Hochstetter’s Frogs Hanging in There on Aotea

JUDY GILBERT (Windy Hill Sanctuary)

The Aotea Great Barrier Hochstetter’s frogs/pepeketua (Leiopelma hochstetteri), which are genetically distinct from mainland populations(1)(2) represent the only island population for this primitive native species – they are unique to us. These frogs are nocturnal and seek refuge during the day in damp spaces underneath rocks, logs, and leaf litter. In dry weather they are typically found around stream beds and banks, although they can also be found away from streams under bush canopies. Unlike the introduced Australian Bell frogs (Ranoidea aurea), Hochstetter’s frogs are acoustically silent. Native frogs are in decline in New Zealand principally through habitat destruction and predation by introduced mammalian species. The global chytrid fungus epidemic is also thought to have impacted on New Zealand native frog populations. 

The last population on Aotea resides in Te Paparahi but until relatively recently no scientific surveys had been carried out to find out how many were there and whether they were successfully breeding. This situation changed in November 2012 and May 2015 when the area where the frogs had been seen was surveyed(3)(4). Fifteen 100 metre transect lines in stream beds were surveyed by a team consisting of herpetologists and Windy Hill Sanctuary field staff. The Windy Hill Rosalie Bay Catchment Trust, with support from Lotteries Environment and Ngāti Rehua Ngāti Wai ki Aotea, spearheaded this investigation following a full frog and lizard survey of the Windy Hill area in 20105, where no frogs were found in what is suitable habitat for them. The idea was that if there was a sufficiently healthy population in Te Paparahi a request could be made for translocation of some of these frogs back to Windy Hill so they could be re-established in a large forest area where pests have been suppressed to very low levels. Hence, a long term survey of the frogs would establish whether the population was high enough for some of the frogs to be relocated without endangering the population at Te Paparahi. 

One problem with monitoring Hochstetter’s frogs is it is very difficult to estimate actual population size because it is difficult to distinguish one individual from another, they are cryptic, and they also move between different streams over longer timeframes. Most monitoring therefore relies on an index rather than an actual estimate of abundance. 

In April 2021 the third Hochstetter’s frog survey took place(6). The Windy Hill Rosalie Bay Catchment Trust, with support from Ngāti Rehua Ngāti Wai ki Aotea, received funding from Auckland Council and the Aotea DOC to survey the same transects in the Te Paparahi Park Area that were previously surveyed in 2012 and 2015(3)(4). The aim of the 2021 Hochstetter’s frog survey Island was to check out the current health of the Te Paparahi Hochstetter’s frog population by collecting more data to add to the nine-year monitoring programme to determine whether this population was increasing, stable, or declining. Special care was taken to prevent or spread any diseases by the survey team. 

These little native frogs are about the size of your thumb nail (30-50 mm) and incredibly camouflaged so finding them is painstaking work. A total of 428 observations of Hochstetter’s frogs were made during the 2021 survey with frogs detected on 13 of the 15 transects - 87%. Of the total frog sightings, 17% were juvenile frogs. The presence of juvenile frogs indicates successful breeding or that sub-adult immigration is occurring. Based on the three surveys the population appears stable. 

Recently, a further exploratory survey by Virginia Morena found two more Hochstetters’ frog sites in the north-western area of Te Paparahi and at much lower altitudes. Knowing that the frogs are found at low altitudes on Aotea means that Windy Hill would be a suitable site for a possible translocation and the data indicates there are sufficient numbers to source some for Windy Hill. 

In terms of protection for these little critters, the recently funded Tū Mai Taonga project, which takes in the area where the frogs are found, will, in time, further protect this unique Aotea population of frogs, but ongoing monitoring will be crucial to the conservation management of this unique and vulnerable population of Hochstetter’s frogs. 

References:

1. Green D.M. (1994). Genetic and cytogenetic diversity in Hochstetter’s frog, Leiopelma hochstetteri, and its importance for conservation management. New Zealand Journal of Zoology Special Issue: Second World Congress of Herpetology 21: 417-424. 

2. Fouquet A., Green D.M., Waldman B., Bowsher J.H., McBride K.P. and Gemmell N.J. (2010). Phylogeography of Leiopelma hochstetteri reveals strong genetic structure and suggests new conservation priorities. Conser-vation Genetics 11: 907-919. 

3. Herbert S., Melzer S., Gilbert J., and Jamieson H. (2014). Relative abundance and habitat use of Hochstetter’s frog (Leiopelma hochstetteri) in northern Great Barrier Island: a snapshot from 2012. BioGecko 2: 12-21. 

4. Herbert S. and Gilbert J. (2015). Hochstetter’s frog population health surveying, Te Paparahi, Aotea/Great Barrier Island, April-May 2015. Unpublished technical report prepared in November 2015 by EcoGecko Con-sultants Ltd, Wellington, and the Windy Hill Rosalie Bay Catchment Trust, Great Barrier Island. 

5. Bell T.S, Melzer S. and Herbert S. (2010). Herpetofaunal survey report and management plan for Windy Hill Rosalie Bay Catchment Trust Sanctuary, Great Barrier Island, New Zealand. Unpublished report, pp 103. 

6. Herbert S. and Sagar J (2021). Hochstetter frog population health survey, Te Paparahi, Aotea/Great Barrier Island, April 2021. Unpublished technical report (No. 5572) prepared in June 2021 by Wild Lands for Windy Hill and Rosalie Bay Catchment Trust.