Endangered terns winning fight against predators

Nationally endangered native black-fronted terns have been handed a big advantage when it comes to the ongoing battle against braided river predators on the Clarence River | Waiau Toa.

Black-fronted tern

Black-fronted tern

Aerial weed control will take place in April after aerial surveys of nesting islands in February indicated weeds were growing back. Thereby reducing the viability of nesting sites monitored as part of a partnership project between us and the Department of Conservation focused on the black-fronted tern restoration.

In an effort to curb the predator population, traps will also be laid every 100m or so along the banks of the river. The aim is to increase the nesting success of the terns.

The riverbed has been degraded throughout its catchment by mammalian predators and weeds which threaten the braided river habitat. In the breeding season, terns pick braided river islands free of woody vegetation to nest, which the likes of broom and gorse can rapidly colonise.

This reduces available nesting habitats and forces terns into less desirable locations where they are more vulnerable to predators and flooding.

What predators are the terns up against?

Hedgehogs, stoats, weasels, cats, rats, mice, possums and the Southern Black-backed Gull | Karoro have all been targeted with the trapping strategy due to their hunger for eggs, chicks and occasionally adult birds.

Karoro are significant predators of birds that depend on braided rivers for breeding grounds and food, such as the black-fronted tern. The terns are endemic to New Zealand and currently have the ‘threatened – nationally endangered’ conservation status.

In 2016, the Kaikōura Water Zone Committee committed $250,000 of Immediate Steps biodiversity funding over a period of five years to protecting indigenous biodiversity in the Clarence | Waiau Toa catchment through ongoing weed control and pest management.

Our Kaikōura land management and biodiversity officer, Heath Melville, said it is important to be providing the native terns a hand up.

“We’ve seen huge benefits to trapping and habitat enhancement over the course of the study,” he said.

“Above average river flows in October created many natural islands and cleared vegetation to the benefit of the birds. But unfortunately, high flows followed, leading to islands being flooded and only 14 chicks fledging this season compared to 142 chicks the season before,” Heath said.

A long project, but a successful one

Aerial view Clarence River - Waiau Toa

Aerial view Clarence River | Waiau Toa

Work over the five-year programme so far has included helicopter control of broom, gorse and hawthorn; control of broom on private and conservation land; ground-based weed control; and the five-year predator control programme.

The Clarence/Waiau Toa is Canterbury’s longest river, running for 209km through its large, rugged catchment. Management is shared between us and Marlborough District Council.

Improving the Clarence/Waiau-toa River ecosystem by controlling weeds is a major focus of the committee’s Kaikōura Zone Implementation Programme.

The zone committee has long supported an integrated weed management strategy for long-term control in the catchment.

Positive results to date:

  • The average number of chicks per nest on both managed and unmanaged black-fronted tern colonies has increased since the project began.
  • In the 2015/16 breeding season, only one chick in 10 nests monitored survived to fledgling stage on the managed colonies. In 2017/18, that number had risen to almost six chicks per 10 nests monitored.
  • In the unmanaged colonies, the average number of chicks per nest has increased from less than one chick per 10 nests in 2015/16 to about 2 chicks per 10 nests this season.