How can a sudden growth of algae in the Thames keep a cockle fleet in the harbour? The Echo discovers how a tiny marine plant can cripple an entire industry.

On July 21, a routine test by experts from the London Port Health Authority revealed something alarming in the cockles off the Southend coast.

A toxin which can cause diarrhetic shellfish poisoning - shortened to DSP - was found in a sample of cockles.

If they were eaten by anyone, they could lead to diarrhoea and, in the interests of safety, the authority decided the only action it could take was to ban Leigh's cockle industry from catching the shellfish in the Thames estuary from July 28.

It is not the first time this has happened. The same problem occurred in 1991 but it only affected isolated areas of the Thames and not the whole estuary and surrounding Essex and Kent coastline.

This time, the ban has been imposed for 28 days but it could end before August 25 if test results show the toxins have cleared from the shellfish.

A sample which fails to show any sign of DSP is classed as a negative result. Two separate "negative results" would lead to the ban being lifted.

However, no single section of the Thames has had more than one "negative result" for the toxin and, for now, the ban will have to stay in place.

London Port Health Authority spokesman Jon Averns said: "We have stepped up our sampling and had a total of 14 positive test results for DSP. We are starting to get some negative results.

"If we can get certain areas where we get two negatives, then we could open up the fishery in stages."

The DSP problem is not just centred on the Thames.

Another 38 stretches of water around the UK have had to be closed off because of the toxin and this in turn has created a backlog of samples which need testing for DSP.

Mr Averns added: "This has posed problems for us in getting the results back."

But what has caused the toxin to get into shellfish in the first place?

The problem has been sparked by one of the simplest forms of life - algae.

The microscopic plant can grow in both fresh and salt water. Under certain conditions, there is a rapid growth in the algae - known as an algal bloom - and at this stage, the tiny plant produces weak acids.

Cockles - like mussels and oysters -- are filter feeders and they suck in seawater and strain out nutrients from it.

But they also absorb any toxins in the water which then gradually build up in the cockle's flesh.

As the cockles have been feeding on the algae, they have also absorbed the acid which has left the shellfish contaminated, giving anyone who eats them diarrhetic shellfish poisoning.

It is because of the gradual build up of the toxin in cockles which makes it perfectly safe to swim or take part in water sports off Southend.

Bathers would have to drink vast quantities of sea water to get poisoned.

Other shellfish like whelks and winkles which do not filter water for food are not affected either.

In addition, cockles caught before the algal bloom and on sale now should also be safe to eat.

Yet the ban has left cocklers reeling because it came just when the industry - which includes 11 boats based in Leigh - was given the go-ahead to work four days a week.

For years, cocklers have struggled to make a living under EU quota regulations and low cockle stocks.

Retired cockler Tony Meddle - a member of the Kent and Essex Sea Fisheries Committee - expressed the exasperation shared by many cocklers.

He said: "The sea fisheries committee has at last been listening to the cocklers and we have been given an increase in the number of days we can fish in August and September.

"Financially it will hurt them unless we get an extra month to work later this year.

"Cockling later in the year won't be as good because the yield is not as good as August and early September."

An algal bloom is normally caused by the long-hours of sunlight in the summer but many, like Mr Meddle, believe the rising water temperatures are another important factor.

He added: "Over the last few years, the water temperature hasn't fallen as it normally would.

"We hope it doesn't occur next year but, of course, if it's water warming causing this - and the earth is supposed to be warming up - who knows what's in store for the whole industry?"

All at sea - cocklers have been left high and dry by the algal growth in the river

By Charlie Mellor

Reporter's e-mail: charlie.mellor@notes.newsquest.co.uk

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