Biota of Seven Islets Off Waiheke Island, Inner Hauraki Gulf
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Tane 37: 99-136 (1999) BIOTA OF SEVEN ISLETS OFF WAIHEKE ISLAND, INNER HAURAKI GULF Michael Lee 29 Omiha Road, Omiha, Waiheke Island "He iti ra, he iti mapihi pounamu " - (although small - very precious) Te Hikitai SUMMARY Seven islets smaller than 1.0 ha lying off Waiheke Island in the inner Hauraki Gulf were biologically surveyed in 1996-97. Koi Islet is an important breeding site for red-billed gulls (Larus novaehollandiae), at least seven other species of seabirds and shorebirds, including reef heron (Egretta sacra), Caspian tern (Sterna caspia) and variable oystercatcher (Haemotopus unicolor). More than 250 birds were present over the summer season. Another islet Papakohatu, has a small coastal forest and is a breeding site for seven species of seabirds though in lesser numbers than on Koi. Lying 1200 m from the nearest land Papakohatu potentially has a significant conservation value. Reef heron also breed on Papakohatu and Passage Rock. Nani Islet is an important breeding site for white-fronted tern (Sterna striata). New Zealand dotterel (Charadrius obscurus) regularly breed on Kahakaha (Frenchmans Cap). Two islets Motukaha and Te Whau contain important Maori archaeological sites. What appears to be the old Maori name (Te Takapu) has been rediscovered for Passage Rock. Papakohatu and Motukaha are badly infested with rhamnus (Rhamnus alaternus) an aggressive exotic tree, which is probably reducing species diversity. These islets may form an invasion route for rhamnus from Motuihe to Waiheke Island. Rhamnus also has a foothold on Nani. Ecological values on these islets have been compromised probably by rodents which are likely to have destroyed original lizard populations, disrupted petrel breeding, suppressed or eliminated larger invertebrates, inhibited plant regeneration and predated bird nests. Ship rats (Rattus rattus) were present on Koi in very high numbers (equivalent to >78 per ha) as well as on Motukaha and Te Whau. Mice (Mus musculus) were present on Papakohatu. Nani and Passage Rock were found to be rodent-free. However, absence of lizards possibly indicates earlier habitation by rodents. Kahakaha is in a similar category - although rat poison bait stations were placed there by Rotoroa Island residents in 1993. Rodents were removed from islets where encountered except for Te Whau which is privately owned. Ownership of the other islets is legally defined 99 as "uninvestigated" (probably customary Maori land) but Kahakaha is owned by the Crown. Apart from invasive animal and plant pests, the immediate danger to the ecological values of these islets is disturbance of nesting birds by humans and their dogs during the summer breeding season. Also see Appendix 1. Keywords: Hauraki Gulf; Waiheke; islets; Koi; Papakohatu (Crusoe); Motukaha; Passage Rock (Takapu); Nani; 'Te Whau'; Kahakaha (Frenchmans Cap); birds; reef heron, Caspian tern, red-billed gull, white-fronted tern, New Zealand dotterel; vascular plants; rhamnus; rodents; archaeological sites. INTRODUCTION The Hauraki Gulf is one of New Zealand's most important regions for nature conservation. Its many islands large and small are set in an extensive body of shallow water, some 5000 sq km, which is plankton rich, warmed by the sub• tropical East Auckland current and fed by streams and coastal estuaries which drain a catchment extending over 100 km inland almost to Rotorua. This benign combination of factors provides the habitats and the nutrients to support a highly productive marine ecosystem which in turn supports millions of seabirds as well as lizards, plants, insects and other invertebrates living on its many islands. Some of the larger islands such as Little Barrier are internationally renown sanctuaries for most of the country's endangered forest bird species. In economic terms the Hauraki Gulf is one of the most productive fisheries in the world - it is also an area of growing importance for tourism and recreation. There are numerous definitions for what constitutes the Hauraki Gulf. For the purpose of this study geographical accuracy is attempted by drawing a line from Bream Head to Cape Colville and including Great Barrier (inner side), Hen and Chickens and Mokohinau Islands. Popular convention has it that there are anything from between 40 to 65 islands in the Hauraki Gulf (e.g. Maddock & Whyte 1966, Owen 1983). Extrapolating from appendix 1 of the Department of Conservation's (DoC) Register of Northern Offshore Islands (Taylor 1989), the total number of islands including reefs, stacks, and sandbars within the area defined is 425 (Fig.l). By far the most numerically significant category are islets of between 0.1-1.0 ha. If one ignores for terrestrial biological purposes the reefs, rocks and sandbars smaller than 0.1 ha, which for the most part do not support permanent vascular plant life, the total number of islands is 351 of which islets constitute 67%. If the 64 small islands 1.0-10 ha are included the figure rises to 85%. 