Thousands of fish have died in waterways across England over past few days in the lead up to tomorrow's opening day of the coarse fishing season, with freshwater species failing to cope with plunging oxygen levels in the hot weather.
The Environment Agency deployed aerating machines on four separate watercourses in the North-west, Anglia and the Midlands in a bid to reduce fish fatalities, and is calling on England and Wales 1.25 million licensed anglers to alert the Agency if they see fish in distress this summer.
More than 5,000 dead fish have surfaced so far in Cuckoo's Hollow, a lake near Peterborough; 3,000 were killed on a commercial coarse fishery in Worcester; Captains Pit in Wirral experienced over 1,500 fatalities; and Old Bedford counter drain in Cambridgeshire had over 1,000 fish.
"In hot weather, fast growing freshwater algae increases its activity which can exhaust much of the dissolved oxygen in streams, ponds and lakes, depriving fish species of the oxygen they need to survive," said Head of Fisheries at the Environment Agency, Dafydd Evans.
"During daylight hours when photosynthesis proceeds rapidly, plants and algae release surplus oxygen and dissolved oxygen concentration in the water may be around 100 percent. However, at night photosynthesis reverses and algae become a net consumer of oxygen leading to severe depletion of dissolved oxygen concentration."
Most fish kills occur during the early morning hours when dissolved oxygen is at its lowest. With Cuckoo's Hollow in East Anglia dropping to just five percent oxygen concentration, where normally it resides between 80 and 100 percent.
In addition, rapid changes in weather conditions from hot days to rain storms can also cause fish kills. Pollutants and run-off result in poor water quality, while cloudy, cooler weather can cause algal blooms to die, with the composing plant-life releasing vast amounts of carbon dioxide into the water.
Mr Evans said drought conditions in the South-east may also make waterways in that region more susceptible to fish kills this summer.
"The combination of hot weather and low water levels can make fish distressed and lead to death," he said.
"It's usually ok to fish during a drought, but anglers should pay careful attention to how the fish are behaving.
"This might mean fish staying in one place near the surface of the water, gasping for air; swimming very slowly in large groups, and obvious over-crowding. Dead fish are easy to spot, as they float on the surface of the water.
"Fish are a vital part of the ecology of our rivers, streams, lakes and ponds, and their health affects other wildlife. Dead or dying fish can pollute water and the fish themselves experience distress."
Mr Evans also urged managers of commercial fisheries to be proactive by planning for hot weather during summer.
"When fish become distressed during hot periods, Environment Agency resources can often be stretched between a number of waterways."
Cuckoo's Hollow in Werrington, Peterborough, saw dissolved oxygen levels drop to five percent, resulting in over 5,000 fish mortalities â€“ including roach, pike eels and other coarse species.
Old Bedford near Welney in Cambridgeshire saw over 1,000 fish mortalities â€“ including roach, perch, and tench. Aeration work by Environment Agency raised the dissolved oxygen levels to between 50-70%.
Captains Pit at Wallasey near Wirral, saw dissolved oxygen levels drop to between 11-18% as the water temperature reached twenty-seven degrees, resulting in over 1,600 fish mortalities. â€“ including roach, perch, and tench.
The Coarse fishing closed season runs from March 15 â€“ June 15 during fish spawning times and applies to all rivers, streams and drains in England & Wales, but does not apply to most still waters.
If you see dead, dying or distressed fish, call 0800 80 70 60.