Riverfly Sampling

David Beeson, December 2021

In Hampshire, we have some unique river systems. With chunks of the county dominated by chalky geology the rainwater is held in huge aquifers and only slowly released. It emerges comparatively warm in winter ( and remains cool in summer) and is enriched with dissolved calcium. The waters are usually crystal clear with the riverbed easily viewed, so the brown trout and waterweeds are easily seen. These should be treasured ecolological areas, yet society too often treats them poorly. Agricultural runoff, deliberate oil spills and sewage effluent all end up where it should certainly not be.

Riverfly Monitoring is an initiative spearheaded by the Riverfly Partnership which ensures that angling and conservation groups can take action to conserve the river environment by monitoring water quality. See: http://www.riverflies.org

I monitor one site on the River Anton, within Andover. The River Anton feeds into the River Test north of Stockbridge. The monitoring is carried out by sampling the river’s invertebrates, some of which are termed riverflies.

Living in the river, and having a thin exoskeleton and possibly ultra-thin gill membranes, invertebrates are potentially killed by falls of oxygen concentrations or environmental pollutants. So, by establishing the ‘norm’ population of the ecosystem any population change could be due to a pollution incident. That data would then be shared with The Environment Protection Agency, who should be equipped to carry out further studies, mitigation and possibly legal action.

For those with local knowledge, my site is behind The Range / KFC just to the north of the inner ring road. It is downstream from Charlton Lakes and Shepherd’s Springs. There are other monitoring sites along the river, so a reasonably precise location of any ‘incident’ can be pinpointed.

It is a three-minute kick sample across the river’s width, plus a one-minute sampling of the larger riverbed stones by washing their surface. The river’s flow will push dislodged creatures into the downstream net.

With a flowing water system, most invertebrates dwell amongst the riverbed’s rocks, stones or weed. Once outside, and in the current, they will be transported away and possibly captured by the resident fish.

Once the capture has occurred the sample needs sorting. Others do this, I’m told, on the bank, I take it home to a more ‘relaxed’ table!

Only specific inverebrates are counted as these are especially common and sensitive. I also captured fish (Bullheads), snails, beetles and worm-like fly larvae and expect to see leeches, caseless (web-spinning) caddis and waterlice at some stage. Lampreys, swan mussels and larger fish may also eventually be spotted.

Of the groups monitored I have yet to see Flat-bodied Hetageniidae and Caseless Caddis, although I have found the caddis previously.

What numbers have I found? Cased caddis in single figures, a single Mayfly ephemeridae, olives in tens, a single stonefly and always hundreds of Gammarus. Clearly, with such a short sampling time and conditions changing with the seasons, numbers will fluctuate, yet a pattern should soon be established.

I’m surprised by the low numbers of caddis and stoneflies as these seemed more frequent on previous studies, but it was a different sampling site with shallower water and more watercress and marsh vegetation. Everything makes a difference.

Homepage is: http://www.nwhwildlife.org. There you will find 130+ ad-free articles. Scroll down to seek out knowledge!

Cheers, David