The eels of Waituna Creek in New Zealand are growing bigger and healthier than almost anywhere else in the country — and scientists have finally figured out why.
Waituna Lagoon sits on the South Island, recognized internationally as a Ramsar wetland site for its rich biodiversity. When the barrier beach that forms the lagoon opens to the sea — either naturally or through human excavation — tiny īnanga fish rush in from the ocean. These small native fish, which make up most of New Zealand's whitebait catch, become incredibly abundant during these periods.
This sudden flood of food supercharges the entire local food web, benefiting longfin eels living in streams upstream. Researchers studied this phenomenon over time and found that during years when the lagoon stayed open, eels in Waituna Creek grew noticeably faster than in years when it remained closed. To confirm their findings, the team analyzed otoliths — the tiny ear bones in fish that grow in rings, much like tree rings record a tree's age. The rings revealed that eels eating plenty of īnanga packed on weight more quickly.
The numbers are striking. Waituna Creek produces nearly 10 times more eel biomass per kilometer per year than streams within protected conservation areas. For an endangered species whose numbers are declining, this difference is enormous.
Growing bigger matters enormously for eels. Females must travel thousands of kilometers to spawning grounds in the equatorial Pacific Ocean. Larger eels are more likely to survive this perilous journey, and when they arrive, they produce far more offspring. A single 4-kilogram female can produce around 8 million eggs, while a female half that size produces only about 2 million. Restoring wetlands that give eels access to abundant, energy-rich food like īnanga could help the species recover across all of New Zealand.
The researchers say their findings point to a new approach for ecological restoration. Rather than focusing only on protecting a threatened species directly, conservation work might achieve better results by restoring food-producing habitats nearby. For streams surrounded by farmland — channels built for flood control rather than wildlife — adding connected wetland areas upstream could unlock benefits that spread throughout entire river systems. It's a reminder that sometimes the smartest way to help one animal is to look after the places where its food lives.