The plight of a few native freshwater fish will mean different things to different people, and it can be difficult for us to consider another person’s – or a fish’s perspective.

New Zealand’s giant kōkopu (Galaxias argenteus) population is at risk and years of planning, collaboration and some good fortune is re-establishing a population at Tāwharanui Open Sanctuary.

The first step was reversing the impact of a dam that seemed like a good idea in 1987, but nobody checked with the giant kōkopu and other native fish living in the stream.

The online dam was created for a stock water pond for the Tāwharanui farm, but it impacted negatively on the habitat and the fishes’ ability to swim up and down the stream.

The few giant kōkopu disappeared, as they have region-wide.

Giant kōkopu, more commonly known for their place in a whitebaiter’s net, live in slow-moving waterways near coastlines and can establish in land-locked wetlands.

They need overhanging vegetation, undercut banks for protection from predators and woody debris on the streambed where they hunt their prey.

Steep banks help thwart introduced predators such as cats and stoats.

In 2012, plans commenced to reintroduce the threatened freshwater fish to Waikokowai Stream, also known as Ecology Stream.

The aims were to establish a self-sustaining population and to study how the threatened fish survive in modified stream environments with fluctuating dissolved oxygen levels.

Years of riparian planting started to improve the water and provided shade for the dam’s outlet drop pipe, encouraging moss and bryophyte growth that migrating fish can climb to reach upstream.

In June 2017, about 8000 young fingerlings were released in two Tāwharanui streams and they were monitored to see how they would react to the variable stream conditions.

“Despite the streamside improvements, building a large dam over a small stream still creates challenges for aquatic life,” says Matt Bloxham senior regional advisor (freshwater) Auckland Council.  

“Particularly in the impounded part of the stream from which stock water is drawn.”

In 2019, five-year-old giant kōkopu with transponders were introduced and their whereabouts is monitored by National Institute of Water and Atmospheric Research (NIWA) receivers located throughout Waikokowai Stream.

Additional probes collect data about the dissolved oxygen levels in the water, which fluctuates throughout the day and night, and when water temperatures change.

NIWA is sent data to identify any emerging trends – observing how our fishy subjects use the stream and how the dammed section caters for fish throughout the year, says Matt.

During the recent drought many of the fish with transponders have, against all expectations, remained in the impounded section of the stream suggesting giant kōkopu display some ability to find localised life-sustaining oxygen gradients.

Land pressures, silt from dirt roads and property developments, and introduced predators all impact on our disappearing native freshwater fish, especially giant kōkopu.

Categories: Fish

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