Wetlands Provide Critical Food Supply Boosting New Zealand’s Endangered Longfin Eels

Researchers examining Waituna Lagoon in the South Island found that the wetland’s periodic opening to the sea dramatically increases the availability of inanga—small migratory fish that dominate whitebait catches. When the lagoon remains open for weeks to months, inanga surge upstream, creating a “natural experiment” that supplies abundant prey for longfin eels (Anguilla dieffenbachii) living in connected creeks. The study shows that eel growth rates and overall biomass are closely tied to these fluctuating food supplies.

The Conversation

Waituna Lagoon is bounded by a barrier beach that can breach naturally or through human-made excavation. Once breached, the lagoon stays open until large swells close it again, a cycle that can last from a month to a year. During open periods, inanga migrate freely from the ocean, dramatically enriching the downstream food web. In closed years, inanga are scarce, and eel growth slows.

The Conversation

Analysis of otoliths (fish ear bones) from local iwi indicated that eels in Waituna Creek grew faster during open years. The creek now produces almost ten times the eel biomass per kilometre per year compared with streams inside the conservation estate. After riparian planting and the addition of woody habitat, longfin eel biomass more than doubled within two years—an increase attributed to the sustained inanga supply. Larger eels also have outsized reproductive value: a four-kilogram female can release roughly eight million eggs, whereas a two-kilogram female yields about two million.

The Conversation

The findings suggest that protecting and creating wetlands can serve as “food baskets” that amplify eel recovery beyond the immediate site. By enhancing energy-dense prey availability, wetlands help more females reach the spawning grounds in the equatorial Pacific, potentially boosting juvenile returns to New Zealand. This ecosystem-based approach contrasts with traditional strategies that focus solely on shielding eels from fishing within protected areas.

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New Zealand’s “Jobs for Nature” programme has recently restored over 6,000 hectares of wetland, aiming to reverse the loss of roughly 90 % of the country’s wetlands—a decline that historically coincided with sharp drops in native freshwater fish populations. Researchers anticipate that such large-scale wetland creation will extend the food-web benefits demonstrated at Waituna Lagoon to other catchments, further supporting longfin eel recovery nationwide.

The Conversation