Clouds of emerging mayflies were once a regular sight on summer evenings and they are a key part of the food chain that supports fish, birds and mammals.
Modest levels of pollution found in many rivers are having a devastating impact on mayflies, new research suggests, killing about 80% of all eggs which are vital to all ecosystems.
A recent study found that the abundance of flying insects has plunged by 75% in 25 years, prompting warnings that the world is “on course for ecological Armageddon”, with profound impacts on human society.
The finding that even pollution well below guidelines can cause serious harm adds to concerns about plummeting insect numbers.
Paul Knight, the CEO of the Salmon and Trout Conservation (STC), which is conducted an in-depth three-year survey of English rivers, said: “The results of this groundbreaking new study are irrefutable. We believe this is just the tip of the iceberg. Lose your invertebrates and other species will follow.”
The prime suspects for this decline are fine sediment, nitrate and phosphate pollution in rivers, which are washed off farmed fields and also result from untreated sewage. Some research has been done on how the larval and adult stages of mayflies are affected by pollution, but not on their eggs.
Experiments in the laboratory found that the fine sediment settles on the eggs and suffocates them, by preventing oxygen transferring into the egg. The sediment can also allow fungus to grow and kill the eggs, while phosphate is known to affect the development of eggs.
At levels very close to existing guideline limits - 25mg per litre of fine sediment and 0.07 mg/l of phosphate - the researchers found 80% of the eggs died. Most rivers contain more phosphate than this – only 17% have “good ecological status” under EU rules.
High sediment levels are also frequently found, with 40% or rivers having more than 10mg/l – a level the new work shows is harmful to the mayfly eggs – and 10% more than 25mg/l.
Mayflies such as the blue-winged olive are a crucial component in the aquatic food chain but numbers have declined substantially in many rivers over the past 30 years.
“Their continuing loss can affect the survival of other important species such as wild fish, bird life and mammals,” he said. “This research shows even modest levels of sediment and phosphate, below current national thresholds, have a significant impact on egg survival.”
Current regulations are simply not rigorous enough to detect the extent of the problem. This latest study supports growing concern about current guidelines.
On a New Zealand perspective it’s not ‘the Silent Spring’ but now ‘The Silent Years’.
A fisherman who lived on the NW Taupo and fished the rivers of the Western Bays remembers great rafts of Manuka beetles in the sheltered bays: Waihora, Waihaha, The Chinamans, Whangamata etc.
He could recall harling on Lake Taupo 30-years ago and seeing huge rafts of manuka beetles which had landed on the lake, floating on the water and having been blown together en masse – with trout surfacing and sucking them in a dozen at a time.
He hasn’t seen even one manuka beetle in the Taupo area in years – despite fishing the rivers there on average three days a month.
Similarly, on the Tauranga-Taupo River, he used to get droves of huhu bugs belting into the windows and ‘helicoptering’ around the rooms if they got in. But again hasn’t seen a huhu bug there now in about five years. Notably also, the calls of the Moreporks sung us to sleep, but they are a rarity also now.
Interestingly another fisher spent some time at Lake Rotorua in the Nelson Lakes this Xmas, and saw no dragon or damsel flies as you would expect, read an old book about the area and it suggests an abundance of those plus mayflies. There were of course wasps and sandflies…
It makes you wonder why, 1080, or wasp predation? In these sorts of pristine areas what else could it be?
Noted the Gowan is quite heavily infested with willow… protected by a conservation order – yeah right… who is it that is managing our fisheries??
Fly hatches on our rivers are a shadow of the 1950s-60s-70s.
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