The following film is of a three-minute timed electro-fishing sample taken on 9th September 2013 from a trout spawning burn which flows into the Yarrow Water, as part of The Tweed Foundation’s electro-fishing surveys of the Tweed catchment.
Electro-fishing is a method of catching fish without harming them: a portable electro-fishing backpack or bankside unit with a hand-held anode (+) and a cathode (-) produces a current in the water which compels nearby fish to swim towards the anode. This allows them to be scooped up by the electro-fishers so that they can be identified, counted and measured. Most fish recover immediately when out of the electric current (and occasionally fish which have been in the current slightly longer than normal may take a few seconds to recover).
There are two main methods of assessing fish numbers used by The Tweed Foundation:-
1. Timed. By applying the same amount of catch effort to similar fish habitats in different areas an indication of fish production can be collected and these can then be compared.
2. Quantitative. A given area is fished in total three times. The depletion in numbers between each run allows the total fish numbers to be calculated.
In total 12 trout fry, 15 trout parr and one salmon parr were caught (with six fish missed). This represents about an average fish density for this type of upland habitat (glide/pool) within Tweed trout burns. This represents an average fish abundance for this type of upland habitat (glide/pool) within Tweed trout burns, where parr dominate in the deeper water. Fry, usually in larger numbers than Parr, are found in the shallower riffle areas.
The Tweed Foundation’s systematic programme of surveys conducted regularly, over a long time period, allow fish populations to be assessed and may give indications not only of trends in fish stocks but also of specific environmental problems in that area.