100 250 I | No. islands S 150 E => 100 z Island sizes in hectares Fig. 1. Hauraki Gulf islands in size categories after Taylor (1989). Despite the unique biological wealth known to be on small islands (Whitaker 1973), the significance of islets and small islands has not been adequately considered in the various conservation management and statutory planning documents. Indeed for most purposes of society (with the practical exception of course of navigation) they tend to be "invisible". Yet these islands as breeding sites for sea birds are the important remnants of a once much more extensive system, which in pre-human times would have included the coastal cliffs, beaches and promontories of the mainland. Many of them, in a curious anomaly, are not formally owned in a property title sense. For nearly 150 years they have existed in a legal limbo as "uninvestigated" which normally presupposes Maori customary land. These fragments of land are also interesting in a social and historic sense as they can be seen as a tiny remnant of pre-European New Zealand. Tiny crumbs left over and forgotten after extinguishment of aboriginal communal ownership and the transformation of the land into individual and Crown property titles. An original intention of the newly formed DoC in 1988 was to carry out a biological inventory of all the islands and islets in the New Zealand area. The register of northern islands referred to above was compiled by Graeme Taylor in 1989 but the biological inventory was never completed. Despite the pioneering efforts of workers like Falla, Turbott, Whitaker, Cameron, Taylor, McCallum et 101 al. many islets and small islands in the Hauraki Gulf have received only minimal inspection. Indeed many have never been legally surveyed nor had any formal management or protection - even islets within a few nautical miles of Auckland City. Many, over 100 in the Hauraki Gulf alone, are "unnamed". The most numerous class of islands in the Gulf are therefore the least cared for and the least known - it can be said they literally "don't count". This study records the findings of a survey of a sample set of seven islets lying off Waiheke Island (Fig. 2). These islets are offshore islands of offshore islands, spray-washed, wild and relatively unknown yet some lie well within sight of the commercial skyline of Auckland City. The survey set out to record the populations of seabirds, lizards and large invertebrates and to record native vegetation, weeds and ascertain the presence of rodents (listed in Appendices 2- 5). Where feasible rodents were eradicated. A further objective of the survey was to compile any human history including traditional Maori names and their meaning, and to record the presence of archaeological sites. Geologically all the islets are made up of Waipapa Group greywacke of Triassic-Jurassic age in common with nearby Waiheke Island (Hayward & Brook 1994). The exposed sides of the islets generally have fresher, harder 175° 00' E Fig. 2. Location of the seven islets studied, inner Hauraki Gulf. 102 greyer rock and the sheltered sides are weathered to more clayey, orange brown "rotten" rock. Steeply folded strata within the greywacke are discernible on Koi for instance with clear traces of metallic manganese sweated to the surface of the orange greywacke. On Kahakaha (Frenchmans Cap) there are beds of red chert, similar to that quarried on Karamuramu (McCallum's) Island. This material contributes to the large red sandy beach. The base and northwestern half of Motukaha Island is greywacke but it is unconformable overlain by weathered basal Waitemata Group conglomerate of early Miocene age. This material is identical to the 20 million year old beach conglomerates exposed in the adjacent cliffs of Waiheke Island (Eagle et al. 1995). METHODS The means of transport between my home on Waiheke and the islets in this study was by sea kayak (a 4.37 m "Spectrum Expedition"). Standard rat and mouse snap-traps with metal covers were used for rodent surveys. To measure rodent population density the standardised method of index trapping developed by Ecology Division DSIR was used (Cunningham & Moors 1988). Rat specimens were measured and weighed and dissection stomachs were stored in bottles of preservative (40% iso-propanol) for later analysis at Auckland University. Lizards and invertebrates were surveyed with the help of pit-fall traps (4 litre paint tins). Rodent poison brodifacoum (Talon WB50) was laid on islets where rodents were found. Plants unable to be identified in the field were pressed for later identification. Islands sizes are taken from Taylor (1989) except for Nani, the stated area of which appeared to include two tidal rocks, and Passage Rock for which no area data was given. In these two cases sizes were calculated from scaled diagrams from the Land Information of New Zealand Survey Data Index (SDI) database. The main survey was from 2 January-1 June 1996.