Physical Environment
The area discussed in this section includes the whole of the Rangaunu Harbour, with specific mention of the eastern shoreline from the Awanui River through to Motutara Bay.
Rangaunu Harbour is the fifth largest harbour in New Zealand. The harbour is approximately 10 km due north of Kaitaia and separates the Aupouri and Karikari Peninsulas. It is 115 square kilometres in size, of which 53 % is exposed as sand, sea-grass and mangrove flats at low tide. The harbour is thus relatively shallow, with large areas of sand-flats, dissected by deeper channels with steep sides, which are maintained by tidal flows. There are 31 square kilometres of mangroves, which represents 15 % of the total mangrove area in New Zealand (King et al., 1985). The Awanui River which drains into the southern harbour is the main freshwater inflow to Rangaunu Harbour, and has been the dominant influence on the shape of the harbour. The extensive areas of mangroves, eelgrass, and salt-marsh mean that the harbour is extremely productive.
The Karikari Peninsula forms the eastern side of the Rangaunu Harbour. The Peninsula is formed from what were once islands, now joined to the mainland by a tombolo. The tombolo is 15 km long, and 5 km wide, and consists of dunes interspersed with swamp, lagoons and lakes. The central dunes lie in a north-easterly direction across the tombolo, with younger fore-dunes running parallel to the eastern and northwestern beaches. The coastal dunes tend to be more mobile, with those bordering on the fore-dunes becoming fixed by light scrub. The eastern shore of the Rangaunu Harbour is thus composed of low-lying dune lands, punctuated in the north by three low-lying (approximately 40 m) rocks (originally islands) between Kotiatia Pt and the Motutaratara Rocks.
Rangaunu Harbour is of international importance as a habitat for migratory waders and has nationally significant wetland and scrub habitats.
Flora & Fauna
The Rangaunu Harbour Study (Northland Harbour Board, 1984) grouped benthic animals of the harbour into three types, largely based on differences in the harbour environment:
Type 1 -animals that inhabit the mobile coarse sand around the harbour entrance, species that are coastal rather than estuarine;
Type 2 - typical estuarine animals, mainly associated with eelgrass - cockles are particularly common;
Type 3 - mainly associate with mangroves, very fine sediment.
Cockles ( Austrovenus stutchburyi ) are known to occur in dense beds near the harbour entrance and are frequently gathered for consumption. In addition to cockles, the benthic fauna of the inner harbour associated with the eel grass ( Zostera ) beds are mud crabs ( Helice crassa, Macrophthalmus hertipes ), polychaete worms, gastropods ( Haminoea zealandiae, Cominella glandiformis, Diloma subrostrata ), bivalves ( Tellina liliana, Solemya parkensoni ) and a variety of amphipods and isopods.
Benthic fauna associated with the mangroves is dominated by animals that are robust to the increased fine content of the sediment (i.e. silts and clays) and include pupu ( Amphibola crenata ), catseyes ( Turbo smaragda ), Pacific oysters ( Crassostrea gigas ), mud whelk ( Cominella glandiformis ) as well as mud crabs ( Helice crassa, Macrophthalmus hertipes ), snapping shrimps ( Alpheus sp. ) and a large diversity of polychaete worms, amphipods and isopods.
Mangrove ( Avicennia resinifera ) occupies over 3,000 ha (27%) of the harbour, and eelgrass ( Zostera sp .) occupies over 2,000 ha (19%). Saltmarsh with rushes ( Leptocarpus similis, Baumea juncea ), glasswort ( Salcornia australis ) saltmarsh ribbonwood ( Plagianthus divaricatus ) and a variety of smaller turfing plants ( Samolus repens, Selliera radicans, Cotula coronopifolia ) lines the foreshore in patches throughout the harbour. Large areas of saltmarsh on the eastern side of Rangaunu Harbour have been reclaimed for farmland.
There is a small area of rocky intertidal at the eastern mouth of Rangaunu Harbour and several small sub-tidal reefs. The sub-tidal reefs are dominated by sponges and provide attractive habitats for oceanic and reef-dwelling fish species. The rocky shore area has low species diversity with characteristic zonation of rocky shores in Northland (Northland Harbour Board, 1984). These areas provide important refuges for juvenile crayfish, which are frequently found there.
Rangaunu Harbour is one of New Zealand's major wading bird harbours, with up to 10,000 waders using the harbour just prior to the northern migration in the early autumn. The major roosting areas in the harbour are Waller Island and Rangiputa Bank, Otakakaha Islands and paddocks on Karikari Peninsula.
The NZ Wildlife Service carried out a survey of birdlife in the Rangaunu Harbour between 1981 and 1982. During the twelve month study, seventy species of birds were recorded and the following species were found to be breeding in the harbour: NZ dotterel, variable oystercatcher, black-backed gull, red-billed gull, white-fronted tern, Caspian tern, black shag, little shag, pied shag. Many other swamp, pasture and bush birds including pied stilts, white-faced herons, ducks and swans were found breeding in areas adjacent to the harbour (Bellingham & David, 1983).
Fish species caught in and around the Rangaunu Harbour in moderate numbers include mullet and parore (frequently found foraging in the mangroves), snapper, shark, flounder, garfish, kahawai, kingfish and trevally. Large populations of eagle rays inhabit the shallow sand-flats. Most coastal fish species make use of Rangaunu Harbour at some stage of their life cycle or are in some way dependent on nutrient input from the harbour (Northland Harbour Board, 1984). The coastal vegetation of Rangaunu Harbour is dominantly scrub (28%) and pasture (68%).
References:
Bellingham, M. & Davis, A. (1983). Rangaunu Harbour wildlife survey. NZ Wildlife Service Technical Report No. 3 (Unpubl. Report).
King, K. J., K. N. Bailey, and M. R. Clark. 1985. Coastal and marine ecological areas of New Zealand - A preliminary classification for conservation purposes. Department of Lands and Survey.
Northland Harbour Board (1984). Rangaunu Harbour Study.
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