“Record River Dee warmth ‘threatens salmon’” shouted the BBC headline. An article on the BBC website today, running only to five sentences, ensured that each sentence contained an alarming statement. First we are told that conservationists are warning that rising water temperatures represent a continued threat to wildlife in the (Scottish) river Dee. Second, we are advised that water temperatures in the river reached 21C earlier this month near Ballater, and that this is well beyond the optimum 16 or 17C needed by Atlantic salmon if they are to be sustained. Third, the fish can stop eating beyond the 16-17C range. Fourth, the high temperatures were reached two weeks earlier than in 2023. Finally, the River Dee Trust said temperatures in the river were on course for a record-breaking month.
Five sentences only, so much hype. No links were provided, so any attempt to dig deeper depends on independent research. Fortunately there is a thing called the internet, and even better, the River Dee Trust has a website, which seems to be shared with the Dee District Salmon Fishery Board. Its contents make interesting reading.
The first thing I noticed is that despite the short BBC piece seeming to be of the type that results from a press release (variations of it can be seen all over the internet over the last few days), the website itself seems to make no such claim about high temperatures in May. The closest to it that I could find is in its report for the week commencing 13th May 2024, which stated:
A scorching hot week of bright sunshine and high temperatures most days presented a challenging week for anglers and for the fish in the river, all the ghillies have mentioned the difficult conditions in their reports. The best time to be on the river was early morning or early evening and so fishing effort was reduced last week.
Temperatures were specifically referred to only twice. First there was a reference to “water temperatures between 57f/60f.” By my calculation, that’s a range of roughly 13.9C – 15.6C. Later in the weekly report we are told “[r]iver temperatures peaked at 18C this week at our monitoring site in Ballater.” The report for the following week (the most recent report on the website) doesn’t mention water temperatures at all, and reads as though all is well (e.g. “it was good to see people fishing in the evenings and enjoying a cast with the extra light nights” and “Honestly I wish I could bottle up the joy – we were on a high throughout”.
That is not to say that the River Dee Trust isn’t concerned about climate change and about river temperatures. Both concerns feature on the website, as do reports in recent years of higher water temperatures. For instance, almost exactly a year ago it published an article with the heading “It’s too warm for salmon”, which bemoaned the fact that river temperatures in summer now regularly exceed 20C for periods of time, with 23C also being exceeded from time to time at 60% of the temperature monitoring sites.
Fortunately, that piece links to another report relating to water temperatures in the summer of 2022 which, to my untutored eye, suggests that the number of such days, although potentially an issue for the health of fish stocks, seem to be relatively few. Despite the summer of 2022 including “an extended period of dry, hot weather,” “[t]he most common temperature (median) across all sites was between 11-14.4°C which is within the optimal temperature range for salmon”. However, the temperature did exceed 20C somewhere in the river catchment (most notably the tributaries of the Callater, Clunie, Gairn, and Geldie) on between 2% and 26% of the 91 days defined as summer for the purposes of the report. One of the key solutions being implemented is the planting of trees – lots of them:
Over 330,000 riverbank trees have been established in the upper catchment to date, on target for establishing 1 million riverbank trees by 2035. These trees are particularly targeted in areas sensitive to high summer temperatures such as the Callater, Clunie, Gairn and Geldie.
Having read that, I wondered if the Dee and its tributaries used to have many more trees in the past, and if the loss of trees might not be more significant than the relatively small increase to date in temperatures caused by climate change. It turns out that this is indeed very much the case. Another paper (the River Dee 2020 – 2025 Fisheries Management Plan) tells us:
Our once salmon-rich rivers have been substantially altered over millennia, with deforestation, agricultural intensification and damming for waterpower causing profound changes. Scotland is one of the most heavily deforested countries in Europe, with only half of the European average of forest cover – and less than one quarter of Scotland’s remaining woodland is natural. From the original estimated 70% forest cover, the upper catchment of the Dee now has only 8% woodland cover (including commercial plantations), whilst some individual catchments such as the Gairn have only 2% woodland.
Perhaps, then, it’s not a coincidence that the Gairn is one of the tributaries where the highest water temperatures have been recorded:
In recent years the Gairn, a tributary to the Dee, has reached a water temperature of 27.5C (81.5F), close to the lethal temperature for juvenile salmon.
The River Dee 2020 – 2025 Fisheries Management Plan is illuminating in other ways too. It blames climate change (perhaps inevitably), but it blames lots of other factors too, for reduced fish (especially salmon) numbers. For instance:
Aberdeen Harbour, one of the UK’s busiest ports, is built around the mouth of the Dee. There is no longer the shelter of a natural estuary for fish passing through, and the confined and noisy environment of the harbour may help predators of salmon and sea trout….Three years of tracking showed that smolt losses in the harbour can on occasions be high (one in three years), with over a quarter of smolts lost in this very small area…Adult salmon and sea trout move through the harbour throughout the year. The potential for disturbance during river entry from noise, sediment and other pollution, and any additional risk of predation during periods of disturbance is, however, poorly understood.
