Cause of fish deaths ‘unknown’
11:46 18 April 2012
The brackish water in Hickling Broad combined with a ready phosphorous content plus nitrate from agricultural run off is the ideal chemical cocktail to trigger blooms of Prymnesium Parvum alga, the latest outbreak held responsible for releasing its deadly toxin killing fish in Catfield Dyke.
A press release from Rita Penman, head of media specialists at the Environment Agency, confirmed that Prymnesium had been found in areas where fish had been found dead and in distress but at this stage she said: “We are unable to say whether or not this caused the death of the fish.”
She added: “The factors that cause the algae to multiply and cause an algae bloom are not fully understood. The presence of blooms of P.parvum does not necessarily mean the algae will produce fish deaths.
“The exact environmental conditions favouring toxic blooms are not clear, even though factors such as water temperature and salinity are helpful in predicting future blooms.
“It does appear that the most important factor influencing the toxicity of P.parvum blooms is the relative amount of nitrogen and phosphorous found in the water with toxicity increasing when both these nutrients become limited (and the cells break down).
“At present there is no evidence linking the Broads Authority’s activity in Heigham Sounds to the fish deaths on Hickling Broad. Recent weather conditions and low water levels are also factors which can contribute to an algal bloom.”
Regan Harris at the Environment Agency has made available to the Eastern Daily Press details of monthly water sampling in Hickling. She explains: “Water sampling in Hickling Broad covers a range of parameters including total phosphorous and the conductivity indication of salinity.”
In Hickling the Water Framework Directive (WFD) classification for total phosphorous is ‘moderate’ which means it fails to achieve the ‘good’ status – the WFD target that applies to all water bodies.
Total phosphorous is known to be elevated in Hickling Broad and has been shown to come from diffuse sources.
Conductivity is not classified under the WDF scheme. Levels of saline in fresh water systems are usually around 1,000 uS/cm. In Hickling the water is brackish at around 6,000 uS/cm.
According to scientific papers written by experts such as Doctor Brian Moss, former head of Environmental Sciences at the UEA, Prymnesium cannot successfully bloom without the saline/phosphorous nutrient often boosted by nitrate from agricultural run off.
Growth is limited by ammonium in the water but unfortunately this chemical content is at its lowest in April.
Sudden changes such as rapid temperature fluctuation in the water column can break down P.parvum cells and release the toxins that have taken a severe toll on Hickling Broad fish stocks on more than 15 recorded episodes since 1984.
This week Graham Gamble, senior EA fisheries officer, based in Norwich, who helped mastermind last week’s massive rescue operation, said: “We are monitoring the situation every day and it appears a vast number of fish have moved into a natural fresh water refuge in Catfield Dyke.”
Norwich Pike Anglers Club Chairman Steve Roberts has always maintained the killer algae would break out from the Heigham Sounds dredging.
“We warned from the start this operation was putting Hickling fish stocks at risk and in our view the fish kill is not a coincidence. However I would applaud the Environment Agency for its rapid response in rescuing many thousands of fish.” he declared.
Broads Authority press officer Hilary Franzen said no dredging had been carried out since last May and the only recent work involved was the completion of the Duck Broad Island situated at least two miles from the trouble spot at Catfield.
• In match fishing completing a hat-trick of three successive victories is rare, so to win four on the trot was something special for Norfolk and Suffolk veteran Mal Dobson.
He first completed a double at Topcroft and Aldeby and did it again in those venues last week.
The 67-year-old retired concrete contractor who cemented together a marvellous run among the roach and bream offered a more subdued response. “It was simply luck of the draw, I was in the right places at the right times,” he smiled.
On the carp waters fish of the week was a 35lb 4oz mirror for James Kemp of Deopham, Alan Flint of Thetford had one of 31lb 3oz and there were twenties for Mark Hartwell of Bury and Carl Kaplinsky of North Walsham.
Best at Taswood was a 34lb common for Wayne Baker of Lowestoft, the third time this specimen has visited the banks since the spring reopening.