Cutting-edge study to save eels

THE European eel is in trouble. 

In the 1960s, our estuaries teemed with elvers, recently in from the sea. A decade later, their numbers began to fall. Since then, the eel population has decreased by 90% and the species is listed as “critically endangered” by the International Union for the Conservation of Nature. Scientists don’t know the cause of the problem; pollution, over-fishing, and changes in ocean currents brought on by rising sea temperatures, are suspects. One definite threat has been identified; eels are killed by the turbine blades of hydro-electric power stations. Researchers at Southampton University have been examining how eels try to cope with obstacles in rivers. Their results suggest that it might be possible to reduce the carnage from turbines.

Eels begin life in an area of the Atlantic, 3,000km long and a 1,000km wide, between Bermuda and Cuba. It’s called after the sagassum seaweed, great quantities of which are found there. Although it hasn’t been proved definitively, it’s believed that all European eels go to the Sargasso Sea to mate and spawn. It’s also their graveyard; once they have bred, the eels die. Their larvae, on hatching from the eggs, drift north-eastwards with the ocean currents for about a year. Arriving off the European coast, after a 5,000km journey, the little eels enter estuaries. There they become tiny versions of their parents and begin battling upstream, surmounting all obstacles; waterfalls, weirs and dams. The term “elver”, short for “eel fare”, refers to this rite of passage. Once at their destinations the eels, now turned brown, will remain in ponds, ditches, and streams for years or decades.

Nobody knows what prompts the behaviour, but an adult eel, having lived a long quiet life, decides one day to leave its freshwater home and head back to the ocean, never to return. During its journey downstream, it may encounter a power station. Swimming with the current, it can be drawn into the intake of a turbine, where its long thin body may be guillotined by the slashing blades. Research published in 2007 showed that between 15% and 38% of the eels encountering turbines on the rivers Meuse and Vecht were killed.

The Southampton scientists wanted to find a way to warn eels of the danger. They began by studying the behaviour of the fish as they travelled down river. A hydro-electric station on the River Stour was taken out of service in the 1970s. It has derelict turbine bays where the team could adjust the flow and velocity of the water passing through them and simulate various conditions eels might encounter approaching a power-station. Forty migrating eels were captured and fitted with acoustic transponders. Eight hydrophones, fitted around the study site, enabled the locations and behaviour of the eels to be monitored. The eels were released upstream of the turbine, five at a time.

Three of the 40 swam upstream and did not enter the study bay. Most of the remaining 37 moved downstream “approaching the intake semi-passively”. They followed the main flow but, on encountering a structure ahead of them, made multiple exploratory approaches before passing it. When the rate of water flow increased gradually, the eels continued to move with it, drifting into the turbine shaft. If the water flow began to increase suddenly, however, they avoided the turbine; sudden acceleration warns eels of impending danger.

If engineers can come up with devices to constrict the flow of water and increase it rapidly ahead of a turbine intake, it will be good news for eels. The sudden change in the speed of the water will alert eels to the impending danger. Engineering a slow steady flow towards a fish-pass, on the other hand, should persuade the frightened eels to use that route instead. This won’t solve all of the eels’ many problems but every little helps.

Adam Piper et al. Response of seaward-migrating European eel to manipulated flow fields. Proceedings of the Royal Society. 2015.