Electrofishing in Hirst Woods

A guest blog by Prof Jon Grey from the Wild Trout Trust.

Back in May, the new footbridges constructed by Aire Rivers Trust in Hirst Woods over the old mill goit made an immediate and obvious impact on the walkers, both two and four-footed. But they were also built to reduce the human impact in the goit itself. Over the past months and with input from the Wild Trout Trust, members of Saltaire AC have been working steadily towards reinstating the flow and reintroducing some habitat features to benefit the flora and fauna of the goit.

This has involved the removal and relocation of accumulated boulders and freeing up the compacted sediments in between to allow fresh water to flow through the goit even under summer conditions. The water bubbling over the riffles (shallow gravelly stretches) provides a very different habitat to the slower deeper stretches which remain quite silty on the bed.

It is hoped that with some higher flows over winter, much of the silt that accumulated when the goit was blocked up will be flushed out or relocated to the sides where we have installed some ‘brash mattresses’. These are densely packed branches and twigs tucked behind chestnut posts on the inside of bends to accentuate the sinuosity of the channel (make it more wiggly) and create dead zones into which the silt will percolate and eventually stabilise the material. As the woody material breaks down over time, marginal plants should colonise and further stabilise the structure.

Brash mattresses also provide excellent nursery habitat for young fish with plenty of protection from both flood flow and predators. More physical diversity in the channel allows for more biological diversity too.

The impacts on fish are much harder to ‘see’. However, part of the funding for the habitat-focussed project from the Yorkshire Water Biodiversity Enhancement Fund was ring-fenced for monitoring pre and post works. On Friday 19^th Sept, we electric-fished the full goit in exactly the same manner as we did last year prior to any improvements.

Last year from the relatively stagnant water of the blocked goit, we caught very few fish (~60) from only three species: three-spined stickleback, stone loach, and minnow. The stickleback is characteristic of slow or stagnant waters and the stone loach favours a more silty bed. Hence, the impoverished fish community reflected the sorry state of the goit then.

This year, we caught six species, adding chub, bullhead, and gudgeon to the list. All of these ‘new’ species favour more flow, especially the bullhead that was typically found around the new riffles on the cleaner gravel and cobble. Furthermore, the goit was teeming with fish this year. We did not bother to net minnow after the first 10m and I would estimate we would have had several thousand if we had continued. The small chub (~100mm) and fry of all the other species indicated that the goit was once more offering suitable nursery habitat.

The results speak for themselves. Good news. No wonder the local kingfishers have been far more frequent visitors of late! I’m excited to see what further changes will bring to the goit in 2022.

Prof Jonny Grey (Wild Trout Trust)