Channel Morphology and Obstructions
DDSFB / RDT / AC / SEPA
Since 2007, the Dee DSFB and River Dee Trust have eased or removed 27 manmade obstructions to fish migration from the river Dee's tributaries to allow fish to gain access to their natural spawning grounds. These manmade obstructions include weirs, bridges, vehicle fords and culverts. Some are completely impassable to fish, others are impassable in low flows; the latter can delay migration and have a knock-on effect of making fish more vulnerable to predators and stress.
Which obstructions to tackle first?
In 2007, RDT assessed all the known manmade obstructions to prioritise them for easement or removal. For example, removing an obstruction that is on a tributary with poor habitat for salmon, or that is located close to the headwater source, offers a lot less potential for increasing fish production than removing an obstruction that is low down on a prime-quality tributary.
Opening up the Culter burn
In 2014, the largest manmade obstruction was eased by installing a fish pass to the face of the Culter dam at Peterculter. Two weirs further upstream in the Culter burn were eased. On 3rd October 2014 the first salmon ascended the Culter Dam in over 250 years.
The fish pass has opened up 76 miles of habitat in the previously inaccessible Culter burn for migratory salmon and sea trout to re-establish natural populations. Previously only the lowest mile of burn below the dam was accessible. Once habitat restoration work is completed in the Culter catchment we expect to see an additional 1,500 salmon returning to the Dee each year. These salmon may be available to the catch and release rod fishery as far up river as Banchory, as radio tracking studies show how fish may wander up to 20km upstream from where they eventually spawn.
A Vaki fish counter is installed at the top of the fish pass to record how many fish are using the pass to ascend the dam. The counter records the length of each fish. All fish longer than 50cm are assumed to be salmon, fish between 30 and 50cm length are sea trout and fish less than 30cm are brown trout. In practice, there is some size overlap, particularly between small salmon and large sea trout. Our scale data show around a 6% error in these classifications.