This summer the trust was involved with the national electrofishing survey. This program was organised by Marine Scotland to produce a detailed picture of the salmon population across the country. The task of surveying was entrusted to local bodies connected to catchment areas in most cases this would be fishery trusts. The Forth Rivers Trust was commissioned to survey across the Forth catchment.
Electrofishing is an effective technique for quantifying fish populations in river systems. It is a particularly good method for detecting juvenile fish, the charge delivered through the water stuns and draws the fish out from hiding. The smaller fish, due to their size, receive a minimal charge compared to larger fish and recovery is quick. All the trust’s science staff are highly trained in the use of electrofishing equipment and in the capture and monitoring of young fish. In addition to the trust’s standard practice, the national survey required further information. The trust takes scales when there is an apparent cross over in age classes of salmon. The national survey requested that all salmon and trout parr had scale samples taken. Added to this was genetic sampling in the form of fin clips from all salmon parr. Both the scale and genetic sample records were linked to each individual fish, a significant and time-consuming detail. This additional sample data was for age classification and genetic sampling to determine local age structure and genetic diversity. It would also serve as a legacy of sample data taken across the country using standardised techniques. Along with the fish data, the survey took information on the sampling site: flow types and vegetation, site dimensions and bank structure were also recorded. Count data was taken for all species of fish captured during the survey. In the Forth area along with salmon (Salmo salar) and trout (Salmo trutta), stickleback (Gasterosteus aculeatus), stoneloach (Barbatula barbatula), minnow (Phoxinus phoxinus), bullhead (Cottus gobio), European eel (Anguilla Anguilla) and lamprey spp. were all noted during the survey.
The trust was issued with the GPS coordinates for the random survey sites. Many of these sites were new and interesting areas where the trust had not electrofished before. One drawback of this approach was visiting sites the trust was unfamiliar with, which, on occasion turned out to be unviable for surveying. This could be too deep, too wide or too shallow. Several land owners prevented access, there was a site with a natural fish barrier and in one site an unpredictable bull prevented a survey from taking place. If sites could not be surveyed, then a new survey site was requested from Marine Scotland. The trust had 30 sites to survey, ten of these would be surveyed annually. To complete the task of obtaining data from 30 sites the team had to visit 50+ sites in total. This underlines the necessity to recce sites prior to surveying, this undoubtedly increases the time to complete the national survey. Reviewing the trusts data, there is an indication of a severe lack of salmon. Although 2018 random sites included many sites known to be poor for salmon recruitment. On reflection 2018’s weather certainly had an impact on all the trust’s electrofishing commitments and hopefully 2019 will be a better year for surveying and not fraught with droughts during the Summer and high water in Autumn.