yellow-eyed penguins on Rakiura?
In November 1999, the Yellow-eyed Penguin Trust started a survey of Yellow-eyed penguin numbers on Stewart Island (Rakiura). Yellow-eyed penguin numbers on the island have never been systematically counted and estimates have ranged between 470 and 600 breeding pairs. In an effort to provide the Yellow-eyed Penguin Species Recovery Group with a more accurate figure, the Trust started a survey of the Island.
The survey took place over three field seasons. In the 1999 season, the north-eastern coast of the island was covered, in the 2000 season the west and south-eastern coasts were covered and finally in 2001 adjacent Whenua Hou/Codfish Island was completed.
The project and the fieldwork was lead by David Blair, the projects officer for the Yellow-eyed Penguin Trust. He was joined by a number of volunteers, including Department of Conservation staff who took annual leave in order to assist. The fieldwork was also supported by the Departments Southern Islands, Stewart Island and Coastal Otago Field Centres who assisted with logistics and provided boats. Searchers were accommodated in tents and huts and in the first season on David Blairs boat Irene.
All areas of known and former yellow-eyed penguin habitat were searched, as were those areas that were considered suitable habitat. Some areas of the Stewart Island coast are cliff-bound and without suitable penguin landing sites and these areas were not searched. Suitable areas were checked for sign of yellow-eyed penguins - tracks on beaches, penguin paths and penguin poo. In some areas, the coastline was cruised along in the late afternoon when penguins are often seen standing on their landings.
Once sites had been identified the real work started. Penguin paths were followed through the bush and scrub in an effort to find nests. This was often an exersize in frustration as paths took tortous routes and often petered out into a roost site. The area had to be throughly searched and every possible path followed. Once the nest sites were found, the location was recorded using a Global Positioning System (GPS), the nest details recorded and the nest physically marked so that others in the party did not "rediscover" the nest. Sometimes nest searches were productive with several being found but on average in took 2 hours of searching time to find each nest.
A mere 79 nests were found on the Stewart Island mainland, a further 38 on adjacent islands and 61 on Whenua Hou/Codfish island. The total of 178 breeding pairs was far short of the 470-600 previously estimated. A clue as to why this figure should be so low is that 99 breeding pairs were found on offshore islands. Although these islands comprise a small percentage of available habitat they hold the majority of the breeding population. It was also observed that very few juvenile (1 year-old) penguins were seen on the mainland, where as on cat-free Whenua Hou numerous juveniles were seen.
The culprit would seem to be the domestic cat. Wild cats may be found throughout Stewart Island and have been implicated in the decline of other birdlife there. Fortunately, they are restricted to the main island and islands such as Whenua Hou remain predator-free.
Although cats may be to blame, further work is needed to rule out other factors, such as food availability. Removing wild cats from Stewart Island would outwardly seem to be a good, but difficult and expensive idea. However given that the primary diet of the cats is rats, the demise of cats may increase the rat predation problems of other vulnerable species.
What we can do is guard our islands from the arrival of predators and to carefully work out how to best protect the penguins on the mainland.
Since 2001 work has been ongoing to try and understand the Stewart Island problem. The Department of Conservation undertook a cat control operation in the northern part of Stewart Island while the Yellow-eyed Penguin Trust undertook monitoring of penguin nests. Unfortunately most of the chicks died of unknown causes before the end of the guard stage, before we expected them to be most vulnerable to cats. Several causes of mortality in young chicks have been identified including a Corynebacterium (or avian diphtheria) infection that killed 50% of South Island chicks in 2004, a Leucocytozoonblood parasite previously only known from Fiordland penguins and chick starvation. All of these are currently under study with Massey University investigating disease issues and the Yellow-eyed Pengin trust and Otago University investigating penguin foraging and diet. Follow the work here >>>
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