National Electrofishing Programme for Scotland (NEPS) 2021: analysis

Introduction

Atlantic salmon (hereafter salmon) are a diadromous species (those spending time in marine and freshwater environments) of high economic, conservation and cultural importance that are protected by international management agreements (North Atlantic Salmon Conservation Organisation; NASCO) and legislation at national (The Conservation of Salmon (Scotland) Regulations) and international levels (The European Commission Habitats Directive, 92/43 EEC). Scotland accounts for ca. 75% of wild salmon production in the UK (ICES, 2021). Brown trout (hereafter trout) are a UK Biodiversity Action Plan priority fish species that exhibit a wide range of life history strategies including freshwater resident and diadromous forms (sea trout). Wild freshwater fisheries, which are dominated by salmon and trout, account for around £79.9m Gross Value Added (GVA) to the Scottish economy each year, supporting ca. 4,300 full-time equivalent jobs (PACEC, 2017).

The abundance of salmon returning to home waters has declined over the last 50 years (Chaput, 2012; ICES, 2021). Early reductions in abundance were largely offset by reductions in exploitation from coastal, estuarine and in-river fisheries (e.g. Gurney et al., 2015). However, adult numbers have continued to decline in recent years raising concerns that this will affect future salmon numbers through declining freshwater production. Between 2015 and 2021 the total reported rod catch for Atlantic salmon in Scotland (caught and released) declined from 56,006 to 35,693. Although likely affected by COVID-19 restrictions, the latter figure is the lowest recorded since records began in 1952 (Marine Scotland, 2022). Sea trout rod catches have declined since the mid-1960s and the rod catch for sea trout in 2021 was also the lowest on record (Marine Scotland, 2022).

In response to declining salmon numbers the Scottish Government published the Scottish Wild Salmon Strategy to promote healthy, self-sustaining populations of wild Atlantic salmon that achieve good conservation status. A detailed understanding of the status of juvenile Atlantic salmon populations and the pressures acting on them is thus pre-requisite to informing evidence-based management of the resource and its environment. Electrofishing data are one of the most commonly collected sources of information on the status of salmon and other freshwater and diadromous species. Where collected to common standards and an appropriate survey design these data can provide assessments at nested spatial scales ranging from individual sites (ca. 100m²), to sub-catchments, catchments, regions or whole countries. This spatial flexibility and potential for upscaling makes juvenile assessment particularly attractive for management decision making.

In 2018 Marine Scotland Science established the National Electrofishing Programme for Scotland (NEPS). NEPS is a collaborative programme of data collection supported by local fisheries managers (Malcolm et al., 2019b). It gathers spatially extensive data on the distribution and abundance of freshwater fish, physical habitat, water quality (to inform assessment of pressures) and levels of introgression; the levels of genetic material in wild populations resulting from wild fish breeding with farm escapees (Gilbey et al., 2021). In 2018 the programme was funded by Marine Scotland (MS), the Scottish Environment Protection Agency (SEPA) and Scottish Natural Heritage (now Nature Scot). In 2019, the programme was run for a second year with funding support from MS and Crown Estate Scotland (CES). There was no national survey in 2020 due to COVID-19. However, a survey was funded again in 2021 using a new survey design, supported by funding from MS and CES.

Previous reports (Malcolm, et al., 2019b, Malcolm, et al., 2020) have outlined the main data collection, survey design and analytical methods developed under NEPS. The current report extends this previous work with the following objectives:

1. Characterise changes in the survey design between the 2018/2019 and 2021 NEPS surveys

2. Provide environmental context for the surveys through a summary of inter-annual variability in rainfall and discharge

3. Model capture probability incorporating the latest data from NEPS 2021

4. Report the abundance of salmon and trout and status of salmon determined from NEPS 2021

5. Characterise variability in habitat, observed salmonid densities and benchmark (expected salmon numbers) between Strahler river orders

6. Harmonise data between NEPS surveys to assess spatio-temporal variability in salmonid densities and the status of salmon.

7. Identify the potential of NEPS data to assess of status of salmon in Special Areas of Conservation (SACs)

8. Characterise relationships between juvenile salmon density and rod catch as a proxy for salmon abundance

9. Characterise spatial variability in water quality to identify pressures acting on salmonid populations and provide an improved basis for understanding spatial controls on salmonid productivity.