The results are in: barriers down, fish up

I’ve been looking forward to this moment for quite some time now…..well, at least a year. The monitoring of my pet project from pre-intervention (weir notching and removal / partial demolition over six structures) to several years post is quite revealing, and I’ll let the data do the talking.

Now, as a scientist, I know there are a few caveats associated with the figure above. But as there was no specific funding pot for the monitoring of the works for this duration, I am making the best of the situation. So, all surveys were carried out in each of the years for roughly the same amount of time (effort), over similar distances, using similar kit, and roughly the same time of year (although 2018 was a little later because of the incredibly dry spring / summer we have just experienced). Ideally, all of these parameters would have been standardised; ie identical each time.

I only have one year of pre data, and two ​‘proper’ years of post, and trout numbers are notoriously variable year to year, but the numbers didn’t fluctuate too much at the reference site (grey bars). I say proper years, as the ¹⁵⁄₁₆ floods annihilated the trout production from that year (I could not find any young-of-year in 2016) and altered the river habitat substantially, maybe even flushing some of the larger trout out of the system. Hence, the dip in 2016 data which was actually when we carried out the majority of the works.

Given the above, where we have removed obstructions to fish passage (blue bars), there are 5x the number of fish in 2018 compared to those same sites in 2015 prior to the works, whereas at the reference site the numbers are at most 2x higher.

I’m fairly confident we’ve had a positive impact just using this simple measure of brown trout abundance, but then the accumulated scientific evidence from all sorts of rivers around the globe is unequivocal! And of course, it’s not all about the trout, as the benefits of weir removal spread way beyond fish.

Some folk are still sceptical about aspects though. For example, there is a perception that removing weirs will remove big fish habitat. As my colleague Paul points out in his blog post, unshackling a river from weirs should allow it to recreate over time a natural sequence of pools, riffles & glides where trout of all sizes and life-stages should find a patch to suit their needs. To come back to my project data, in the 2015 survey, the largest trout measured ~320mm. In 2018, there was a considerable number that were >400mm.

That’s made my week. Cheers!