In New Zealand at least 30 types of fish use estuaries at significant times in their life. Some, such as sand flounder, yellow-bellied flounder, common sole, kahawai, grey mullet and yellow-eyed mullet, move in and out of estuaries each day. They swim over the tidal flats on the incoming tide searching out shellfish, crabs and worms. Others – snapper, red cod and gurnard – are seasonal visitors. They enter the estuary as immature fish and spend some months feeding in the rich, sheltered waters before heading back out to sea.
Estuaries are vital to the 17 native fish that migrate between fresh and salt water. They serve as gateways through which the fish must pass to complete their life cycle.
Adult whitebait or īnanga (Galaxias species) come down rivers to lay their eggs among the plants of the upper estuaries in late summer and autumn, and then die. On hatching, the young are swept out to sea, where they spend five or six months. They then enter estuary mouths in spring, and swim upstream to fresh water further inland.
In contrast, adult eels come down the rivers and through the estuaries to spawn at sea somewhere in the tropical Pacific. Their larvae then make their way back to New Zealand’s coastal waters in spring. They transform into a juvenile stage known as glass eels just before they enter estuaries, where they settle and feed for a while before swimming upstream into fresh water.
Tidal flats hold a bounty of food for wading birds, which gather there in large numbers to feed; for individuals, there is little protection on the exposed flats. Mud probers like oystercatchers and godwits push their beaks deep into the sediments in search of shellfish, marine worms and insect larvae. Wrybills feed from the surface of the mud, sweeping their beaks across it in search of insects, small crabs and prawns. Pied stilts search the water and marshy rushlands for aquatic insects, snails and crabs. Others such as herons and Caspian terns are fishers, stalking the estuary waters in search of their swimming prey. Waterfowl such as ducks and teal sieve estuarine waters and mud through their beaks to extract plankton and small animals. The rushes, reeds and mangroves provide sheltered breeding grounds for swamp birds such as the pūkeko, bittern, marsh crake, banded rail and fernbird.
The Ramsar Convention is an intergovernmental treaty which was signed at Ramsar, Iran, in 1971 for the conservation of wetlands used by migratory birds. Through this treaty, estuaries in New Zealand are linked with sites in Australia, Japan, China, Korea, Siberia and Alaska.
Each year thousands of godwits, plovers and lesser knots travel 11,000 kilometres from their Siberian and Alaskan breeding grounds, arriving in spring at New Zealand’s estuaries. Sheltered estuaries, from Pārengarenga Harbour in the north to Paterson Inlet on Stewart Island in the far south, provide rich feeding grounds for 102,000 eastern bar-tailed godwits and 60,000 lesser knots, as well as smaller numbers of ruddy turnstones, red-necked stints and golden plover. In recognition of their critical role in the life of migratory birds, four large estuaries – Firth of Thames, Manawatū River, Farewell Spit and Waituna Lagoon – are designated Wetlands of International Importance under the Ramsar Convention.