Trout key indicator of waterway health

Recent drift dive surveys of one of Wellington’s main river systems – the Hutt River – suggests the water quality in the catchment is okay, though concerns remain around low flows and water quality issues in the lower reaches, according to Fish & Game.

Fish & Game’s drift dive programme gathers data on trout numbers in key rivers to increase the knowledge of population change, and also gauge how each river system is responding to influences such as natural fluctuations in flow, or nutrient inputs from land use in the catchment, for example.

“Because trout require very good water quality to survive, they are an important indicator species for the health of our rivers, lakes and estuaries,” explains Wellington Fish & Game manager Phil Teal.

He says trout and salmon habitat is specifically referenced in the RMA, and this ensures that decision makers are obliged to require any planned activity must not adversely impact on the level of water quality needed to sustain viable trout populations.

“Trout are a well-studied fish and their water quality requirements and tolerance for pollution is relatively well understood. Looking after trout and trout habitat will generally look after habitat for most aquatic life.

“Trout and salmon in the RMA are a type of safeguard for matters of public interest – we all want rivers that are swimmable, fishable and safe for food gathering.”

This year has been a tough one for Fish & Game staff trying to survey trout numbers in the rivers around the Wellington region – while there haven’t been any large floods, there has been just enough rain, and of enough frequency, to make water clarity an issue.

Divers surveying fish populations need particularly good water clarity (4m+) to enable them to accurately count large and medium sized trout.

Some of Wellington’s rivers are surveyed on an annual basis, some every three years, and some simply cannot be dived as visibility is too poor or the river too large, such as the main stem of Manawatu River (in which case the tributary streams are surveyed).

Mr Teal says Fish & Game’s drift dive surveys carried out over the years provides one of the few long-term data sets collected on river life and the health of waterways.

The Hutt River was recently surveyed, with seven divers drifting down the river from the Akatarawa confluence (near Upper Hutt) causing a few onlookers to scratch their heads.

Preliminary analysis of results shows the Hutt River’s trout population appears to be in “pretty good shape under the circumstances”.

“I wouldn’t state that the Hutt River is a picture of health – water quality deteriorates as you go downstream and flow is restricted to a trickle at times during the year from draw-off for town water supply. This stresses all aquatic life.

“The number of medium-sized trout [1kg] observed is encouraging though, which shows good recruitment from two years ago and good retention of the fish in the population,” says Mr Teal.

“We just need the regional council to address the public’s concerns about the low flow regime if we want a truly healthy river system with what could be a magnificent trout fishery.”

ENDS

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