A fish counter network for monitoring Scottish salmon stocks

2. Introduction

Stocks of Atlantic salmon are widely exploited including during both the marine and freshwater phases of their lifecycle. However, populations are declining across their native range and the species was recently classified as ‘Vulnerable’ by the IUCN, whilst the Great Britain subpopulation was classified as ‘Endangered’ (Darwall and Noble, 2023). Scottish salmon stocks are managed at a local level by District Salmon Fishery Boards and the River Tweed Commission, at a national level by the Scottish Government and at an international level by the North Atlantic Salmon Conservation Organisation (NASCO), which is advised by the International Council for the Exploration of the Seas (ICES), following work undertaken by the Working Group on North Atlantic Salmon (WGNAS).

In Scotland, stocks are managed nationally under the Conservation of Salmon (Scotland) Regulations 2016 which aim to ensure that any harvesting of stocks in Scottish domestic waters is sustainable and to secure the additional conservation requirements of Special Areas of Conservation (SACs) designated for their Atlantic salmon populations. The Regulations require an assessment of the conservation status of salmon in specified inland waters that also has regard to the conservation objectives of the SACs.

The Regulations also prevent salmon from being retained in all coastal waters and in inland waters where stocks are not assessed. The Scottish Government’s Marine Directorate conducts an annual assessment of 173 salmon stocks on 215 rivers, and assesses the conservation status by determining the probability of each stock achieving their conservation limit over a five-year period (The status of salmon in Scotland: 2025 - gov.scot; SG, 2025). Annual amendments to the Regulations are then made by the Scottish Parliament to regulate the killing of salmon. Caught salmon cannot be retained where stocks are in poor conservation status, i.e. they have <60% probability of achieving their conservation limit over a five-year period. Although salmon may be retained on rivers in good or moderate conservation status, 98% of rod-caught salmon were returned to the river in 2024 (Scottish salmon and sea trout fishery statistics 2024 - gov.scot; SG, 2024).

Under the Habitats Directive (Council Directive 92/43/EEC on the Conservation of natural habitats and of wild fauna and flora), 17 Special Areas of Conservation (SACs) have been designated in Scotland for the protection of Atlantic salmon. Salmon are a primary reason for the designation of 11 SACs and are a qualifying feature in the designation of 6 SACs (Table 1).

Annual stock assessments are conducted by the Marine Directorate and feed into statutory SAC site condition monitoring undertaken by NatureScot every six years. These include assessments for three SACs which are sub-catchments: the River Moriston, which is a tributary of the River Ness, the River Teith, which is a tributary of the River Forth, and the Endrick Water which discharges into Loch Lomond and eventually to the Clyde estuary via the River Leven.

In responding (Salmon Interactions Working Group Report: Scottish Government Response - gov.scot; SG, 2021) to the independent Salmon Interactions Working Group Report, the Scottish Government committed to a programme of installing new fish counters across Scotland over a four-year period to produce a strategic network of counters. This commitment was restated in the Wild Salmon Strategy (Scottish wild salmon strategy - gov.scot; SG, 2022) and its Implementation Plan (Wild salmon strategy: implementation plan 2023 to 2028 - gov.scot; SG, 2023).

To date, work has been ongoing to ensure counters owned by Scottish Goverment are fit-for-purpose, installing new sonar cameras for use as counters on the rivers Laxford and Deveron, and awarding Marine Fund Scotland grants to third parties for the purchase of fish counting equipment and to develop machine learning approaches to processing sonar and video data.

In relation to the assessment and management of stocks, this report reviews and summarises the Scottish Government’s requirements for data on adult salmon abundance, available fish counting technology, and count data. It also identifies some locations, from fieldwork undertaken in 2021, where equipment could be sited and additional data obtained.