Thousands of young trout released into Lake Tutira in September could be affected by the algal bloom now appearing prominently in the popular recreational lake north of Napier.
Hawke's Bay Regional Council said the algal bloom had been seen in the lake for a few weeks but the overall water quality was unchanged and the lake was in "the same state" it had been for the past few years.
Napier man Gary Bowling has fished at Lake Tutira for 30 years and he described it as being "in the worst possible state ever" when he visited last week.
"The lake has a thick sludge of blue, white, green, brown and blotches of red so thick that when you throw a stone into the lake, it does not splash," he said in a letter to Hawke's Bay Today.
The council and Hawke's Bay District Health Board has decided to permanently signpost the lake warning the public of the algal blooms, asking people to avoid contact with the water.
A hi-tech buoy on the lake was used to monitor the water and it showed no major change in quality overall.
Data from the buoy showed a trend for algal bloom to occur a few weeks after heavy rain had washed more nutrients into the lake.
Heavy rain over winter meant lake levels were higher than normal and many areas frequently used by campers were now under water.
The algal bloom had been reported as cyanobacteria which was potentially toxic. A sludge could form in areas when the bloom was driven by the wind and there was a definite smell which could cause health problems for people with asthma or other respiratory issues.
Fish and Game released 2750 trout into Lake Tutira a few months ago as part of an annual restocking programme.
The regional council's principal scientist for water quality and ecology Adam Uytendaal said and there was some evidence the toxins in the water could accumulate in the muscle of fish.
"So eating fish from the lake is not advised. Fish in the lake, such as the recently released trout, could be affected by the algal bloom if the bacteria breaks down and lowers oxygen levels in the water.
"The monitoring buoy data shows oxygen levels in the surface to be healthy despite the algal bloom."
The council said another factor stimulating the algal bloom could be a reduction of cold and warm layers of water in the lake.
"A natural phenomenon in the lake is tipping, whereby the layers change place during autumn/winter as the water cools, bringing nutrients up from the bottom of the lake."
The council and land owners were working to reduce sediment going into the lake by managing the land, putting in hillside and wetland plantings, which would take many years to bring about change.
An annual recreational water quality monitoring programme began last week providing people using beaches and rivers for swimming, boating and other water activities with information on water quality at 32 popular spots.
Results are available at the B4U Swim line, 0800 248 794, or the council's website.