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Helping the little Bignose grow

Since 2015 Environment Canterbury’s water quality, ecology and biodiversity teams have been investigating the effects of stock exclusion on Bignose galaxiids (Galaxias macronasus).

These tiny fish are only found in small spring-fed streams in the Upper Waitaki catchment in the Mackenzie Basin. They are classified as ‘nationally vulnerable’ due in part to their extremely confined distribution and extensive loss of habitat.

In 2016, we funded the fencing of several spring-fed streams that contain Bignose populations on Grays Hill Station. This is part of a five-year study comparing fenced with unfenced sites to investigate the impact of stock grazing on populations and stream habitat.

We hope that the information gathered in this ongoing study will help us understand how to effectively manage the habitat of these unique fish.

Example of a fenced stream with abundant aquatic plant growth

The Bignose only grows up to 90mm long and belongs to the same genus as other native whitebait such as the īnanga, but it doesn’t migrate to sea. Initial surveys from 2015/16 using electric fishing techniques dramatically expanded the known distribution of the Bignose, but also raised key questions about its habitat and the effects of high country land use on fish populations.

Left: Surface water science technician Roland Eveleens taking some in-stream measurements. Right: The team retrieving fyke nets in October 2018. In stream plant growth is in its early stages and should reach thigh-height by January.

Our studies have shown some negative implications when stock are excluded from grazing in the stream. One issue is that it leads to dense growth of instream plants which raises water levels and can allow predators of the Bignose, such as juvenile trout and eels, near the headwaters. Furthermore, these plants die from frost in winter and subsequently rot in the water, which potentially changes the water chemistry. This is thought to be detrimental to the survival of the fish, but we don’t yet know to what extent.

Senior surface water scientist Duncan Gray holds a mature adult Bignose which can reach up to 90mm in length

Riparian planting helps reduce the growth of instream plants by shading the stream, but it’s a slow process and must be considered in the context of the Mackenzie Basin landscape.

Another issue is how hard the Bignoses are to trap and monitor. Traditional techniques using electric fishing machines or traps were not doing the job well enough. So we undertook a catch efficacy study which found that fyke nets (see photo) were far more efficient at catching these tiny fish who love to hang out deep in the substrate of the stream bed. Once we started using fyke nets we found hundreds of fish near spring-heads where we’d previously only caught or witnessed dozens.

In summer 2016/17, we measured and tagged some fish using safe fluorescent dyes in a unique colour pattern, so we could identify each individual fish. This allows us to measure growth rates and determine the relative size of the population by looking at the number of fish being recaptured. We now believe that Bignose populations are much larger than we initially thought, but that growth rates for adults may be very slow.

A custom-made double-ended fyke net used for capturing fish in very shallow water. Nets are left overnight, and fish are counted and measured in the morning.

Since the sites were fenced, we’ve completed multiple fishing and habitat surveys. We visit the sites every year and continue to discover new and interesting things about the little fish and where they live.

By understanding the impacts of stock fencing on this species we hope to manage their habitat in a way that could help them come back from the brink of extinction.

Credits:

Words by surface water scientists Jarred Arthur and Duncan Gray, photos by Jarred Arthur

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