Salmon Smolt Tracking

Tracking Salmon Smolts in the Nith and out to sea

In 2021, the Nith Catchment Fishery Trust ran a project called the Nith Salmon Smolt Tracking Project. The project entailed trapping 50 salmon smolts from the Upper Nith, in the Crawick and Mennock Waters, and fitting them with acoustic tags. Eight acoustic receivers were positioned down the length of the River Nith which meant that the salmon smolts could be tracked as they migrated downstream and out into the Solway. (Fig. 1)

Fig. 1 Geographic locations of freshwater receivers in the river Nith (red dots – West Coast tracking project; purple dots – Nith Smolt Tracking Project). The WCTP release site is marked by a red star, the Nith smolt tracking project release sites are marked by purple stars.

The Nith Salmon Smolt Tracking Project took place alongside the Atlantic Salmon Trust’s West Coast Salmon Smolt Tracking project. This was a larger national project that tagged salmon smolts from across ten rivers along the West Coast of Scotland with the aims of understanding the broad migration routes taken by salmon smolts up the west coast. To achieve this, 224 acoustic receivers were deployed in arrays in likely migration routes up the west coast of Scotland (Fig. 2). On the mainstem of the River Nith at Auldgirth, 130 salmon smolts were acoustically tagged and released. These smolts passed four of the lower receivers before entering the Solway and starting their migration up the west coast.

Fig 2. Geographic locations of where acoustic receivers were deployed for the West Coast Tracking Project 2021. The red dots are acoustic receivers which were deployed for the West Coast Tracking Project. There were a few other projects which were running simultaneously on the west coast which include COMPASS (orange dots), Sea Monitor (blue dots) and MEFS MPA project (yellow dots) and the Nith Smolt Tracking Project (purple dots).

Of the 50 salmon smolts tagged on the Crawick and Mennock Waters, 48% of them were detected leaving the River Nith at Kingholm Quay. They took an average of 17.78 days to migrate down the river from the release site to the last receiver at Kingholm Quay, a distance of 56kms. The slowest part of the journey was between the release site and the first receiver, located below Sanquhar. The fastest section of the smolts journey down the Nith was between receivers Ar3 and Ar4, between Thornhill and Auldgirth. Most salmon smolts migrated during the hours of darkness. The salmon smolts then reduced their speed once they reached the tidal waters at Kingholm Quay, where they stayed in proximity to receiver Ar8 for 11-77 minutes. Once at sea it is not possible to gauge survival rates as the arrays of receivers are “leaky” meaning that not all salmon smolts will be picked up on these receivers (Fig. 2). Three of the Crawick salmon smolts were detected on the COMPASS array (between Larne and Stranraer) and one Crawick salmon was detected on the Sea Monitor array (between Malin Head and Islay). The smolt detected on the Sea Monitor array had not been detected on the COMPASS array.

Of the 130 salmon smolts that were tagged at the AST trap at Auldgirth, 50.77% made it to the Solway. Overall, the percentage loss was 2.22% fish per km. It took them an average of 8 days to migrate from the tagging site to receiver Ar8 at Kingholm Quay, a distance of 22kms. As with the Crawick smolts, the Nith smolts travelled mainly during the hours of darkness. Thirteen of the Nith salmon smolts were detected on the COMPASS array (between Larne and Stranraer) and three of the Nith salmon smolts were detected in the Sea Monitor array (between Malin Head and Islay). No smolts were detected on any of the arrays between the Outer Hebrides and mainland Scotland so it is likely that they are migrating on the seaward side of the Outer Hebrides.

When we first proposed the Nith Salmon Smolt Tracking project, one of the parts of the river identified as a potential high-risk area for the smolts was the Cauld at Dumfries. The high number of predators observed at this location taking fish has always been of concern to fishery managers and anglers. However, the results from this study show that of the 96 smolts that were detected directly upstream of the Cauld, 91% of them were detected at the receiver Ar8, at Kingholm Quay. Only 8 smolts were lost whilst migrating through this section. This is similar, if not better, to the percentage losses experienced in other sections of the river.

The data discussed in this summary report were taken from a report compiled by Jessica Rogers from the Atlantic Salmon Trust who very kindly analysed our data along with the AST data and provided all the information about the seaward leg of our smolts journeys.

Thanks to our funders including the Nith District Salmon Fishery Board, Holywood Trust, Dumfries and Galloway Council, Atlantic Salmon Trust and all those organisations that have supplied information and support to enable this project to take place.

For further information on the The West Coast Tracking Project – The Atlantic Salmon Trust, please follow this link.