Dozens of dead fish were found along the shores of Lake Tutira by the Hawke's Bay Regional Council's science monitoring team yesterday.

"We saw potentially over 100 pan-sized trout," said Vicki Lyon, a water quality and ecology resource technician.

Prior to these latest deaths, the lake's monitoring buoy recorded surface water temperatures approaching 33C — the warmest water ever recorded in the lake.

Phycocyanin levels, which indicate the presence of cyanobacteria, peaked at around the same time. Cyanobacteria is the toxin that led to the death of 4-year-old labrador Marley after she drank from the Tukituki River last month.


The conditions are the worst recorded in the lake since the buoy was installed in 2008.

Dr Andy Hicks said algae blooms of this magnitude raise pH levels in the lake, which can also be lethal to fish.

"The buoy does not monitor pH, but our field staff measured a pH of 9.9 when they were collecting samples which could be enough to kill trout, especially when combined with such warm water."

Dr Hicks said extremely high temperature and extremely low dissolved oxygen, combined with the ongoing algal blooms, are making the lake a difficult place for fish to live, especially trout.

The council have sent water samples and dead fish to the Cawthron Institute for toxin analysis. Lake Tutira is the subject of long-term monitoring and modelling programmes by the council to find solutions to the water-quality problems that have plagued the lake for decades.

Maungaharuru-Tangitu Trust is finalising a funding application to Te Mana o Te Wai for a hapu and community-driven project to improve the water quality and health of Tutira.