Data from the dataset entitled 'Thesis Chapter 2 dataset' were collected by performing paired time- and area-delineated single-pass electrofishing surveys, with the former forming the initial component of the latter. The first 5 minutes of the area-delineated survey were timed and fish captured/missed recorded separately from those from the second component. The total area covered in each of the area-delineated surveys was measured using 5 cross channel transects and with a fixed site length of 20 m. An estimated area coverage metric was then calculated for the initial time-delineated part of the survey, based on upstream distance covered during this survey component and the mean site width. Habitat variables including proportions of substrate made up of particles in various size classes (silt-boulder/bedrock - recorded via modified Wolman pebble counts), macrophyte cover (recorded via modified Wolman pebble counts) and mean site depth (recorded via 5 depth measurements within each width transect) were also recorded for each electrofishing site.

Data from the datasets entitled 'Thesis Chapter 4 dataset' were collected during inter-catchment translocation experiments involving juvenile European eels in Wales, UK. Juvenile glass eels were collected from the River Severn in April 2015 and reared for one month in captivity. Eels were released in batches of approximately 900 within 50 m release sites at the centre of 13 stream reaches, each measuring approximately 1 km in length, in June. Experimental treatments were defined by the presence (n = 7) or absence (n = 6) of brown trout within these stream reaches. Stream reaches were then revisited and electrofished using area-delineated single-pass electrofishing surveys in September 2015 to determine surviving eels numbers. Initial surveys covered the release site, as well as 20 m sections of stream at 30 m intervals upstream and downstream of this site. Lengths of captured eels and trout were measured. Site-level habitat variables were recorded for each electrofishing survey while stream level and stream reach level habitat and water chemistry variables were recorded at a single point within each stream reach.

Data from the dataset entitled 'Thesis Chapter 5 dataset' were collected during intra-catchment translocation experiments involving juvenile Atlantic salmon (0+ age class, or 'fry') within the River Erriff catchment in 2019 and 2020. Salmon fry densities in the immediate post-emergence period were determined from area-delineated single-pass electrofishing surveys, as described above, in June/July 2019 and June 2020. Following these salmon fry were translocated from identified high fry density sites (high density source 'HDS' sites) to identified low fry density sites (low density recipient 'LDR' sites) in July 2019 and June/July 2020. These were performed by electrofishing of HDS sites, followed by adipose fin excision to mark translocated fry, and subsequent release of translocated fry at LDR sites. Individual translocation batches involved capturing fish from several HDS sites and releasing these at several LDR sites, with individual site-level capture target and released fry numbers based upon predetermined fry density targets, as outlined in more detail within the thesis. Each translocation batch was performed in a single day, with no fish held overnight. Identified high- and low-density sites which had no density manipulations were used as controls (HDC and LDC respectively). Repeat area-delineated electrofishing surveys were then completed in September 2019 and August/September 2020 at all sites to determine numbers and densities of translocated and non-translocated fry, as well as of older conspecifics and brown trout. All sites covered a single riffle and measured 20 m instream length with habitat variables recorded as described for the Chapter 2 dataset. Additionally, a number of 'hotspot' surveys were performed on short (5 m in 2019, 10 m in 2020) sections of riffle habitat upstream and downstream of LDR sites to determine patterns of dispersal for translocated salmon fry.

Data from the dataset entitled 'Thesis Chapter 6 dataset' were collected during intra-catchment translocation experiments involving juvenile Atlantic salmon (0+ age class, or 'fry') within the River Erriff catchment in 2021. As in 2019 and 2020 salmon fry were translocated following identification of high and low density sites via area-delineated single-pass electrofishing surveys in June 2021. In June 2021 all salmon fry were translocated and released into the centre of 2 LDR stream reaches, approximately 500 m in length, with approximately 1500 fry released per stream reach. Additionally, 2 LDC stream reaches. measuring approximately 250 m in length. Both LDR and LDC stream reaches were then split up into individual sites based upon habitat units (i.e. pools and riffles). Translocations followed the same method of capture, adipose fin excision, and release as in 2019/2020, with these being completed in June. Repeat electrofishing surveys were performed in August/September 2021 to determine numbers and densities of translocated and non-translocated fry, as well as older conspecifics and other species. As in 2019/2020, habitat variables were recorded for each experimental site including proportions of substrate made up of particles in various classes (silt-boulder/bedrock - visually estimated), proportion of site area made up of pool, glide and riffle habitat and mean site depth.