6. HOKIANGA HARBOUR

Physical Environment

While only the southern coast of the harbour is included in this study area (from South Head to the Waihou River), the Harbour, being one body of water, is discussed as a whole.

The Hokianga Harbour is located on the west coast, and is formed from a drowned-valley system.   The harbour can be described as a "headland enclosed estuary" i.e. an inlet constricted by rocky headlands, with the harbour entrance maintained by strong tidal currents (Hume & Herdendorf, 1988).   There are strikingly scenic beach dunes at the mouth of the harbour which form the North Head - these have acted as "sinks" for sediment transported along the open coast as the shoreline has built out to form a stable smooth coast.

The harbour has a narrow mouth, shallow channels, and banks along much of its shoreline.   The perimeter of the harbour is characterised by numerous shallow estuarine embayments lined with mangrove forest and saltmarsh, although extensive areas have been drained or reclaimed for farmland.   Where beaches are present, they are fairly straight, and vary in width between 15 m and 20 m, and are backed by small raised beach deposits with steep dune slopes.

The harbour has a total area of approximately 115 km 2 , of which 65 km 2 are tidal flats (Heath 1975).   It is fed by many rivers - Mangamuka, Waima, Operehu, Orira, Omanaia, Whirinaki, Ohuri, and Waihou rivers. The largest lake in Northland, Lake Omapere, is also connected to the Hokianga Harbour via the Utakura River.   Numerous creeks and streams also feed into the harbour.

The Hokianga Harbour has a large catchment in relation to the size of the harbour.   Table 1 provides a comparison of the catchment areas relative to harbour sizes for several North Island harbours:

The catchment area of the Hokianga Harbour includes large areas which have been farmed for many years, including sheep, cattle and dairy farming.   Higher up in the catchment area are the Paratiko Range to the south, the Warawara Range to the north, and the Maungataniwha Range in the northeast.   All these ranges retain extensive areas of native vegetation, including remains of kauri forests, and vigorously regenerating shrubland.

The basement sedimentary rock which predominates in the catchment area, particularly in the lower hill country, generally produces saline brown clay soils.   As most of this land was originally covered in kauri forest, the soils were often heavily podzolised with a strongly developed silicate pan reducing water infiltration.   Clearance of native forest from soils left them very prone to surface erosion due to the poor infiltration, particularly during periods of very heavy rainfall, such as during cyclonic storm events.   The muddy sediments lining the upper part of the harbour are a result of ongoing soil erosion.

In addition to mud resulting from erosion of the land in the catchment, there are several other sediment types in the harbour.   The outer harbour has a predominance of sand of the same composition as previously described on the open west coast, but of a slightly smaller grain size - 20 to 24 mm on average (Applied Geology Associates, 1982).     Further up the harbour, and close to where rivers enter the harbour, are deposits of alluvium.

Flora & Fauna

About one-quarter of the Hokianga Harbour is covered with mangroves and saltmarsh (DOC, 1991).   The upper Hokianga Harbour has some of the finest areas of large mangroves ( Avicennia marina var. resinifera ) in New Zealand (Chapman, 1978a).   Mature mangroves in the Hokianga Harbour average 7 metres tall (Dr. Bob Creese, University of Auckland, pers. comm .).   Areas of mangroves occur at Koutu, at an unnamed bay north of Kauwhare Point, Opara, Whirinaki River, Karuhiruhi Stream, Wharekauere Stream, the west bank of the Omanaia River, Oue Creek, south-west of Rawene, Perunui River to Mangungu, Wairere River, Ruapapaka Island, and Horeke to Rangiahua.   In many of these areas, there is a transition from mangrove to saltmarsh (consisting of Juncus maritimus var. australiensis ) in a landwards direction - for example, at Koutu.   In some areas, Leptocarpus sp. also occurs with the Juncus (e.g. at Rangiahua, Oue Creek, Whirinaki River).   Spartina alterniflora , and Spartina townsendii , both exotic (i.e. not native) species, occur in some areas (e.g. Perunui River, mouth of the Wairere River, Rawene sub-estuary, and near Ivydale), and appear to be replacing indigenous (i.e. native) salt meadow species.

White faced herons, black shag, little shag, little black shag, mallard and grey ducks, white fronted tern, pied stilt and red-billed gulls are common in the harbour.   In addition, low numbers of international migratory waders (godwit and knot) use the mudflats for feeding during the summer.   Birds categorised as rare or endangered are also found: banded rail, white heron, little egret, reef heron, eastern little tern, caspian tern, NZ dotterel, fernbird, and (over winter) the cattle egret.   The harbour has New Zealand's largest population of the endemic (i.e. found only in New Zealand) banded rail, as well as the spotless crake and the marsh crake - this is the only site where these species are known to occur together (DOC, 1991).

There have been no ecological studies of the Hokianga Harbour undertaken, so information about the presence/location of other flora and fauna arise mostly from local knowledge, and thus focus on species of interest to man.

Fish found in the harbour include flounder and mullet (particularly in the upper harbour), parore, dogfish and snapper (particularly in the lower harbour), and trevally near the entrance.   Stingrays also occur here.  

The sea floor is home to a wide variety of shellfish: Horse mussels are commonly found in the upper harbour, and from Rangi and Koutu Points southward to the harbour entrance pipi and cockles are common. Oysters are found attached to rocks or mangroves.   Scallops are present in channels near the harbour entrance. Sub-tidal reefs provide habitats for mussels, rock lobster and kina in the outer harbour.

References:

Applied Geology Associates. 1982.   Coastal sand and shingle resources of Auckland and Northland. Unpublished report to the Auckland Regional Authority, April 1982.

Chapman, V. J. 1978(a).   Mangroves and salt marshes of the Herekino, Whangape, and Hokianga Harbours.   Department of Lands and Survey, Auckland. 32 pp.

Department of Conservation (DoC). 1991.   Coastal wetland inventory: Volume 2: Coastal wetlands north of the Kaipara Harbour catchment (Draft).   Department of Conservation, Wellington.

Heath, R. A. 1975.   Stability of some New Zealand coastal inlets.   New Zealand Journal of Marine & Freshwater Research.   9 (4):449-57.

Hume, T. M. & Herdendorf, C. E. 1988.   A geomorphic classification of estuaries and its application to coastal resource management - a New Zealand example.   Ocean and Shoreline Management.   11 :249-274.

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