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Stopping whitebait from frittering away
With the whitebait season in full swing until 30 November in most parts of the country, scientists from the Marine Ecology Research Group at the University of Canterbury and from the University of California are studying how to make sure there are enough of the delicacy in our streams and rivers.
Whitebait are a variety of native fish species that spend six months at sea and then make their way up rivers and streams. The study will take four years, and the researchers will investigate where our most common whitebait (īnanga, or Galaxias maculates) lay their eggs and the issues that threaten their spawning habitats.
The researchers have already discovered that whitebait favour very specific sites for spawning (laying their eggs), with dense riverbank vegetation and very damp conditions that are increasingly difficult to find along the banks of many of the waterways being studied.
One of the researchers, Dr Mike Hickford, says that where whitebait lay their eggs is much more specific than just amongst bank vegetation in the tidal areas of streams.
“What’s important is what is happening underneath the vegetation. The īnanga spawn where grasses clump together and form a dense root mat and where there is almost 100 percent humidity – conditions that are as close to being under water as you can get out of the water. It’s not uncommon for different generations of fish to choose the same clump of grass for egg laying.”
The vegetation provides protection for the eggs, which are laid on riverbanks on autumn high tides and stay there until the next high tide when the larvae hatch and are washed down the river and out to sea. They remain in the ocean over winter, returning to freshwater locations in spring, when whitebaiters are ready with their nets for them.
The whitebait season in most of New Zealand runs until 30 November. On the West Coast, the season is until 14 November, and on the Chatham Islands, the season is from 1 December to the end of February.
It is estimated that over 99.5 percent of the larvae die between hatching and returning from the sea as whitebait, highlighting the importance of ensuring plentiful spawning sites if populations are to be sustained.
Dr Hickford says factors threatening existing spawning habitat include flooding, sedimentation, grazing livestock, bank disturbance and deforestation.
Large rivers, such as the Buller on the West Coast, are often popular whitebaiting spots but they actually have very small spawning areas because the desirable habitat has been destroyed through flood control measures and the impact of urbanisation.
“By and large, it is probably smaller streams that are more vital for the health of the population as their spawning habitat tends to be more intact.”
The researchers are also trying to establish how many ideal spawning sites are needed to protect and grow whitebait stocks.
Another strand of the research involves using techniques developed at the University of California to determine important spawning sites, by detecting and analysing chemical traces in the otoliths (ear bones) of whitebait and tracing those to the river or stream where the larvae hatched.
The team is experimenting with enclosing trial areas on riverbanks and streambanks on the West Coast and in Canterbury to see if they can fast-track the process of establishing optimum spawning conditions.
In other experiments, riverbanks have been modified, grazing stock removed and grass left to grow rather than being mowed to help re-establish natural conditions. It’s hoped that whole sections of streams known to be important whitebait spawning areas will be fenced off in the future.
