The Last Piece of the Jigsaw: monitoring a whole Salmon population on the River Tweed

The Tweed Foundation is monitoring a whole Salmon population in a Tweed tributary by counting them out and counting them back.

Knowing how many young fish are produced and how many get back to their home spawning areas as mature adults to keep their populations going are fundamental questions for fisheries management. 

The Tweed catchment area is 5,000 km2, so how is it possible for us to check on what is going on? To do exactly that, the Tweed Foundation (the fisheries trust for the Tweed river system) has made the Gala Water into an Index river using an industrial relic, the Skinworks Cauld (weir) in Galashiels to monitor fish stocks. 

In 2019 our biologists started acoustically tagging salmon smolts in the Gala Water to understand their progress and survival downriver to the sea. Continuing this work over the next few years will add significantly to our knowledge of the Gala Water population and where and how its fish are lost. 

We tagged a further 1,000 Salmon smolts with a PIT tag.  Working in a similar way to a barcode reader, these tags will be automatically read when the fish return back up the fish ladder. This will show us if survival to adulthood is related to size of smolt, timing of migration or other factors, and at what age fish are returning.  In future years, Sea-trout smolts and juvenile Brown-trout will also be tagged. This will tell us how many times Brown-trout return to the Gala to spawn and what the survival rate of these fish is from year to year. The first ever “life-table” for a river population of Brown-trout in Scotland will be created - a basic requirement to track survival over time and allow us to adapt management practices as required.

Our counter in the Gala Water fish ladder now counts adult fish returning to spawn, with the videos taken allowing identification to the different species. A smolt trap – trialled in 2018, catches Salmon and Sea-trout smolts heading out to the sea – and juvenile Brown-trout heading down to the main river. This enables us to calculate the total of young fish leaving the river from the samples trapped, giving the first ever estimates of the contribution that a tributary makes to the overall Tweed fishery. In 2018 we calculated that the Gala Water produced around 25,000 Salmon and 14,500 Sea-trout smolts. The fish counter figures show that around 1,000 adult Salmon return to the Gala on average, which gives a return rate of c.4%. This is the first time such a statistic has been available for Tweed Salmon and we can now continually monitor this to show how survival changes over time and whether restrictions on the salmon fishery need to be increased – or can be eased.

This work contributes to our understanding the Salmon population of the Tweed as a whole, improving its management and protecting its hugely important value to the local rural economy.