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Massive Fish Die Off In The Whangamarino

Fuseworks Media
Fuseworks Media

The deaths of thousands of fish over the past week in the Whangamarino wetland have been caused by very low levels of dissolved oxygen in the water, The Department of Conservation [DOC] says.

Department spokesperson Kevin Hutchinson said numerous reports had come in from concerned residents and wetland users about the deaths, numbering possibly in the hundreds of thousands. While most are pests such as koi carp and catfish, native species mullet, bullies and eels have 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. 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."

Kevin Hutchinson says the decomposing plant matter started a bacterial process which depletes oxygen in the water. The warm humid weather experienced over summer has kept water temperatures and 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.

"This is further evidenced by the layer of oily scum present on the surface of the water, which are natural oils released by decomposing plants, the dark black colour of the water and the soft, wilting emergent vegetation starting to break down. The decomposition is also identifiable by an unpleasant smell, very similar to that from a compost bin."

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. 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," Kevin Hutchinson says.

"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.

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