One of the UK's foremost authorities on fish and fishery management, Dr Bruno Broughton, has warned of the extreme danger that the current hot, stormy weather is posing to the nation's fisheries.
With several high-profile fisheries having closed over recent days, it is thought that the current conditions have already started to take casualties.
Bruno explained: "The premature mortality of freshwater fish during the hot summer weather is a common phenomenon. The hotter the weather, the longer it lasts and the larger the fish stocks, the greater the potential risk.
"At the heart of the problem is the growth of large quantities of water plants where there are ample supplies of nutrients – especially phosphates – and long periods of sunshine. In these conditions, the rooted vegetation or the suspended algae which stain the water green or green-brown become prolific. During the day, they photosynthesise by using carbon dioxide and giving off oxygen, which dissolves in the water.
"However, this process ceases after dark and the plants behave like animals by respiring… using up oxygen and releasing carbon dioxide. When the plant life becomes very abundant, the night-time fall in dissolved oxygen can drop below that required by fish, which can suffocate as a result. The problem is exacerbated by the warm-water conditions because the warmer the water, the less oxygen it will be able to hold.
"Clearly, if this process is likely, some form of oxygenation of the water may be required to help the fish during the danger period. This oxygenation needs to take place AFTER DARK because the lowest dissolved oxygen concentrations under these conditions will be during the night and at their lowest ebb at the end of the night (ie just before dawn).
"The onset of thundery weather and intense rainfall are red warning periods for fishery owners. The sudden influx of cooler water (as heavy rain) into warm fisheries causes the water column to become unstable, with warmer water rising rapidly on top of the cooler water. The consequence is that the water column flips upside down, with warmer water rising to the surface and, often, bringing with it sediment lying on the lake bed. This can cause the water to turn dark brown or black in colour.
"This colour prevents light penetration and starves the plants of sunlight, causing them to die. Where there are dense algal blooms, for example, the algae then sink to the lake bed in huge numbers and become a new food source for bacteria, which multiply rapidly and quickly strip oxygen from the water.
"This can cause sudden and almost complete deoxygenation. Fish will try to overcome this by gasping at the water surface, and they may do so en masse, but this offers only temporary relief. Within a short period of time, deaths can occur and large or complete fish mortalities can ensue.
"It is therefore very important that fishery owners prepare an emergency plan that can be enacted at once if problems occur. There will usually be insufficient time once fish begin gasping to discover from where pumps can be hired or how they can be powered.
"I helped write an Environment Agency booklet on this subject (Deoxygenation – Practical Self-Help For Fishery Owners & Managers) which explains how to prepare an emergency plan and enact it if an oxygen ‘crash’ takes place. The guide provides lots of useful advice and tips which should help prevent disastrous summer fish kills.
"The guide is available as a download from the Environment Agency website. Alternatively, it can be viewed and downloaded from my website at:
"Many summer fish kills are avoidable with proper planning and rapid action. It is essential that a contingency plan is ready to go in case the worst happens; sadly, it will almost certainly prove to be too late if you have to start from scratch once fish are in trouble."
For further, comprehensive information on how to improve the management of your fishery, and advice on most fishery-related matters, contact Bruno using the information below:
Dr Bruno Broughton B.Sc. (Hons), Ph.D., F.I.F.M., C.Env.
Lower Bromstead Lane
Shropshire TF10 9DQ
Tel: 01952 691515