With the number of Salmon caught by rods and nets in decline throughout their natural range indicating decreasing marine survival, there is limited long term data from U.K. rivers that allow us to monitor return rates. To answer this question for the Tweed, the Tweed Foundation are using Passive Integrated Transponder (PIT) tags to monitor the return rates of Salmon to the Gala Water.
From 2019, we have been tagging a sample of 1,000 Salmon smolts leaving the Gala Water each year with PIT tags. Tagging involves making a small incision into the stomach cavity while the fish is under anaesthetic and inserting the tag. Each PIT tag has a unique number which can be used for identification purposes. Tagged fish can be detected in the river when they pass close by a PIT tag antenna. The PIT loops are connected to a PIT reader which, when triggered, records the unique identification code of any tagged fish and the time that they pass by. We have installed three PIT loops in the Gala Water, so any tagged Salmon can be identified and recorded as they leave the river (and hopefully when they return).
The first tagged Salmon returning to the Gala were recorded in 2020. However, one of the challenges in monitoring the return rates of Salmon to our rivers is that they can spend anywhere between one and three years out at sea. Therefore, to get the total return rate of Salmon for a cohort leaving the Gala, the numbers of returning tagged adults have to be recorded over the following three years.
By carrying out PIT tagging, we will also increase the information available from the Gala fish counter as it will show us the proportions of the Gala run that return as Grilse (having spent one winter out at sea) and as Salmon (having spent multiple winters out at sea). With Grilse tending to carry fewer eggs than the larger Salmon, this will improve our estimates of how many eggs are being deposited in the Gala.
Monitoring the survival rate of Salmon while they are out at sea has long been a challenge for fisheries management. PIT tagging is giving us an insight into their survival out at sea, and in doing so is helping us to understand the number of smolts that need to leave the Gala Water in order to ensure that enough adults return to spawn.