Scottish Marine and Freshwater Science Vol 10 No 2 | Assessing the status of Atlantic salmon (Salmo salar) from juvenile electrofishing data collected under the National Electrofishing Programme for Scotland (NEPS)

Atlantic salmon (Salmo salar) are a species of high cultural, conservation and economic importance that are worth ca. £80 million per annum to the Scottish economy. Quantitative and scientifically defensible assessments of population status are required to inform evidence based fisheries management across a range of spatial scales. Juvenile electrofishing data are one of the most commonly collected sources of information on the status of Atlantic salmon in Scotland. However, their interpretation is fraught with technical challenges that include the need to 1) estimate salmon densities from electrofishing count data 2) develop a suitable “benchmark” against which to compare observed densities 3) obtain appropriate (representative) annual monitoring data 4) develop approaches that allow site-wise estimates of observed and benchmark densities to be scaled to larger areas. This report presents an approach for assessing the status of Atlantic salmon in Scotland from juvenile electrofishing data.

Observed salmon counts were obtained from the National Electrofishing Programme for Scotland (NEPS), an unequal probability Generalised Random Tessellation Stratified (GRTS) survey design with 27 strata (assessment regions). Capture probabilities were modelled from available multi-pass electrofishing data collected between 1997 and 2018 and used to predict densities of salmon fry and parr for 1-pass and 3-pass NEPS electrofishing sites. Observed abundances from NEPS were then compared to a recently published juvenile salmon density benchmark at site, catchment, regional and national scales to determine salmon performance and status. A rule based approach was used to classify regional electrofishing data into three grades based on performance against the regional benchmark and uncertainty in the mean observed regional densities. Further work is required to investigate, and where possible harmonise, the outputs of this juvenile assessment approach with existing adult based assessment methods used to classify the status of Scotland’s salmon rivers under Conservation Regulations. A number of future developments are identified to improve models of capture probability and benchmark density. Future developments of NEPS should consider current and future monitoring requirements at national and local scales, alongside available resources to improve the overall survey design.