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A Christmas Bonus from the Acoustic Tagging

As a major part of The Tweed Foundation’s work in the Living North Sea programme, Sea-trout smolts have been acoustic tagged so that they can be tracked down the River to the estuary to show up any problems, and to give an idea of their success rate in reaching the sea.

There has been a bonus from this work however: two of these tagged smolts have been picked up on an automatic listening station in the estuary of the River Tees at Middlesbrough that was part of another tracking programme, being run by CEFAS, to look at fish movements around the Tees barrage.

As these smolts were detected on our own automatic listening station in the Tweed estuary when they left, the exact amount of time that they took to travel down the coast to the Tees can be calculated.

A Sea-trout smolt caught in the Gardo fishery nets in the Tweed estuary on 17th June 2010. This smolt was hanked up in seaweed and so brought in by the net. It shows the marine dress of Sea-trout smolts, and why a local name for them was "Orange fins".

Smolt no. 220, released at Yarrowford, on the Yarrow Water, left the Tweed estuary at 09.25 hrs on the 10th May 2011 and reached the Tees at 23.42 hrs on24th May 2011.

Smolt no. 255, released at Philiphaugh on the Ettrick Water, left the estuary at 20.07 hrs on 9th May 2011 and reached the Tees at 10.13 hrs on 27th May 2011.

The direct distance along the coast from the Tweed estuary to the Tees is 141kms, so smolt no. 220 travelled at a minimum speed of 2.48kms /hour and no. 255 at a minimum of 3.33 kms / hour. If they took a less direct route, they will have had to travel at faster speeds.

This is the first direct indication of what sort of speeds Tweed Sea-trout smolts can travel at on their journey south. Tag returns from 1950s work shows that their route is down to East Anglia, across to the Frisian Islands / Waddensee and then, if they are returning after just one winter at sea, back to the Tweed. If spending two winters at sea, tag returns show that they can go as far as the very tip of Denmark, at the mouth of the Baltic.

A map showing the migration route taken by Tweed smolts; one sea-winter fish taking the 'short' route back to Scotland, and two sea-winter fish going all the way up to Denmark before returning.

On average, these 1950s smolts took 60 days from tagging in the Tweed estuary to get to the Great Yarmouth area of East Anglia (where most recaptures were made), a distance of about 445kms along the coastline – and that works out at around 3.37 kms / hour, similar to the direct measurements given by this year’s two acoustic tagged smolts.