Fish Surveys

A trout fry (left) and salmon (right) caught below Carry bridge along Colne Water

A trout fry (left) and salmon (right) caught below Carry bridge along Colne Water

The data that we collect here at the Trust helps us to monitor fish numbers and in turn helps us to monitor the success of the schemes that the Trust has undertaken.

To carry out our surveys we use a technique called electrofishing which involves the use of a weak electrical field; the voltage and frequency of the electric fishing equipment are set depending on the conductivity of the water (and the species that are being targeted.)

The electrical charge used provokes the fish to swim toward the electric field, as the fish swim toward the field a second person catches the fish using a dip net. This process is completed in reaches, the reach (length ten times the width) is netted to prevent any fish entering or escaping the survey area, the reach is then fished over a five minute period, after this period the captured fish are sorted by species, measured, weighed and released.

Sea trout captured during a survey

Sea trout captured during a survey

Using the number of captured fish plus those fish that are seen but not captured and the total fished area we can calculate our Catch Per Unit Effort and population estimate. The data collected from semi-quantitative surveys is calibrated through our quantitative surveys.

These quantitative surveys use the same principle but on a larger scale, each reach will be fished three times allowing us to remove the whole fish population ensuring the accuracy of the operators. This additional information provides us with adjusted values for our semi-quant surveys. The final value of fish is calculated per 100m², and in turn classified using the National Fisheries Classification Scheme.

To read more about electrofishing please click here.

To access our report on 2015’s fish surveys please click here.

Andy Croft (survey and monitoring) holding the salmon parr that was captured on the River Loud

Andy Croft (Environment Agency – survey and monitoring) holding the salmon parr that was captured on the River Loud