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ETTRICK FISH COUNTER
(click for latest counts)
Until recently there was no suitable model of fish counter available for use on the Tweed due to the particular circumstance that there is no size difference between Salmon and Sea-trout here. The Resistivity Counters used elsewhere in Scotland cannot measure the size of fish passing, they can only be set to count above a minimum size. On the North Esk, for example, the Resistivity Counter at Logie has a cut-off point of 50cms, all fish above this size being regarded as Salmon. If this was done on the Tweed, whilst only 0.5% of Salmon would be excluded, 65% of Sea-trout would be included!
The type of counter installed in 1998 in the fish pass of the Howden Cauld near Selkirk on the Ettrick, about 250m downstream of the junction with the Yarrow, is a RiverWatcher counter made by Vaki Aquaculture, of Reykjavik, Iceland.
This works by passing two infrared beams across a narrow “Scanner” set into the furthest upstream step in the fish ladder of the cauld.
The “scanner” in the fish pass is connected by cable to the counter computer which has been housed in a small cabinet on the bank (there is a window in this through which the counter display can be seen).
The direction in which the fish are travelling is given by which beam they first break and their size by the “shadow” they cast on the receiving side of the scanner. A silhouette of the object that causes each count is also stored in the counter’s memory and these are used to check whether they were fish or not
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Closed Circuit TV At The Ettrick Counter
All fish counters need to be calibrated – which means that their efficiency has to be worked out. No fish counter is 100% effective, but if their “miss” rate is known, it can be allowed for in the final estimates. The calibration set-up used at Howden is an underwater CCTV camera that can be mounted over the exit from the fish counter scanner in the fish pass. A white slab has been set in the base and the camera looks down onto this, through a depth of water of only about 50cms. A cable is strung the 40m to the shore from the fish pass and the monitor is placed under the fish counter. In this way, when a fish goes through the scanner, it can be seen on the television whether the counter has registered it or not.
The various calibration tests that have been carried out since installation show the counter to be 100% efficient for fish of 40cms and above going upstream and 88% efficient for fish going downstream - the lower downstream rate is due to the very fast speeds that fish can sometimes drop back through the scanner at. No Salmon have been seen going over the face of the cauld and therefore missing the counter, nor are they seen coming back down over it – the fish that do drop back do so just after passing the scanner. At particular water levels a few Sea-trout can get over the face, but this is a very rare event.
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