The Environment Agency is continuing its investigations into the unusually high number of carp mortalities being reported for this time of the year.

“Overall, we have seen fewer carp mortalities in 2007 than in previous years. However, an unusually high number in August and September have been investigated by the Environment Agency’s Fish Health, Ageing and Species Team based at Brampton,” explained Environment Agency Fisheries Advisor Nigel Hewlett.

“We have seen large carp dying in a number of waters in the Midlands and Thames regions. As well as investigating individual cases, we are gathering information that allows us to look for common causes. However, our investigations have so far revealed a number of possible factors, either acting alone or in combination. We have not detected a single common cause”

Some of the possible causes are:-

Notifiable disease: DEFRA has imposed restrictions on six waters due to the notifiable diseases Spring Viraemia of Carp (one case) and Koi Herpesvirus (five cases). Other suspected cases of KHV are being investigated and this figure is likely to rise, although some suspected cases have already proved negative for this disease.

Flooding: Some fisheries have lost small numbers of large carp following this summer’s flooding. Very large and rapid changes in water quality caused by the floods may be the cause. In many cases only dead fish were found. No live sick fish have been seen, making it more difficult to determine the cause. 

Algae: There have been some reports of changes in water colour, notably to a tea-like colour, before losses occur. This is caused by changes in the algae in the water and is unlikely to kill specific species of fish unless individuals are already debilitated. Algal blooms can cause fish deaths, but they are usually caused by other actions that cause disturbance of the lake beds or margins. Recent flooding could have caused this problem.

Stress: The carp in our waters have faced, like the rest of us, a very unusual summer. Many carp may have started to reach spawning conditions in the very warm April we had. They then faced cooler conditions, flooding or rain until mid-August. This could have led to stress, which increases the carp’s susceptibility to naturally-occurring diseases.

Poor fisheries management: It is not a palatable message to put across but many of the cases we have investigated this year were preventable. Losses caused by overstocking, poor habitat or inappropriate stocking can all be controlled by fisheries managers.

“Unfortunately, we are aware that not all cases have been reported,” confirmed Nigel Hewlett.

“It is very important fisheries owners report problems to us. Our investigations can help prevent future problems and help us study the wider picture of losses to find trends and common causes. All of the cases we investigate receive the same prompt and high standard of examination. This not only provides samples for examination but also gives the fisheries owners the opportunity to review fisheries management practices and reduce or prevent future losses.”