Juvenile monitoring

Juvenile monitoring

In 1990 the Board initiated a program of yearly electro-fishing surveys to monitor the status of juvenile salmonids throughout the catchment.  Two approaches are used, electro-fishing a known area and timed electrofishing.

Electrofishing

The electrofishing team in action!

For area based electro-fishing survey sites are selected at random; although in practice vehicle access often limits the areas available to survey. Each survey  site is approximately 100m2 and is electro-fished three times to allow fish population densities to be calculated (Zippin, 1958) or only once to provide a more basic dataset. The captured fish are measured, weighed and a sample of fish scales is collected for later age determination. A range of physical parameters for each site are also recorded and each site is photographed to allow accurate revisiting in future years. This approach is typically used for tributaries where detailed data on fish densities is required.

Timed electro-fishing involves fishing a given section of river for a given period of time to provide catch per unit effort (fish per min). The approach provides a relative index of abundance, range of species present and the age distributions. The approach is less rigorous than the area-based surveys but they are often quicker allowing more burns to be surveyed. On the Spey this approach is used to examine the mainstem and where an initial survey is used to characterised the fish populations present.

The techniques used on the Spey follow those developed through the Scottish Fisheries Co-ordination Centre.

Juvenile Salmon Distribution
Juvenile salmon are well distributed throughout the mainstem and tributaries of the Spey. They were absent above impassable waterfalls or dams without fish ladders.  Age classes ranged from 0+ to 3+ although the older 3+ parr are pretty scarce and limited to the upper tributaries.

The density of salmon parr varies throughout the catchment and in general tributaries such as the Fiddich, Livet, Avon, Crombie show the higher densities while those draining the Cairngorms, such as Druie, Feshie, Tromie, Truim, are lower in density. Growth rates also vary with juvenile salmon in the lower tributaries showing faster growth rates than those further upstream.

This related to altitude, river temperature and underlying geology. The parr from the higher altitude tributaries such as the Loin have slower growth rates than the lower tributaries. Salmon parr from the Feshie, which flows over granitic rocks and is fed by snowmelt for much of the year, show much slower growth than equivalent altitude sites on the Livet and Dorback. These latter tributaries drain richer limestone substrate.

One surprise from the surveys was the abundance of juvenile salmon in the mainstem, in particular from the Fochabers to Grantown.  Excellent numbers of salmon fry and parr were captured in this reach of the river indicating its importance as a nursery area as well as a premier angling area. Upstream from Grantown salmon numbers were less abundant and this reflects the slower flowing nature of the river.

Surveys also indicate that in the smaller burns (width <2m) although salmon will often be present these burns tend to offer better habitat for trout.

Why not watch our videos of electrofishing on our Vimeo channel?

Juvenile Trout Distribution
Information on the distribution and population densities of juvenile trout is also gained from the electro-fishing surveys. Juvenile trout are distributed widely through the catchment. In particular the larger tributaries such as the Fiddich, Avon, Dulnain  and Dorback produce good densities. Trout were also present in the streams draining the harder rocks such as the Feshie, Upper Avon and Druie, however, densities were generally low. Similarly although a few trout were often captured at the mainstem survey sites numbers here were also lower than salmon.

More recent surveys have shown the importance of smaller burns for rearing trout. These have often been neglected in the past but clearly protecting trout stocks in the future must ensure good management of the habitat in these burns. Resident populations were also found in burns above impassable waterfalls.

Age classes ranged from 0+ to 4+ with older trout occasionally caught and growth patterns were similar to salmon with better growth rates in burns in the lower catchment and those draining richer limestone areas.