Timed electrofishing surveys on the Luce and Urr


GFT have been undertaking timed electrofishing (fry index) surveys on two Galloway Rivers in recent years.

These surveys are used to pick up trends in salmon fry densities, primarily within the larger main stem reaches of a river catchment.  The surveys can be undertaken quickly and with less resources.  In Galloway, we undertake a five minute survey using a two-man team which includes an SFCC team leader operating a back pack electrofishing kit assisted by an experienced single hand netter.  Using this method, entire main stem river catchments can be surveyed in very little time.

In Galloway, we first trialled the method back in 2014 on the Urr.  At the time, we only had an electracatch backpack system with no automatic timer, but in 2015, we acquired our first 500 WATT EFish backpack electrofishing kit and were able to accurately measure our electrofishing effort.  In 2016, with support from the DFSB, we decided it would be beneficial to use timed surveys upon the Luce catchment and we now sample up to 19 sites across the Cross Water and Main Water of Luce annually as well as 10 sites on the River Urr. 

The results from time delineated surveys are very different to semi and fully quantitative (density dependant) surveys which we have been undertaking in Galloway since 1990.  In timed surveys, results are displayed as number of fish caught per minute, whilst in quantitative surveys, results are displayed as a density of fish caught per 100m2 of water.  We have found that the timed survey method allows trends in species and year-class distributions to be determined very quickly, which has been of particular interest during years of adverse weather conditions (i.e. prolonged hot and dry springs or exceptional winter flooding events) when redd wash-out and fry survival has been a concern. 

Fully quantitative surveys –v- Time delineated surveys

To obtain fully quantitative information on the fish populations within the river, each survey site is fished through up to four times consecutively to allow the calculation of a more accurate Zippin estimate of the fish population.  This is an estimate of the fish population density per 100 m2 of water, including the 95% confidence limits. 

A classification scheme for densities of salmonids was previously generated by the SFCC using data collected from 1,638 Scottish electrofishing survey sites, covering the period 1997 to 2002 (Godfrey, 2005[1]).  From this, regional figures were created to allow more accurate local ranges.  The categories are based on quintile ranges for one-sample electrofishing surveys in the Solway region.  Table 1 shows the quintile ranges that we use to classify Galloway River juvenile fish population data. 

Table 1: Quintile ranges for juvenile salmonids (per 100 m2) based on one-sample electrofishing events, calculated on densities >0 over 291 sites in the Solway Statistical Region

  Salmon 0+ Salmon 1++ Trout 0+ Trout 1++
Minimum (Very Low) 0.22 0.38 0.38 0.35
20th Percentile (Low) 5.21 2.86 4.14 2.27
40th Percentile (Moderate) 12.68 5.87 12.09 4.71
60th Percentile (High) 25.28 9.12 26.63 8.25
80th Percentile (Very High) 46.53 15.03 56.49 16.28

 

In timed electrofishing; rather than determining fish density per unit area, sites are electrofished for a given length of time and the number of fish are regarded as an index of abundance; a catch per unit effort (time). Since there are only a handful of Fishery Trusts in Scotland that use timed surveys as part of their annual electrofishing programme; we do not have an SFCC recognised table of quintile ranges as we have for quantitative surveys (Table 1). However, with the assistance of the Spey Foundation, who have been undertaking fry index surveys for a number of years and who have developed a Spey specific set of standards; we have been able to develop a table of Galloway standards using the Luce and Urr data collected since 2016. Table 2 shows these quintile ranges. We feel this set of quintile ranges accurately reflects the abundance of fry and parr from our timed sites on the Urr and Luce and we will be using this table to classify our existing timed data collection from the Luce and Urr and further timed sites we undertake in Galloway that specifically target juvenile salmonids and in particular, fry.

Table 2: Quintile ranges for juvenile salmonids caught during 5 minute time delineated surveys on Galloway Rivers (2016-2019)

Breakpoint (salmon fry/min) Class Breakpoint (salmon parr/min)
0 Absent 0
<3.4 Very Low <1.0
3.5 to <7.0 Low 1.1 to <1.8
7.1 to <11.4 Moderate 1.9 to <2.6
11.5 to <23.2 Good 2.7 to <4.6
>23.2 Excellent >4.6

 

1 Godfrey, J. D., 2005; Site Condition Monitoring of Atlantic Salmon SACs: Report by the SFCC to Scottish Natural Heritage, Contract F02AC608.

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