Main photo: University of Canterbury (UC) School of Biological Sciences researcher Dr Ben Crichton with a giant kōkopu.
Te Whare Wānanga o Waitaha | University of Canterbury (UC) School of Biological Sciences researcher Dr Ben Crichton has studied kōkopu – whitebait species that can live for 10 years or more. Whitebait is the collective term for the juveniles of six species of freshwater fish. Nocturnal, territorial and competitive, the three kōkopu species have received less research attention than īnanga, which make up most of the whitebait catch.
Dr Crichton spent two years surveying Te Tai Poutini West Coast streams after dark, counting, catching and tagging the fish, often into the early hours of the morning.
“The whitebait stage is only one part of the life cycle. We wanted to understand what happens after juvenile fish move upstream, and how juvenile supply and habitat conditions together shape adult populations,” Dr Crichton says.
The research compared streams that were open to whitebaiting with streams that were closed to whitebaiting. Across eight streams, Dr Crichton surveyed 150-metre reaches every two months, tracking individual fish to better understand their survival, growth and population patterns.
The research showed that unfished streams had more juveniles migrating upstream, while adult numbers were similar across both fished and unfished streams.
“Even though more juveniles reached unfished streams, indicating that fishing was influencing juvenile availability, adult kōkopu populations were often already near the maximum capacity of the habitat. This highlighted that the availability of quality adult habitat can be just as important as the number of juveniles arriving each year,” Dr Crichton says.
UC School of Biological Sciences Professor Angus McIntosh says the work provides rare insight into a fish species many New Zealanders have never seen.
“I think if you ask most people who have eaten a meal of whitebait, they would be surprised that they could have eaten six different fish species,” Professor McIntosh says.
“Some of those tiny juveniles can grow into big, chunky adult kōkopu, 30 or 40 centimetres long. Most people don’t associate a large fish like that with whitebait.”
Dr Crichton says the research highlights important differences between whitebait species and why management cannot focus on a single life stage alone.
“Īnanga have a short life cycle, so their adult populations depend heavily on juveniles migrating upstream each year. Kōkopu live longer and need fewer juveniles to maintain their numbers; all species still rely on quality habitats to survive, grow, and spawn.
“If the management of these species only focuses on the whitebait stage and ignores adult habitats upstream, juvenile fish may swim upstream into rivers that can’t support them.”
Professor McIntosh says the findings show why managing whitebait is complex, particularly when the harvested species have different life cycles and habitat needs.
“What happens at the river mouth during the whitebait fishery is connected to what happens upstream. Managing the fishery is important, especially for īnanga, but we need to improve the availability of habitat for adult kōkopu.”
Dr Crichton and his supervisory team agree that the aim is not to stop people from catching and eating whitebait, but to encourage a better understanding of the adult fish behind the catch.
“I think people should ask themselves, in the spirit of knowing what you’re eating: what sort of fish could be in here? What do they turn into?” Professor McIntosh says.
“These are old, typically rare fish. They are the key to this wonderful harvest that people love.
“We owe it to the fish, and to that part of New Zealand lifestyle, to do a better job of understanding the fish and their needs so we can manage them better.”
UC School of Biological Sciences Distinguished Professor David Schiel praised Dr Crichton’s commitment to his research.
“It is a testament to Ben’s interest, perseverance and insights that he spent many months scouring through streams by headlamp to collect the data behind this thorough study.”
