Juvenile Surveys

Undertaken by electrofishing in the rivers and burns in the summer. At each site the presence and abundance of salmonid fry and parr is recorded, allowing an assessment of the health of the population and early notification of problems.

What is Electrofishing?

An electric current is passed through the water which attracts and stuns (but not kills) the fish, allowing them to be netted out for identification and measuring. This is relatively unbiased and by selecting sites to cover a range of available habitats, it is possible to determine the population density of juvenile salmon and trout within the catchments. The main issue with electrofishing is that it is only suitable for shallow areas and as such is used for fry and parr rather than adult assessment. However electrofishing is also often used for broodstock capture for hatcheries.

Different factors can affect these types of survey, for example the electrical conductivity of the water. While the impacts of the current vary between species, it is aimed particularly at salmonids. Electrofishing surveys have however also enabled us to map the presence of other species such as eels and minnows in the area.

Why are juveniles important?

 
Without babies there are no adults! Juvenile salmonids occupy very different habitats to the adults and as such are often overlooked by the angler and even proprietor. However, by sampling the juveniles we get a good indication of the health of the system as this type of sampling is not affected by marine factors, weather conditions or angler efficiency. At present salmon and trout fry and parr occupy most available habitats across all catchments indicating that populations are healthy, if small.  Some catchments including Loch nam Brac, Loch Innis and Gleann Leireag are primarily trout systems with few or no salmon.

Upon capture the fish are lightly anaesthetised for ease of processing. Salmon and trout lengths are noted to assess the numbers in different year classes and to determine between fry and parr in some cases. Salmonid fry appear very similar although salmon have much larger pectoral fins than trout and the mouth does not extend beyond the pupil of the eye.

Salmonid densities in Sutherland catchments are compared to previous years to identify any significant changes that might need to be addressed. Reports detailing electrofishing surveys from previous years are available under publications.

Differences between trout (left) and salmon parr