Fish zapped and tagged to track them in blue yonder


Fish zapped and tagged to track them in blue yonder

Jerome Starkey, Countryside Correspondent

February 10 2018, 12:01am,

The fish went belly-up in a fuzz of electric current as scientists on the riverbank laid out a makeshift surgery: scalpels, forceps and surgical thread on top of a plastic storage box that served as the operating table.

The sea trout was hauled out, measured, weighed and tranquillised so that scientists could insert three high-tech tracking devices into its belly.

The operation, on the River Frome, in Dorset, was the first part of a five-year study to track sea trout and salmon using microchips, sonar and depth gauges, in an attempt to solve the mystery of where and why they are dying at sea.

Stocks of sea trout and Atlantic salmon have collapsed by 70 per cent since the 1970s and only 5 per cent of juvenile salmon that swim out of British rivers make it back to breed as adults.

“We have very little information about salmon and almost zero information about sea trout when they go to sea,” Dylan Roberts, head of fisheries at the Game and Wildlife Conservation Trust, said.

The charity is leading an £8 million, EU-funded project called Samarch to tag fish in four rivers including the River Tamar, the Devon-Cornwall border, and the rivers Scorff and Bresle in France. If they can work out where the fish are dying they might be able to work out what can be done to help them, Mr Roberts said.

Catching the 2.2kg sea trout on the River Frome involved zapping the water with 170 volts from a generator.

Bill Beaumont, the author of Electric Fishing: a Complete Guide to Theory and Practice, laid the fish on a plastic tray and made a two-inch cut in its flank.

He inserted a microchip the size of a grain of rice; a sonar beacon the size of a baked bean, which emits a ping every 30 seconds; and a bright orange sensor tag that measures depth and temperature every two minutes. The tag was the most important monitor: scientists can plot the data against ocean temperature charts to trace its route at sea.

“These are, hopefully, going to provide something totally new to science,” Rasmus Lauridsen, head of fisheries research at the conservation trust said.

But the challenge is getting them back. Céline Artero, the Samarch project’s lead scientist, said if the fish returns to breed this summer the sonar beacon will trigger sensors in the estuary. Once it swims upstream, past a microchip antennae, they will hunt it with a listening device and catch it.

If it is caught by an angler an external tag, which hangs next to the dorsal fin, offers a cash reward if the fish is handed in and if the fish is eaten or killed, the tag floats so that it will eventually wash up on a beach where it is hoped that, being bright orange, it will be found.

A similar scheme to tag sea bass had two tags returned from beaches in Weymouth. One tag tracked a fish for 300 days, and showed it swam along the Channel and into the Celtic Sea. “The scale of movement that we are seeing is even greater than we had previously imagined,” a preliminary report by the Centre for Environment, Fisheries and Aquaculture Science said. It plans to tag 18 sea trout this year, as a trial, and at least 450 over the next four years.

Salmon can spend up to four years at sea but die soon after they have spawned, which means scientists would be unlikely to learn much by tagging an adult they found in a river. Juvenile salmon, called smolts, are too small to carry the largest tags, but researchers plan to fit 240 of them with smaller tags.