The natural environment
The 'wet' part of the Harbour is rich in shellfish and fish and has significant beds of seagrass, a critical habitat for many marine animals. A rich diversity of wading birds, some migratory and some resident, make their homes in the areas of harbour bed exposed at low tide. The lower reaches of the harbour contain beds of mussels, cockles and pipi. The saltmarshes on the harbour margins are dominated by sea rush and oioi, providing habitat for such birds as fernbird, bittern and banded rail. The muddy upper reaches of the harbour, home to titiko (mudsnails) and mudcrabs, are being increasingly colonised by mangroves, which provide habitat for banded rail.
Read about the State of the Ohiwa Harbour and Catchment (3.65MB, pdf).
Kiwi population on the rise
A kiwi population is slowly growing in reserves, forest and farm land around the Ohiwa Harbour, thanks to the Whakatane Kiwi Project. Since 1999, when four breeding pairs were found in the Ohope reserves by the Department of Conservation, over 300 North Island brown kiwi chicks have been raised in or released to local reserves.
Whakatane is the only place in New Zealand where kiwi live in such numbers so close to an urban area. Many of the birds are now migrating southeast from Whakatane and Ohope reserves towards Wainui and sometimes beyond.
The Whakatane Kiwi Project works closely with landowners, foresters and residents in these areas, so that kiwi are afforded greater protection in their travels. Long-term, the Project aims to establish a network of predator control activities on private and public land, so that Ohiwa is a safe haven for all native wildlife.
Read more about kiwi management around Ohiwa Harbour in the case study - Kiwi management around Ohiwa Harbour.
Streams and reserves
The streams feeding the Harbour are home to significant numbers of native freshwater fish such as inanga (adult whitebait). While most of the flat and rolling land in the catchment is cleared for farming, there are many important remnants of coastal forest, dominated by pohutukawa and puriri. Many of these special places are set aside as reserves and managed by either the Department of Conservation or the district councils. Community groups help to look after some of these reserves.
There are the usual threats to native birds (some quite rare) such as rats and stoats. Various invasive weeds, too many to list, compete with native vegetation from the edges of the mudflats to the hills. Some pests are controlled by landowners under Regional Pest Management Plan obligations. The Ohiwa Strategy partners are working towards better protection of all the ecological values of the Harbour and better coordination of pest control work.
Several aspects of the Harbour’s ecology are monitored by the Regional Council; you can find out more about this research here.
Geology and soils
The Harbour itself is unusual in that it has a spit on either side of the entrance. These spits are subject to regular changes according to currents and tides. The Harbour was once fed by a much larger river; it is slowly infilling with sediment and will one day become dry land. This natural process is accelerated by human activities, which cause increased sediment to enter the Harbour.
The spits at the mouth of the Harbour are constantly changing and in the 1970s a number of houses on the Ohiwa spit were lost to the encroaching sea. The sea has since retreated and left a wide band of sand dunes. The site where a hotel and wharf once stood is now in the middle of the Harbour entrance.
Evolution of the Ohiwa Harbour, in response to earthquakes and tsunami, is explained in this Waikato University podcast at 23:59.
Geologically, the area is predominantly greywacke of Late Jurassic to Late Cretaceous age, draped by loess (wind-blown silt) during the Pleistocene Ice Ages and more recently covered in volcanic ash and pumice from the Rotorua and Taupo volcanic centres.
It is from this air-fall rhyolitic volcanic ash that most of the soils of the Ohiwa Harbour catchment are derived. However, there are also other parent materials and processes that have formed soils in different parts of the catchment. Soils on the floodplains and lower flats are formed from a mixture of colluvium and alluvial rhyolitic ash and greywacke. Soils in the dunes are formed from air-fall tephra on windblown sand. The soil in the hills and steep headwater areas are Whakatane Hill soils and associated steep land soils, formed from Tarawera ash, Kaharoa ash on Taupo pumice, and Whakatane ash over sandstone or greywacke.
The Ohiwa Harbour is a visually spectacular landscape covering approximately 26 square kilometres. The tidal waters, estuaries, mudflats and islands contrast with the surrounding predominantly rural landscape. There are great views of the Harbour and further afield from Tauwhare Pa and Onekawa Te Mawhai.
There are two main islands in the Harbour; Uretara and Ohakana Islands, plus the four smaller islands of Whangakopikopiko, Te Motuotu, Hokianga and Pataua Islands.