The National Electrofishing Programme for Scotland has been designed using a stratified, unequal probability, generalised random tessellation stratified (GRTS) sample design for survey over time with an oversample. GRTS designs ensure that the sampling is spatially balanced (approximately equally distributed over the area of interest), while maintaining random site selection at finer spatial scales. The National Electrofishing Programme includes the following design features:
Only rivers that had a high chance of being sampled by wading and electrofishing were included in the sample design. Large rivers (> Strahler river order 4), very small rivers (Strahler river order 1) and lochs were excluded.
Rivers above impassable barriers for salmon were excluded from the design.
Stratified according to 27 geographic regions which correspond to groups of hydrometric areas (groupings of similar adjacent river catchments) with thirty samples in each region.
Three panel sample design for monitoring over time. Ten sites per panel and region. Annual panel visited annually to improve trend detection, 3 year panel visited on rolling 3 year programme, 9 year panel visited on rolling 9 year programme.
Unequal sample weighting (based on the National Density Model) whereby rivers that are expected to have greater salmon densities are associated with a higher probability of being sampled.
Over Samples obtained for circumstances where health and safety did not allow sites to be sampled thus maintaining the statistical integrity of the overall design.
The sample design was obtained using a series of GIS layers (Digital River Network (DRN); Hydrometric Areas (HA); Loch polygons; Fish barrier dataset) and the R package spsurvey (Kincaid et al., 2018).
The resulting sample design is shown in Figure 1. The location of sampling sites for 2018 can be downloaded in “csv” and “shapefile” formats. The dataset will be updated in October 2018 to reflect any changes in sample locations due to site accessibility and feasibility of sampling over the summer electrofishing season.
Figure 1. Flow diagram showing the procedure that was followed in the design of the National Electrofishing Programme for Scotland.
Figure 2. The National Electrofishing Programme for Scotland in year 1 (black – 1 pass sites; red – 3 pass sites). Note that some sample locations could change during the sample season depending depending on the need for oversamples.