EELS TAGGED IN BID TO UNDERSTAND THEIR RIVER MOVEMENTS AND MIGRATION

Eels tagged in bid to understand their river movements and migration. The Game & Wildlife Conservation Trust’s (GWCT) fisheries team have begun a new project to catch and tag eels in the River Frome in Dorset in a first ever bid to better understand the movements of this fish species in our river and waterways and monitor the start of their migration from the UK and across the Atlantic Ocean.

The European Eel has declined by around 90% since the 1980s and is one of only a few UK species considered to be ‘critically endangered’. It is disappearing faster from our rivers than any other fish despite being considered one of the toughest and most resilient of fish species.

Eels hatch in the Sargasso Sea near Bermuda and spend about three years drifting across the Atlantic Ocean on the Gulf Stream to arrive in Europe as glass eels that then become elvers and swim up rivers and waterways across the continent. They can spend decades living in the riverine environment before they migrate back across the Atlantic to the Sargasso Sea, where they spawn and die.

Despite being one of our longest living river inhabitants, little is known about their life and what triggers the start of an eel’s migration.

The GWCT fish research team spent a week in late June setting eel traps along a stretch of the River Frome near Wareham in Dorset. The traps were left overnight and emptied the following day. They caught 74 eels and each one was weighed, measured and tagged with an acoustic or PIT tag to enable the research team to track their movements.

The project is a collaboration with the Environment Agency, the Centre for Environment, Fisheries and Aquaculture Science (Cefas) and Bournemouth University.

The second phase has now begun where the team is trapping, catching and tagging mature eels, known as silver eels, as they start their migration. The team will then track them on their way to the sea, and with luck, some way along the Channel.

Eel tracking is an entirely new study area for the team, which has been monitoring the Wild Atlantic salmon stocks on the River Frome for the past 50 years, as well as carrying out research into many other fish species.

‘A bit of a mystery’

Will Beaumont, leading the fieldwork for GWCT, says:

“The project is looking into eels, their behaviour and movements while in the river and harbour environments. For a fish that has been around for millions of years and has been such a culturally and economically important fish, we know very little about it. The lives of eels are a bit of a mystery. We do know they are one of the toughest fish species we have, but we still seem to be managing to wipe them out. If we want save them, it is extremely important we get a better understanding of them so we know how we can stop them from disappearing forever.”

To be able to monitor the eels in the River Frome and Poole Harbour, the GWCT team, in collaboration with researchers from Bournemouth University, spent five days setting traps and running a mobile riverbank laboratory near Wareham.

This is done under a strict home office licence and involves netting them overnight and processing them the next day, before releasing them back into the river. The process is done under the highest possible welfare standards and the eel are not harmed.

“For the larger eels we were using acoustic tags that transmit an ultrasonic ping every 30 seconds and PIT tags, which are the same as the ‘chip’ you may have in your pet cat or dog. The smaller eels only get a PIT tag.”

The eels carrying acoustic tags will have to swim within 400m of a receiver to be detected, and the smaller eels need to be caught again in future surveys and scanned.

The team have placed 35 receivers along the River Frome and in Poole Harbour. There are also some in the Channel, run by Plymouth University, which the eels may also be detected on. These include one outside Plymouth and another near the Channel Islands.

“Now we just have to hope that the eels we have tagged swim near enough to these so we can pick up their signal,” Will says hopefully.

The data collected will help researchers see what habitat the eels are choosing, how they move around the river catchment, what the transit times are through different environments when they decide to start moving down the river, through the harbour and down the Channel. It can also give them clues as to whether the eels prefer to stay higher up near the surface or lower down near the bottom when migrating and what the survival rates are for different environments.

The data collected from this collaborative project will go on to form the basis of a PhD thesis by a Bournemouth University student. The Environment Agency, which is providing much of the funding, is looking to gain a greater understanding of eel behaviour to better inform how they manage the species.

Ros Wright, from the Environment Agency’s National Fisheries Services team, says:

“We are working with GWCT, University of Bournemouth and Cefas to understand more about the status of populations of eels in our rivers using the River Frome as a study site. This detailed study will monitor all life stages of eels using a variety of methods including PIT and acoustic telemetry, traps, fyke nets, silver eel racks and fish counters. This will improve our estimates of the number of adult silver eels migrating out to sea to spawn.”

Migration barriers a big problem

There are several threats to eels that are thought to be contributing to their decline.

A parasitic worm – known as Anguillicoloides crassus – started spreading among European eels in the 1980s, probably having been inadvertently transported from Taiwan where it occurs naturally in the Japanese eel. The worms infest the swim bladder of the eel, making it more susceptible to disease, slowing its growth rate, and even killing it if the infestation becomes severe.

Since the swim bladder is the buoyant organ which allows the eel to swim, a severe parasite infestation can hamper its ability to reach its spawning grounds.

In England it is thought that a major factor impacting eels is the impact of migration barriers, such as weirs and other in-rivers structures, which prevent eels from reaching many kilometres of their historic habitat. Along with barriers, there are many thousands of water abstraction structures that can trap eels (and other fish) causing damage or mortality.

Illegal fishing in some parts of Europe, to supply the Asian markets with European eels is another big problem. According to a UK government source elvers (young eels) can fetch up to £4,000 per kilo on the black market and this has led to extensive illegal exports, with people even arrested trying to smuggle elvers in their hand luggage out of UK airports. Due to the amount of money involved, there is a high involvement with organised crime.

Water pollution is also a problem, as their long life makes them vulnerable to the build-up of chemicals in their bodies, and eels have been found to contain heavy metals and other toxins.

Another issue is thought to be habitat loss with wetland across Europe being drained for agricultural and other uses over the past century.

“There are probably many other things going on at sea which are not helping either, but we know so little about that part of their life that it is hard to gauge,” says Will. “Eels are very tough, but they are also a very little fish in a very big sea.”