In other words, there are difficulties for returning fish as soon as they reach the river mouth. And they face a myriad of other problems:
In-river construction works can harm fish [and] their habitat…. Underwater noise and vibration impacts are particular areas for concern…
Interestingly, hydro scheme proposals earn a mention here. Poaching is an issue (though seemingly not a large one), while natural predators are an issue, and in the case of goosanders, apparently a relevant new problem:
Goosanders are a sawbilled duck that have relatively recently arrived in the UK. Their UK population is stable but counts on the Dee show an increase in these birds over the last 10 years. They feed on juvenile salmon and other fish – an average goosander eats 10 smolts/large parr each day. Tracking programmes show that high losses of smolts occur in many Scottish rivers. Evidence from acoustic tracking of Dee smolts shows high losses of smolts too, and most likely caused by goosander predation.
Seals are another problem predator (though seemingly a relatively small problem) as are cormorants, which “are becoming more of a threat as they encroach inland now even as far as Invercauld and take smolts and adult sea trout.”
Dolphins, it seems, are an unknown quantity, but the harbour area at Aberdeen might be exacerbating the issue:
It is possible that the artificial channel of the river as it passes through the harbour influences an otherwise balanced relationship between dolphins and salmon.
Humans are a big problem in many insidious ways:
There has been a huge decline in water quality from a historical perspective, and whilst regulations in recent years and decades have improved the situation, water quality is far from what it would be without the impact of our rural and urban communities. It is probably the biggest factor influencing the middle and lower catchment fish stocks and river life…Asthe riversupplies water for over 300,000 people and their homes and businesses, balancing water demands is a major challenge for the future. Although the principle abstraction is for domestic use there may be future demands on water resources for agricultural irrigation.…The biggest visible impact on water quality is sediment run-off. This is damaging to juvenile and spawning habitat and invertebrate habitat, thus affecting food supply for fish. Chemicals such as pesticides and road salt may have at least as much impact on water quality assediment, as they are washed off agricultural and urban land. Even low concentrations of chemicals in the river damage fish by affecting their homing, spawning and predator avoidance skills, ultimately reducing survival…Poor management practices outside of the riparian zone can overwhelm riparian buffering, or field drains can bypass them. To deal properly with pollutants affecting water quality requires a national change in land management practices and the use of chemicals.
The loss of floodplains and damage to peat cause yet more problems:
87% of the UK’s wetlands have been lost in the last 300 years as land has been drained to accommodate housing, industry and agriculture. The reduction of wetlands, like deforestation, peatland drainage and disconnection of floodplains, reduces the river’s capacity to deal with high and low flows events, reduces water quality, groundwater storage and the complexity and biodiversity of habitats and species….Drainage of peatlands was widely practiced in the past in an attempt to improve the land for agriculture, forestry and grouse moor management. However, it has led to significant negative impacts on rivers, by contributing to increased runoff during floods, reduced water storage for supply during droughts and reductions in water quality.
The above probably encompasses the main problems. There are others, though compared to the ones mentioned already, they seem to be relatively minor – invasive species (mink; pink salmon; the aquatic plant water crowfoot or ranunculus and others).
Then we return to the loss of woodland:
The accelerated hillslope erosion and rapid run off from the cleared land has reduced floodplain functioning and suitable stream habitat. Such drastic alterations mean the river is now more susceptible to extreme flows and temperature fluctuations. Whilst the loss of native woodland and functioning floodplains may not seem to be a direct cause of the decline in salmon stocks, it is likely they have been the first and most lasting impact on the rivers’ ability to produce salmon. A lack of trees has meant a direct reduction in habitat complexity and nutrient input and a lack of shade which can cause dangerously high water temperatures. Deforestation has reduced the resilience of the river to changes and contributed to a negative spiral in fish stocks….Climate change impacts have also been compounded by the historic impacts of deforestation and agricultural intensification. Scotland was once heavily forested and salmon have evolved to thrive in forested river catchments. Therefore, the expansion of native woodland is a major factor in giving salmon the rich, robust habitat they need….Wooded stream banks effectively reduce summer water temperatures, whilst increasing winter temperatures, making the streams more suitable for juvenile salmon. Trees also increase food supply directly through insect drop, and indirectly through leaf litter. Eventually fallen trees and branches will provide physical habitat for fish species and tree cover will reduce run off rates during heavy rain events.
And so we have come full circle. What have we learned? Mostly, I think, that if and to the extent that climate change is a problem for fish in UK rivers (and specifically in the Scottish river Dee and its tributaries), it is close to being the least of the problems. We humans have caused many issues, and the greatest of these, specifically so far as concerns high summer water temperatures, appears to have been deforestation on a fairly spectacular scale.
I venture to suggest that we have also learned something else – a BBC cut and past of a scaremongering five line press release, without investigation, validation, or context is close to misrepresentation if and insofar as its function is yet again to pump up climate change hysteria. Where is the BBC Verify team when we need it?
Footnote – the featured image comprises my first attempt to generate a picture using AI.
via Climate Scepticism
May 30, 2024 at 02:38PM
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