29 January 2016

Lethal conditions for fish in Tūtira Lake

More dead fish have been found at Lake Tūtira by HBRC’s science monitoring team this week.

“We saw at least dozens, and potentially over a hundred pan-sized trout along the shores of the lake today,” said Vicki Lyon, a Water Quality and Ecology Resource Technician who was on site.

Prior to the latest fish kills, HBRC’s lake monitoring buoy recorded surface water temperatures approaching 33oC. This is the warmest water ever recorded in the lake. Phycocyanin levels, which indicate the presence of cyanobacteria, peaked at around the same time.
“Algae blooms of this magnitude raise pH levels in the lake, which can also be lethal to fish” said Dr Andy Hicks. “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.”

In the last 24 hours, the lake has also experienced another dip in dissolved oxygen levels, which was associated with a large decline in both temperature and phycocyanin levels.

“The buoy provides very useful data to help explain the fish kills” says Dr Hicks.

“The start of 2016 has been unusually harsh. Extremely high temperature and extremely low dissolved oxygen, plus ongoing algal blooms, are making the lake a very difficult place for fish to live, especially trout, which are quite sensitive to such things”.
HBRC has sent water samples and dead fish to the Cawthron Institute for toxin analysis. Fish are not known to be affected by cyanobacterial toxins, but HBRC is getting samples analysed to be sure.

“We have not seen conditions like this in the lake since the buoy was installed in 2008. It’s not the typical environmental conditions in the lake for summer. Although the algal blooms are not unusual, we have not seen fish deaths at large scale during previous blooms,” said Dr Andy Hicks.

Lake Tūtira is the subject of long-term monitoring and modelling programmes by HBRC to find solutions to the water quality problems which have plagued the lake for decades. Maungaharuru-Tangitū Trust are finalising a funding application to Te Mana o Te Wai for a hapū and community driven project to improve the water quality and health of Tūtira.

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