National Electrofishing Programme for Scotland (NEPS) 2023: status of juvenile Atlantic salmon and brown trout populations

Glossary and abbreviations

DRN

A spatial dataset consisting of edges (lines) and nodes (points) that represent rivers. Attributes associated with edges can provide additional information such as Strahler river order, width etc.

Genetic introgression

Levels of genetic material in wild populations resulting from wild fish breeding with farm escapees

GRTS

Generalised Random Tessellated Stratified survey design

Inclusion probability

Probability of a site becoming part of the sample as part of the survey design process

Non-response site

A site that cannot be sampled

NEPS

National Electrofishing Programme for Scotland

NIPS

National Introgression Programme for Scotland

Occupancy

The proportion of the river network where fish are present

Oversample

A replacement site

Post-stratify

Statistical method to re-analyse data using strata (grouping factors) that were not part of the original survey design.

Region

NEPS reporting regions. Regions consist of hydrometric areas, which were aggregated together to create regions of broadly comparable size. In the 2018/19 NEPS survey NEPS regions were also strata.

SAC

Special Area of Conservation under the Habitats Directive (Conservation of Habitats and Species (Amendment) (EU Exit) Regulations 2019)

Sample frame

Simplified representation of the target population or metric to be sampled. In the case of NEPS this is the rivers on which samples can be located and is determined from the DRN.

Sample size

Number of samples, in the case of NEPS this relates to numbers of electrofishing sites

Strahler river order

Metric of river size whereby a source river is Strahler river order one. When two rivers with Strahler river order one join they become Strahler river order two etc. Larger numbers denote larger rivers

Strata

Groupings of more homogeneous sub-populations. In the context of NEPS strata are used to group contiguous nearby rivers to provide regional assessments. Sample numbers can be adjusted up or down within strata during a particular survey year while maintaining the overall integrity of the survey design.

Stratified sampling

Sites, and data collection, across distinct strata (e.g. different river catchments or regions) within the larger sample population (e.g. a country).

Target sample

The feature of interest. In the case of NEPS this is defined as all river reaches that can be sampled by wading and electrofishing, are located below impassable barriers to fish migration and where salmon fisheries operate.