Death of rare frogs linked to Brynderwyns roading work

One of 153 Hochstetter's frogs moved out of harm's way by ecologists during this year's roading recovery work to the Brynderwyn Hills leg of State Highway 1.

Experts say the deaths of two rare Hochstetter’s frogs during this year’s roading recovery work in the Brynderwyns were likely due to changes in water acidity caused by a newly installed concrete culvert.

An examination of the frogs by an Auckland Zoo vet was inconclusive but further studies by the national veterinary laboratory Gribbles Laboratory found “both frogs exhibited [acute] skin cell degenerations that were consistent with trauma potentially caused by pH change or a chemical trauma”.

The discovery and probable cause of the dead frogs was detailed in a recent ecological report released online by NZ Transport Agency Waka Kotahi (NZTA) after the completion of its joint venture with Fulton Hogan to restore the cyclone-damaged leg of State Highway 1, which saw the road closed for four months earlier this year.

The Brynderwyns Recovery Hochstetter’s Frog Compliance Report, compiled by WSP consultancy, was part of a wider ecological compliance report, due for release soon, which NZTA was required to produce as per the resource consent granted for the road work.

Consent conditions required mandatory surveys and salvage of at-risk and declining species, such as the frogs. Ecologists were onsite throughout the project, their findings and other relevant information contained in the reports.

Extensive work done with heavy equipment this year to make the Brynderwyn Hills route into Northland more resilient, included consent conditions that required ecologists onsite to survey and shift at risk and declining species, such as Hochstetter’s frogs. Photo / Supplied

According to the frog report, ecologists found 153 Hochstetter’s frogs during a baseline survey before the road works began. They ultimately spent 327 hours during the road’s closure between February and June salvaging 150 frogs within 744ms of impacted streams. The relocation area was out of the impact zone but in the same general area as the project footprint. Exact locations were redacted in the publicised report.

One of the frogs that died was discovered dead in a pool above the mouth of a culvert that had been “relivened” with water previously diverted away, on the evening of March 13. A second frog was found there the following morning in a distressed state and died later despite efforts to flush it with fresh water from upstream.

Because of the “extreme sensitivity” of Hochstetter’s frogs, ecologists suspected the two dead frogs had been affected by water chemically altered during the curing of the concrete culvert.

“The other possibilities that we can think of are injury or natural causes, but there were no obvious signs of injury and natural causes would be quite a coincidence, given that these were the only two frogs that were in the pool,” the ecologists said.

“The construction area had been thoroughly searched for frogs, but we either missed a handful of frogs, or (more likely) frogs moved down from adjacent habitat into the construction area,” they said.

In the course of a gully having been pumped dry for the installation of culverts, the over-pumping dam started leaking and a pool formed in contact with the newly laid concrete, just upstream of the new culvert. The new concrete leached hydroxyl ions into the pool, increasing the pH.

“We think that a large part of the problem was that the water in the pool was in contact with the fresh concrete for many hours, and the pH change should be much smaller if we have proper flow through the culvert,” the ecologists said.

Each of the frogs located during the Brynderwyn's roading work were examined and given a reference number for an ecological compliance report. Photo / supplied

According to an ecological compliance report, water that leaked from an over-pumping dam into freshly installed culverts turned too alkaline for native frogs to survive and two died.

The culvert was quickly taken back offline and water diverted to reduce further potential impacts around the culvert mouth as well as downstream.

Spot measurements of residual water where the frogs died showed an upstream baseline pH (the norm’ for that environment) that was close to neutral (7.3). The small pool that formed at the mouth of the culvert following the over-pumping dam leak was at 9.3 and the water in the offline culvert was 9.8.

The report said that after discovering the change of pH levels, precautionary steps were taken to prevent any further harm to frogs and all other newly set concrete structures remained offline until a protocol (detailed in the report) was developed.

Daily measurements were taken to understand the time frame needed for the pH levels to return to neutral following offline flushes of the new culvert. A water cart was used to isolate contaminated water to be treated offline.

Ten days after the spike in pH levels, the upstream pH was 7.3, the pool formed was 7.4 and the water in the culvert had dropped down to about 7.5. The culvert was subsequently re-livened.

No further dead Hochstetter’s frogs were found at any subsequent culvert re-livenings.

As a further part of the consent process, NZTA was to make a financial contribution of between $100,000 and $200,000 towards a project that increased the knowledge and understanding of Hochstetter’s frogs. This week, project director Mark Ware said the payments were yet to be made as the transport agency was “still working through what research might be undertaken”.

Ware also confirmed iwi and hapu had been informed about ecological issues as they arose during the project, saying communications with the relevant parties had been “prompt”.

He said an earlier report from roading staff to media that 600 frogs had been discovered was incorrect.

Sarah Curtis is a news reporter for the Northern Advocate focusing on a wide range of issues. She has nearly 20 years experience in journalism, much of which she spent court reporting. She is passionate about covering stories that make a difference.