The big event of the month was an unprecedented fish mortality. This got extensive publicity locally. Here, I reproduce the summary of my interim report on the incident. I await further clarification from LB Merton, when they respond to my freedom of information request.
The nature and cause of a mass mortality of fish on 8th July 2026 is described. Some 100 mature Carp, a few European Eels and a few other bottom-feeding fish were killed. There have been other mass mortality incidents there in the last 25 years, in which various fish species were involved and all have occurred close to the discharge points of the lake tributaries. In contrast, this event was lake-wide, and so was not caused by a short-lived discharge from a tributary. None of the previous mass-mortalities corresponded to high temperatures, but the water temperature this time peaked at 30oC, or above. A shortage of oxygen (anoxia) was suggested by the behaviour of the fish and by similar incidents elsewhere in the country. Previous monitoring showed plenty of oxygen in the lake water, so anoxia is apparently rare here. LB Merton proposed to control Blue-green algae (cyanobacteria), which were abundant at the time, but were equally abundant earlier in the summer and in previous summers, as were heat waves. In contrast to previous heat waves, this mass mortality corresponded to record low wind speeds, so I consider that three factors came together to kill the fish: abundant cyanobacteria, high temperatures and lack of wind. I checked for other possible correlates with the die-off and found none. This work was impeded by the absence of published information on the lake, something that LB Merton could remedy readily. The ultimate cause of cyanobacterial abundance was eutrophication of the lake by phosphate, but the main source of this remains unknown. Candidates were: waterfowl defecation, excess feeding of birds and fish, arrival down the tributaries, atmospheric deposition, and disturbance of sediment on the bottom of the lake. A special study is needed to identify the principal factors. The best way to avoid a recurrence of mass mortality would be to reduce phosphate pollution, starting with the easy ways to do this and moving on to the less tractable sources. Meanwhile, LB Merton have trialled aerators and ultrasonic devices, methods without a convincing track record in lakes as large and shallow as ours. Given that oxygen comes back naturally, there is a real risk that the devices will be confirmed as the cause of a recovery that was, in fact, a natural occurrence. It is vital that LB Merton publish the studies and advice they have had, so that this event can be better understood.
I did my monthly bird count on the 16th. I was on the lookout for numbers or behaviour that were affected by the continuing drought, or the mass fish kill. The 100 Canada Geese were well above the long-term average for July, if not a record number. This was an about-turn after the low numbers in the last two years. One wonders whether the birds immigrated to the park because people feed them and their natural grassland food is desiccated in the drought. The 80 Greylags were somewhat more than expected, although not such an excess as the Canadas. Interestingly, 30 of the Greylags were out in the Great Field and making their way back to the lake before gate opening time and dog disturbance. So, they were finding some food in the grassland. As usual, there were many, now well-grown, young Greylags and a remarkably late brood of 2 recently-hatched goslings. In contrast, the 25 Egyptian Geese were around the expected number, despite their similar ecology. The resident pair of Mute Swans was still there. 10 Coots were a remarkably low count for July. Coots feed on the grassland of the golf course, which would have been poor forage in the drought, but also on waterweeds. This could indicate that waterweed growth was poor, forcing the Coots to leave for better foraging elsewhere. There were plenty of fish-feeding species: Cormorants, Grey Herons and Great-crested Grebes, an indication that the mass mortality hit the big Carp harder than it did smaller fish.
It was nice to see Wild Carrot in flower in the Revelstoke Road car park verge and a Nursery Web Spider in the Lofthouse Glade. There are many reports of the Jersey Tiger Moth on social media, and there was one on tree in Horse Close Wood. Bramble and Elder fruits were ripening. Unlike last year, the small patch of Dog's Mercury in Horse Close Wood was withering in the drought. The Lime trees in Horse Close Wood had already shed their seeds. Squirrels and Wood Pigeons will be short of food. No water was flowing down from the lake outfall, but the surface water tributary from Home Park Road suburbia had a trickle. The park gates were opened at 07:00.kkk
