To the casual observer, an estuary may appear devoid of animals for much of the day: a few wading birds out on the tidal flats, or perhaps a couple of gulls fighting over a mud snail. But these birds are just one part of an intricate web of living creatures that may number in the millions or even billions.
To appreciate the abundance of estuary life you must consider what is living in the mud. For it is here that detritus (dead organic matter) accumulates, attracting animals in search of food. When the tide retreats, thousands of burrows, tracks and deposits belonging to various types of animals become apparent.
New Zealand’s native mosquitoes are irritating pests, but they do not transmit diseases to humans. In 1998 an Australian mosquito – the southern marsh mosquito – was found breeding near Napier. Subsequently it has been found in estuaries from Kaipara Harbour in the north to Wairau Lagoon in the South Island. Adult females are aggressive biters and in Australia carry the Ross River virus. Humans infected with the virus may develop flu-like symptoms that last for months. The New Zealand government has allocated $40 million to eradicate this unwanted visitor.
Enormous numbers of worms live in estuarine muds. Known as mud worms or bristleworms, these small segmented creatures are related to earthworms, but are characterised by a pair of short bristly structures on each segment. Some live in permanent burrows, feeding on the organic matter in the mud; others are more mobile and scavenge on the surface or through the mud. Like earthworms, they leave casts of undigested sediment at their burrow entrance or along the trails they make. Mud worms are a significant food source for wading birds and fish.
Three kinds of crab are commonly encountered on tidal mudflats.
Kairau, the tunnelling mud crab (Helice crassa), dominates the upper tidal zone. This extraordinary little crab is found in high numbers on many estuaries, but is easily overlooked. It retreats into a burrow as soon as it detects movement and its grey-olive shell merges with the mudflats. Kairau scoops up mud with its nippers and uses its mouth parts to locate organic matter.
The mid-low tide zone is the realm of another tunnelling crab – the stalk-eyed mud crab (Macrophthalmus hirtipes). It is larger and greener than kairau, and mainly active at night.
The hairy-handed crab or papaka huruhuru (Hemigrapsus crenulatus) is found throughout the intertidal zone. It is not a tunnelling crab and may be found under rocks or just under soft sands and mud. Densities of 255 crabs per square metre been recorded in the Avon–Heathcote estuary.
The cockle or tuangi (Austrovenus stutchburyi) is a shallow-burrowing shellfish, found from subtidal to mid-tide levels. It is not related to the northern hemisphere cockle, and is found only in New Zealand waters. The cockle is one of the most important creatures of the estuary: it provides food and modifies the habitat, playing a critical role in filtering water.
In some areas the native cockle or tuangi reaches great numbers. At Pāuatahanui Inlet near Wellington, a density of nearly 550 million cockles to 1 square kilometre was recorded in 1976. This number of shellfish would produce 3,500 tonnes of meat – about the same amount as 7,000 head of cattle.
Cockles are favoured by oystercatchers and sand flounder, and by people. If these shellfish manage to escape the attention of the many predators in the estuary, they can live for up 20 years. They are commercially harvested from Whangārei Harbour, Golden Bay and Otago Harbour – the total catch limit in 2001–2 was set at 2,280 tonnes.