Another Life: Tracking the way to Sargasso for Ireland’s silver eels

Living through an autumn of near-forgotten vintage – tranquil, golden, benign – it has been hard to quell some measure of paranoia. Lulled in the embrace of those Scandinavian high-pressure zones, one peered west beyond their contours for the tight-wound watch-springs of “extreme weather events” that must surely lie in wait. Even normally (whenever that was ), October and November would bring at least one night of black Valkyrian storm and rain, rocks rumbling in the hill stream, leaves whisking from the trees.

I could lull myself to sleep by counting all the silver-bellied eels rushing out of the estuary through the crash of surf and blinding, stirred-up sand, to start the long swim to Sargasso. For this is their departure time and plenty of rain their weather. Instead, my bedtime reading has included the Empirical Observations of the Spawning Migration of European Eels: the Long and Dangerous Road to the Sargasso Sea, research by 16 European marine biologists, published online by Science (iti.ms/2eNLgRa).

Their main discovery from tagging hundreds of migrating eels has been well advertised. Instead of a century-old assumption that eels swim at high speed to meet up in the Sargasso Sea for a single, springtime orgy, it seems some are quite happy to take their time and may even arrive at Sargasso for next year’s spawning season. There are, it seems, useful implications for the management of Europe’s critically endangered remnants of its once-abundant Anguilla anguilla populations. There is fascination, meanwhile, in the detail of the operation and much new knowledge of the journeys of the eels.

Satellite-tracking has been attempted before, including Irish eels, but not with the scale and success of this five-year operation. It tracked the eels to the Azores, in some cases 5,000km towards their spawning destination. Its pop-up and archive tags yielded 4,883 days of depth and temperature data from which to reconstruct migrant behaviour. Tracking began with some 700 female silver eels, captured at their "escapement", mainly between October and December, from 20 European rivers spread from Sweden to Spain.

They included some from the Shannon and others from Lough Mask and Lough Owel. Patrick Gargan of Inland Fisheries Ireland was one of the research team. A map reconstructing the migration paths shows eels from Baltic rivers and Sweden's west coast swinging north bunched together in the Norwegian Sea, then west, well north of Ireland, into the Atlantic before spreading out. Those from Ireland and France headed southwest and one tagged Irish eel, "at liberty" for 280 days, reached the Azores, where the different paths were converging.

A good many of the eels never left the Atlantic shelf, being eaten by predators, so that much of the best data came from some 80 of the original 700 eels that reached the deeper Atlantic. It was here they added to their journeys by migrating by day to the shadowy depths and swimming in shallower water by night – a shift between 1,000 metres and 200. Such vertical migration is common among ocean organisms as a defence against being eaten and a good reason for the eels’ silver bellies, helpful camouflage from below. While their maximum depths could not be measured, the tags’ monitoring showed temperatures as low as zero degrees.

The most significant information, however, came from matching the eels’ “escapement” times and swimming speeds with surveys of newly hatched larvae, the size of pine needles, in the Sargasso Sea. In the long-accepted model of migration, eels swam rapidly in a straight line to a Sargasso gathering starting in late winter, with spawning at a peak in March and April and extending into May. Their actual swimming paths are not the direct “great circle” routes of transatlantic planes, but often meander and make pauses.

The well-remarked Irish eel, for example, took 10 months to swim more than 6,900km, having doubled back to the Azores after travelling south. With anything up to 10,000km to travel, depending on where they start, Europe’s eels were hitherto presumed to take between 80 and 170 days to reach Sargasso. However the new study found that for half the eels to arrive in time for peak February spawning, they would have to travel up to an extra 53km a day, at speeds much faster than those measured at body lengths a second.

It concluded that the Sargasso spawning season must begin in December and that a lot of eels don't make the earliest round. How this helps the management of national eel fisheries is far from clear. With eel stocks down to a 10th of historical levels, Ireland has banned commercial fishing (although catching goes on illegally) and the ESB has been urged to engineer better passage for eels up and down the Shannon, rather than mincing migrating silver eels in its hydroelectric turbines.

Michael Viney's Reflections on Another Life, a selection of columns from the past four decades, is available from irishtimes.com/irishtimesbooks