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Fish populations

Organisation: Various
Location: National
Type: Tools & approaches

Electrofishing is one of the most effective methods available to quantify, assess and monitor fish populations; within and between waterbodies.
Electrofishing involves creating an electric field in the water that temporarily immobilises the fish or influences the direction in which they swim, making them relatively easy to capture with a net.
Electrofishing surveys can either be qualitative or quantitative. Qualitative electrofishing is used to capture a snapshot of the fish population, giving an indication of the species present (or any notable absences) and their age classes.
Quantitative sampling requires a more structured approach, with a defined area, typically 100m2, fished repeatedly having been isolated using stop nets at the top and bottom of the river section.
This method allows an accurate count of the number of fish to be made and, by recording the species, age and size of the fish caught, the results are entirely comparative between different sites and over time.
Semi‐quantitative electrofishing is a method that concentrates on recording the presence or absence of different species, the sizes of the fish caught and the abundance of juvenile fish. Unlike quantitative electrofishing surveys, semi‐quantitative surveys are comparable because they are always undertaken for a fixed time period of five minutes and each site is surveyed with the same level of effort. The advantage of this approach is that it is quick to undertake a survey and so multiple sites across a catchment to be surveyed each year without prohibitive cost implications.
Results recorded from electrofishing sites across a catchment can be used to assess the distribution and density of juvenile fish, which in turn enables us to estimate the number of adults that were present the previous year and the health of the fish population. The results can also be used to compare tributaries in the same catchment or sections within the same river, which is particularly important in identifying where density is below desired levels and ensures that river improvement works are targeted into the right locations and that any improvements are accurately recorded.

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