reaching European rivers is down to about one per cent of its peak in the 1970s.

Some years ago, I was a member of a multi-national group which looked at all the quantifiable possibilities we could think of. We considered the impact of water pollution, of the building of dams, of increased fishing and so on. None of these correlated with times of abundance and scarcity. The conclusion was that it must be something happening with ocean currents. Oceanographers pointed out that, as we knew nothing about the route taken by the eels and they didn't know enough precise details of ocean currents, they couldn't help. Since our paper was published in 1994, oceanography has progressed a long way and knowledge of the travels of the eel, thanks to this tagging programme, is increasing dramatically. Perhaps an explanation may be forthcoming sooner rather than later.

Meanwhile the International Council for the Exploration of the Sea has concluded that the eel is a 'threatened species'. It is recommending the application of the 'precautionary principle' and taking steps to ensure that fishing effort is substantially reduced. Thereby, greater numbers of eels may be left to set out on their astounding migration to spawn in peace in those deep, distant, warm waters.

LIFE CYCLE

The life cycle of the European eel is one of the most remarkable in the world of nature - and still holds an element of mystery. Little eels swim into lagoons and rivers from the Atlantic and the Mediterranean. Some stay close to the seaside, others penetrate large and small continental rivers. There they feed and grow until some unknown impulse tells them to stop feeding and set off on a migration to spawn. In Ireland, this period of growth extends for anything between 10 and 20 years, with variations from 5 to 50. With a small, but significant, bit of help from his counterpart in Ireland, George Farran, the Danish oceanographer Johannes Schmidt embarked just over 100 years ago on a hunt for the eel's breeding ground. Nearly twenty years later he established that small larval eels could be found in the Sargasso Sea and nowhere else. His study was one of the great classics of marine biology.

Irish people have fished for eels for about ten thousand years. In spite of the fact that they have a better flavour than most other fish and command some of the highest prices, they have not been popular in 20th century Ireland and can rarely be found in retail outlets or restaurants. Eel fishing in Lough Neagh provides a livelihood for about 200 men and there are smaller fisheries on the waters of the Shannon, the Corrib and a few other lakes and rivers. The entire catch is exported. Although eels can be induced to spawn artificially in laboratory conditions, the fishery, together with eel culture, depend on the natural supply of elvers. Concern for the survival of the species throughout Europe has led to a ban on commercial eel fishing in the Republic.