Lets talk about the weather

Alex Deacon

We Brits love to talk about the weather, it’s just one of those things that define us as a nation. But it matters. Just take a moment to reflect briefly on the increasingly extreme weather patterns we’ve experienced in recent years – record breaking summer temperatures, wet and wild winter storms, and winter temperatures barely dropping below zero here in the southern counties. Packed beaches and flooded streets grab the headlines; the impacts on our communities and economy are becoming more apparent with every heatwave and flood event. But what affect does this kind of weather have on life beneath the surface of our rivers?

Last summer the Test & Itchen catchments were in a state of drought, with the Avon and Stour not faring much better. Our chalk streams and rivers were running perilously low. The Trust’s network of water temperature loggers recorded highs in excess of 20 degrees Celsius on our chalk streams. Fish are ectotherms and have little control over their body temperate. This means temperature plays a major role in determining both the health of our fish populations. Salmonids, including salmon, brown trout and grayling favour cool, clean water, with our rivers not very far off their southerly limit. Research shows river flow (a product of rainfall) also plays a key role in determining the health of our fish populations. Local data collected and analysed by our Environment Agency’s (EA) survey teams helps us to understand just how the weather of recent years has affected our fish populations.

One of the most sensitive life stages of these species are the months not long after hatching. Survey results indicate low summer flows and high summer temperatures has resulted in a reduction in the size of juvenile trout and salmon in a number of our chalk streams. As a result, local trout populations suffer, and less salmon will reach adulthood.

The relationship between autumn/winter flow and the number of juvenile salmon recorded in surveys the following summer on the River Test. Low flows correlate negatively with salmon numbers. Graph courtesy of the Environment Agency Solent and South Downs: Fish monitoring report 2019.

And then from one extreme in our weather to another. Last autumn and winter’s stormy downpours seemed at times to be relentless. The aquifers which steadily feed our chalk streams went from being near empty to inundated, and rivers followed; the Avon and its tributaries busting out into the floodplain. These high flows coincided with the highest number of adult salmon and sea trout passing through the Knapp Mill fish counter on the Lower Avon since records began in 2006.
A 65cm long salmon migrating upstream through the Knapp Mill fish counter on the Lower Avon. Image courtesy of the Environment Agency Hampshire Avon Knapp Mill Fish Counter Report Q4 2019.

Elevated flow levels tend to make in-channel barriers such as weirs and hatches more passable to fish, thereby improving access to their spawning grounds upstream. Conversely, sustained periods of low flows can result in fish being unable to reach their spawning grounds, or in poor physical condition; both are likely to impact upon the recruitment of the species. Whilst it’s encouraging to see an increase in salmon entering the river, the North Atlantic salmon population is a species in crisis. We must do everything we can to protect our salmon stocks. Since 2015, the Test and Itchen salmon populations have fallen below their conservation limit, whilst the Avon and Stour are worse off.

It’s now mid June, and all that rain seems like a distant memory. Spring rainfall seemed non-existent, with yet another record broken – this time, the ‘sunniest spring on record’ (Met Office)

The Environment Agency Water Situation Report for May 2020 highlights both the dry spring and wet autumn/winter of previous months.

Nature has adapted to withstand the occasional heatwave or exceptionally stormy winter; the weak perish and the strong prevail. But we appear to be heading into unchartered territory, with our climate regularly producing such extreme anomalies, we are seeing our fish populations put under even greater strain.

The data collected by the Trust and our partners highlights the significant role our climate plays in determining the health of our fish populations, not to mention the other vital components which make up the unique biodiversity our river systems. This also highlights the value of monitoring and research data in helping us to understand the direction we need to take to ensure our rivers more resilient to our changing climate. With the support of our partners, the Trust are working throughout our catchments to restore the health of our rivers with climate change in mind.