Mud, mud, glorious mud...

Press, 16 August 1980, Page 16

Mud, mud, glorious mud...

The monotonous stretches of tidal mudflats in the estuaries and harbours around our coasts are a vastly undervalued resource. That is the view of George Knox, Professor, of Zoology at Canterbury University and head of the Estuarine Research Unit., Naturalists know, of the enormous numbers of birds that find a home on mud? flats, flounder fishermen appreciate ■ their , bounty, and the solitary stroller may enjoy their beauty. Most of us ignore them. Yet, they are not just unsightly wastelands fit only for the dumping of rubbish and for reclamation. They are among the most productive areas in the world. : “It is only in recent years that we have come to realise the importance of estuaries in the over-all economy of our coastal waters,” Professor Knox says. “Recent research has shown that the production of estuaries is equal to that of tropical forests and four times that of good ryegrass pastures.” Although little of this vast productivity is of

immediate use, it is vital for our present arid future prosperity. The often tiny and apparently insignificant plants and. animals are all part of the long and complex food chain that supports commercial At the base of the food chain are the submerged grasses, and the rushes and sedges from the salt marsh. Mangroves, sea - lettuce, large algae, and the muddwelling micro-algae are also important producers. . Direct grazing by marine animals consumes only about 10 per cent of this material; Most of it dies and rots, and the resulting detritus is colonised by bacteria, fungi, protozoa, and other micro-organisms. These in their turn become the basic food for crabs, shellfish, worms, and fish.

Some fish can feed directly from the detritus and others eat the detrituseaters. A 20cin mullet will filter 1500 g of estuarine sediments a day, or more than 450 kg a year, to get its food. '

Filter-feeders reach very."' high concentrations ini many of our mudflats. In

the Avon - Heathcote estuary there can be .as many as 500 large mudflat snails per square metre, while cockles can exceed 2500 per square metre. Many of the smaller marine animals are found in even greater numbers. Researchers have found 12,250 polychaete worms, 47,750 amphipods, and nearly 138,000 small snails, all in a square metre; of mudflat. These enormous numbers of shellfish, crabs, “and worms support huge -'flocks of birds, particularly gulls,ducks,' and many ■Species?'- of-- waders. The 4000-odd. pied oyster batchers studied at the Avon-Heathcote Estuary were found to have consumed in total an average of nearly one million and a half cockles a day during the winter months. Estuaries are the main feeding and breeding ground for many commercially valuable species of fish. Natural beds of pipis, cockles, and mussels are exploited in many estuaries . and commercial oyster and mussel farming is becoming increasingly important. ' The Arapaoa River, a 4750 ha branch of the Kaipara Harbour,, was.. found in a recent survey to be supporting the equivalent of 12 family units and contributing to the income of many more. There were six full-time fishermen, two part-time, fishermen, and three fulltime oyster farmers.: There were also many recreational fishermen taking moderate catches . of kahawai, parore, mullet, and snapper. Total annual production in 1976 was valued at $lOO,OOO. Estuarine sediments contain a high percentage of . very absorbent clays, and the filter feeding animals themselves remove enormous quantities of suspended matter from the water. This they deposit in the mild with their excreta. And their bodies add their

accumulated store when they die. : Because of this estuaries : are far richer in organic matter and nutrients than either the surrounding land or the open sea. They- are also naturaltraps for pollutants. Petroleum by-products, persistent pesticides, radioactive wastes, heavy metals, and other toxic man-made chemicals are becoming increasingly concentrated,in our estuarine waters arid mudflats, with devastating effect on the wildlife; t . ■. . The plants and animals that live in the estuaries are extremely sensitive'.to' changes in their environment. : Unsatisfactory conditions can destroys whole species. The destruction of. one seemingly insignifi-, cant species somewhere in the food chain can affect in . turn the ’edible shellfish 5 , commercial species of fish, and the seabirds 'arid' Waders that feed on them:, ’ Estuaries are also very . vulnerable to direct human impact. The grazirig of farm animals on the salt marshes bordering an estuary cah destroy the rushes arid sedges at. the base of the food chain. In time, the entire, chain will, be affected. . ' ■* The building of' causeways can cut off inlets andupset the tidal flow, bringing slow but certain extinction to the delicately balanced, natural . community that depends on n. The dumping of rubbish is riot merely "unsightly; it can have serious consequences. Pollutants in the rubbish . can poison the immediate neighbourhood or . flow along the creeks . . with .the . outgoing tide, carrying destruction further afield. Professor Knox believes that maintaining the carry-- ; ing capacity of the estuarine ecosystem shopld. -be- a major natiorial, goal. “Time is riot on bur side. We must act quickly if we want to Save our estuaries from destruction.” ’

Report by the National Conservation

Week campaign committee