Description of Area: Hokianga, Whangape and Herekino Harbours
Hokianga is the largest of the three harbours in this unit and the fourth largest in New Zealand. This drowned river valley extends well inland and has 50% of its 11,500 ha area composed of tidal mudflats with Pleistocene dunes at the entrance. Whangape (850 ha) and Herekino (630 ha) harbours are significantly smaller in area but also have tidal mudflats occupying approximately 50% of their area. Whangape is a small Y-shaped harbour which opens to the sea via a 4 km long, 150 m wide channel bordered by steep hillsides. It is formed by the confluence of two estuaries or arms of two rivers. The Herekino Harbour entrance is 200 m-wide and lies between huge dunes to the north and a prominent spur to the south. The harbour is a drowned former stream valley, approximately 6 km long that branches into two in the upper reaches. The northern shore is mainly sandy but the southern entrance consists of a relatively sheltered, rocky shore.
Tidal flow is the major factor influencing water circulation in the Hokianga Harbour rather than the freshwater inflows. The maximum tidal range of the three harbours is 3 m.
Hokianga Harbour comprises three major areas – the lower harbour (from the entrance to Te Karaka/Onoke) is characterised by high salinity oceanic water, soft substrates dominated by sands, numerous areas of boulder and rock, strong tidal currents, low water turbidity and relatively short water residence times. The middle harbour (Te Karaka/Onoke to Matawhera) is characterised by soft substrates dominated by fine sand and mud and moderate to strong tidal currents in the main channels. The upper harbour (Matawhera inwards) is characterised by lower salinity estuarine water and soft substrates dominated by silt and clays, strong tidal currents in particular areas, numerous backwaters and lengthy river arms, relatively long water residence times and high water turbidity.
Whangape Harbour comprises four areas – the entrance channel (a former steep sided river gorge), a shallow central basin (between Whangape and Pawarenga) that at low tide comprises 2 – 5 m deep channels linking the estuaries with the entrance channel, and two estuaries formed from the Awaroa and Rotokakahi Rivers
Herekino Harbour is mainly empty of water at low tide, with only a shallow subtidal entrance channel.
Hokianga harbour contains large stands of mangrove (Avicennia marina var. resinifera) and saltmarsh species (Juncus spp., Leptocarpus spp., and Muehlenbeckia spp). Twelve intertidal habitats and substrates have been recognised for the Hokianga Harbour, including the presence of quite large beds of introduced cord grass (Spartina sp.).
Both marine and estuarine species occur in the harbour. Truly marine species found as far as Onoke, some 12 km from the harbour entrance include brown algae (common kelp, Ecklonia radiata, and sea wracks, Carpophyllum flexuosum and C. maschalocarpum), topshells (Melagraphia aethiops and Diloma subrostrata), oyster borer (Lepsiella scobina) and cat’s eye (Turbo smaragdus). Horse mussels (Atrina zelandica) are found subtidally in the lower harbour and provide nursery habitat for juvenile species of fish such as snapper (Pagrus auratus). Truly estuarine species present include the mudflat whelk (Cominella glandiformis), cockle (Austrovenus stutchburyi), mudsnail (Amphibola crenata), estuarine snail (Potamopyrgus estuarinus) and mud crab (Helice crassa). The introduced Pacific oyster (Crassostrea gigas) is also common.
Fish species in Hokianga Harbour comprise coastal and estuarine elements. In the vicinity of the harbour entrance, banded wrasse (Notolabrus fucicola), spotty (N. celidotus) and P. auratus are common coastal species. Estuarine species found within the harbour include yellow-eyed mullet (Aldrichetta forsteri) and grey mullet (Mugil cephalus). Rock lobsters (Jasus edwardsii) also occur within the harbour.
Whangape Harbour has some of the tallest mangroves (10 m) in New Zealand, however the soft sediments generally support a low diversity of biota. Sand dollars (Fellaster zelandiae) are numerous just inside the harbour mouth and pipis (Paphies australis) dominate the remaining 3 km of the entrance channel. Hermit crabs (Paguristes pilosus and Pagurus novaezelandiae) and scavenging whelks (C. adspersa, C. glandiformis, C. maculosa) are common. The central basin contains the most diverse fauna of the harbour. Pipis, hermit crabs and whelks dominate the medium grained sandy sediments nearest the entrance channel, while wedge shell (Macomona liliana), nutshell (Nucula hartivigiana) and cockles (A. stutchburyi) are common in the finer substrate of the basin. The Awaroa estuary supports rich populations of cockles while the Rotokakahi estuary is dominated by rich pipi populations. The only introduced species is the Pacific oyster, Crassostrea gigas.
The southern rocky shore of Herekino Harbour between Owhata and the harbour entrance support zone forming species such as periwinkle (Austrolittorina antipoda), barnacles (Chamaesipho brunnea and C. columna), flea mussel (Xenostrobus pulex), tube worm (Spirobranchus carniferus) and algae (Apophlaea sinclairii, Gigartina alveata, Xiphophora chondrophylla, Corallina turf). The medium grained sandy sediments support intertidal and subtidal populations of pipis and cockles. Other common bivalves include wedge shell, nut shell and Theora lubrica. The upper reaches of both arms of the harbour support mangrove forests.
The harbours are of considerable wildlife value and are important for birdlife. In the Hokianga Harbour there is a small brown teal (Anas aucklandica) roost in the Mangamuka arm and high numbers of banded rail (Rallus phillippensis) and spotless crake (Porzana tabuensis)