I expected to find reduced fish numbers in the upper Avon but somehow it is still a disappointment when you see it in the flesh. We surveyed 8 sites today from the bridge at Delavrorar up to the shelter hut upstream about 6km upstream of Inchrory. We found salmon fry at each site but in declining numbers.
The size of the fry decreased also.
The parr were also small, at some sites the mean size of the one year old parr was only just over 50mm, the same size as many of the fry in the Spey mainstem. There was the occasional larger parr, including one who ate a fry whilst in the bucket awaiting processing. This is the first time we had ever seen a parr eat another fish after capture; I have seen it with eels several times but never with parr, a hint at the lean pickings on offer in the upper Avon perhaps? The parr were generally small but they were also lean as the photo below shows.
The physical habitat quality was excellent throughout but the conductivity dropped as we progressed upstream. The meter may have been playing up but the reading at the upper site was about 6, the lowest I have ever seen. Tomorrow we will finish the upper Avon mainstem surveys but before we finish we will carry out a bankside invertebrate survey to assess the food resource. Maybe we will find higher fry numbers at the end of the track, there the gradient lessens slightly and there are better spawning gravels.
Some of the small burns joining the Avon had much higher conductivity readings, one was over 100; they must have areas of richer geology within the catchment. These richer water intrusions were visible in the Avon due to the higher level of algal growth on the river bed. The two photos below highlight this.
The upper photo could also explained the unusual colouring of the salmon fry. In the same sweep one fry would be sandy in colour, whilst another would be dark. We figured this was because of the variety of colours of rocks present in the lower reaches of todays survey stretch. Some fry must have been lying on dark rocks and others on light. However as we progressed upstream the substrate became dominated by reddish granite boulders and cobbles.
The question is has the juvenile fish density in the upper Avon always been so low? There are anecdotal reports referring to big runs of fish into the upper Avon in the past, including a comment on the blog last year. The physical habitat as mentioned above is excellent but the productivity is known to be naturally low. Low fish densities often means fast growth but not in the upper Avon. Today was an eye opener for me, I’d never surveyed such small fish at this time of year. When scale reading Steve often refers to the first winter check being very close to the centre of some parr scales. Some of the fry we found today will barely be big enough to have grown scales by the end of the first summer. Judging by the size of the fry it appears that the overall productivity of the river is limiting the fish density. If only the Cairngorms were made of limestone, what a salmon river the Avon would be! There are few others rivers with salmon present at such high altitude and with such low conductivity with which to compare the Avon – the upper Dee maybe. I must consult with my colleagues over the hill.
Todays upper survey site was immediately upstream of a huge granite boulder that must have weighed 30-40 tonnes, a spectacular site for a survey. The rest of the scenery wasn’t too shabby either – what a great day out – as always in the upper Avon.