For two years, Dr. Ben Crichton surveyed New Zealand's West Coast streams after dark with a headlamp, wading through water and catching fish by hand into the early hours of the morning. What he discovered transforms how we understand whitebait—a delicacy most New Zealanders know only as tiny, silver juveniles on their dinner plate.
The research reveals that those small whitebait can grow into chunky adult kōkopu 30 to 40 centimeters long, living for more than a decade. Yet most people who have eaten whitebait would be astonished to learn they may have consumed six different fish species in a single meal, each with entirely different life cycles and habitat needs. This gap between what people eat and what they know about their food matters profoundly for how New Zealand manages one of its most iconic fisheries.
Working across eight streams on Te Tai Poutini West Coast, Dr. Crichton from the University of Canterbury School of Biological Sciences surveyed 150-meter reaches every two months, tagging individual fish to track their survival, growth, and population patterns. He compared streams open to whitebaiting with protected streams closed to fishing. The patterns that emerged challenged conventional thinking about the fishery. Unfished streams had more juveniles migrating upstream, yet adult kōkopu numbers were similar in both fished and unfished streams. This suggested something unexpected: habitat capacity matters as much as juvenile supply.
"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," Dr. Crichton explains. The implication is stark. Juvenile fish may swim upstream into rivers that cannot support them if management focuses only on the whitebait stage and ignores the quality of adult habitats upstream.
The three kōkopu species have received far less research attention than īnanga, which dominate the whitebait catch. Yet their differences are crucial. Īnanga have short life cycles, so their populations depend heavily on annual juvenile migrations. Kōkopu live longer and need fewer juveniles to maintain their numbers. Both rely on quality habitats to survive, grow, and spawn—a seemingly simple point with profound implications for policy.
Professor Angus McIntosh, from the same university, says the work provides rare insight into a species most New Zealanders have never seen as adults. "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. 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 and his team emphasize they are not calling for an end to the whitebait fishery—a practice woven into New Zealand's culture and lifestyle. Instead, they are asking people to pause and wonder: What sort of fish could be in here? What do they turn into? The research shows that managing whitebait is far more complex than numbers at the river mouth suggest. What happens downstream is intimately connected to what happens upstream. These are old, typically rare fish. Understanding them better, and improving the availability of habitat for adult kōkopu, is the key to sustaining both the wonderful harvest and the fish that make it possible.