Lost Youth

As the tail end of Hurricane Lorenzo was flinging rain against my window, I finally had chance to reflect on the electrofishing data I had collected during 2019. Many of the projects I have instigated since joining WTT in Yorkshire have included an element of rudimentary monitoring, mostly the assessment of the fish stock within a particular reach, and in particular with a focus on young-of-year (YOY). In some places, these have been the first data collected, while in others there have been some Environment Agency data to draw upon. My aim for specific reaches is to gain at least one year prior to any interventions (ie habitat restoration or weir removal) and then several years post; ideally for as long as possible. From an analysis perspective, we must accept that having only one year pre-works is less than ideal, but that is simply a reflection of the relatively reactionary nature of the work. Longer term data from multiple sites will allow me to draw better inference about the causes of any patterns that may emerge.

To recap on some of the projects monitored, the longest dataset I have accrued so far is for Eastburn Beck where, together with the EA’s Pete Turner, the aim was to improve accessibility (and habitat) through notching of a series of small step weirs. Below is a figure of the number of brown trout caught of a given length (for clarity I’ve made 5mm size-bins so eg all fish between 170 – 174mm get put into one bin). The grey data are from 2018 and the black data are from 2019. There are three peaks or clusters in the data representing three age-classes: ~90mm = YOY; ~175mm = 1+; and ~240mm = 2+ fish). 

Working backwards through those ages, there were virtually no fish in the 2+ cohort in 2018 and in a previous blog I ascribed this to the unprecedented flooding of the 2015 – 16 winter. Thus, it was good to see a much healthier number of 2+ present in the 2019 data. Maybe the hot dry summer of 2018 had taken a toll on the number of YOY from 2018 because the number of 1+ fish was almost 50% down in 2019. However, the most marked decline was clearly in the YOY: an 88% reduction in numbers.

On Lothersdale Beck, a tributary of Eastburn, I hatched a plan with the owner of the village pond to open the sluice-gate at the bottom of the dam (a formidable structure >5m in height) and leave it open from October to March. There were two aims: for the empty’ dam and sluice-gate to act as a choke on the system, holding back spate flow and reduce the risk of flooding in the village below; and to allow trout to potentially migrate further upstream over the spawning period. 

Electrofishing the beck with keen volunteers revealed trout above the dam for the first time since I started monitoring 3 years ago, so that aspect has apparently worked. Unfortunately, we did not record any YOY above the dam but there are a few reasons why that might be. Too few fish might have made it through this year to find each other, or simply there was such a low density of progeny from this initial (re)colonisation that they were impossible to detect. Time will tell. However, a similar story to that on Eastburn emerged from the reaches below the dam, ie a ~60% reduction in YOY.

Further afield but still within the Aire catchment, annual surveys of habitat improvements with Bradford City Angling Association were, to put it bluntly, disappointing. We didn’t recover any YOY from around the woody structure placed several years ago. But what of beyond the Aire? With a keen group of committee members from Knaresborough Anglers, we returned to Cockhill Beck with high expectations after having installed some woody material throughout a 1km reach to retain and sort gravel for spawning habitat. While the beck looked in better condition, and other fish species had all increased, we did not capture any trout in 2019, so another 100% decline in YOY. Only Dauber Gill on the Nidd had anything like a balanced’ population structure, ie lots of YOY and proportionately decreasing numbers of older age-classes. I discussed this briefly with Dr Mike Forty at Ribble Rivers Trust and the picture was similar from their catchment wide surveys. What could have affected trout fry density across such a spatial scale?

The winter of 2018 – 19 looked spot on for so long. Conditions early on were relatively benign and actually it was the best year of my (relatively short) time in Yorkshire to go out redd-counting. The spates that we did have tended to rise quite slowly and ease off equally so…..until Storm Gareth. After what seemed like a perfect incubation period, Storm Gareth blitzed through at exactly the wrong moment as eggs hatched into weakly-swimming emergent fry, perhaps the most vulnerable life-stage. I went out specifically during Storm Gareth to see what the rivers looked like at various project sites. It was certainly dramatic! One or two gauges peaked at 2015 – 16 heights but without the duration. 

While this is not great news of course, nature can prevail. Cue compensatory growth of those fish remaining that had little competition for resources. Individuals in better body condition are generally better equipped to survive the rigours of winter and spawning. I hasten to admit that I do not have data on body condition, but my goodness some of the trout I have caught in 2019 with both electrofishing kit and on rod and line have been in fine fettle and dare I say it, plump. 

Fingers crossed they get an easier ride this year. Certainly, Lorenzo should have helped them on their way!