The deaths of thousands of fish over the past week in Waikato's Whangamarino wetland have been caused by very low levels of dissolved oxygen in the water, says the Department of Conservation (DOC).
Spokesman Kevin Hutchinson said numerous reports had come in from people living in the area, near Te Kauwhata, and wetland users about the deaths, numbering possibly in the hundreds of thousands.
While most were pests such as koi carp and catfish, native species mullet, bullies and eels had also been found dead.
"The drought at the end of 2010 exposed large areas of the wetland and rapid plant growth occurred in areas usually under water," Mr Hutchinson said.
"High rainfall in January compounded by the baked dry ground in the catchment meant water rapidly ran off into the wetland and water levels remained consistently high for about three consecutive weeks."
The decomposing plant matter started a bacterial process which depleted oxygen in the water. The warm humid weather experienced over summer had kept water temperatures high, thus enhancing bacterial growth.
Tests conducted with an oxygen meter by DOC rangers yesterday confirmed the very low levels of oxygen present in the water.
Water levels in the wetlands were kept higher than natural during the recent big rain events by the operation of established regional flood control measures, Mr Hutchinson said.
The gate letting water from the wetlands to the Waikato River was closed in order to reduce the risk of flooding of farmlands around the wetlands.
"Things are unlikely to improve until either the water level drops right down exposing the wetland floor or we get another significant rainfall event flushing the water through the wetland system," Mr Hutchinson said.
"This event has a positive side with the removal of a large amount of pest fish biomass. However, large scale fish diebacks have been known to cause negative impacts on some bird populations, for example shags and heron. A similar event occurred in summer 2004."
