Introduced Marine Organisms in New Zealand and Their Impact in the Waitemata Harbour, Auckland, by Bruce
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Tane 36: 197-223 (1997) INTRODUCED MARINE ORGANISMS IN NEW ZEALAND AND THEIR IMPACT IN THE WAITEMATA HARBOUR, AUCKLAND Bruce W. Hayward Auckland Museum, Private Bag 92018, Auckland SUMMARY Sixty-one exotic marine organisms are listed that appear to have arrived in New Zealand with human assistance in the last 150 years and have become established. Of these, four were deliberately introduced, 25 probably came in as fouling on vessels, 10 possibly in ships' ballast water, three probably as deck cargo and the remainder as either fouling or in ballast water. The majority have come in from Europe (11), east Asia (10), eastern North America (6), Australia (6) and western North America (5). At least 12 species are known to have been exported from New Zealand and become established in other countries. These numbers are conservative estimates of the real number of introductions. It is not possible to assess the risk posed by any exotic marine organism prior to its arrival and establishment in New Zealand. Two surveys of the fauna of the Waitemata Harbour made sixty years apart, provide an insight into some of the changes that have occurred as a result of the establishment of at least 39 introduced exotic species. Many live in relatively low numbers and seem to have had little significant impact on the original ecosystems. The largest diversity of introduced organisms (especially Bryozoa) are fouling species on hard substrates, but the greatest environmental changes can be attributed to four bivalves introduced in the last 30 years. The Pacific oyster, Crassostrea gigas, is having a major impact on intertidal hard shore communities. Thickets of the small Asian mussel, Musculista senhousia, accumulate mud which temporarily smothers extensive areas of low tidal and shallow subtidal flats in the upper harbour. The small, fragile bivalve Theora lubrica, lives in billions in shallow-water muddy substrates around the harbour edges and is one of the few organisms that thrives in highly disturbed and polluted environments under the wharves and marinas. The file shell, Limaria orientalis, is now one of the dominant molluscs in the muddy shell gravels of the main harbour channels (10-30m deep) and has become a significant component of the diet of bottom-foraging fish, such as snapper. Keywords: Introduced marine organisms; marine invaders; ballast water; fouling organisms; New Zealand; Waitemata Harbour; Orakei Basin. 197 INTRODUCTION New Zealanders are aware of the numerous exotic terrestrial plants and animals that have been assisted immigrants to our country since the arrival of humans less than 1000 years ago. Some, such as sheep, cattle, pine trees and pasture grasses, are of great economic benefit to our country. Many others, such as possums, goats, rabbits, Old Man's beard and Kahili ginger, are an economic and environmental disaster. The enormous impact of these introduced pests and weeds on New Zealand's natural terrestrial and freshwater ecosystems is well documented. Many millions of dollars of public funding are spent each year trying to control or eradicate them. New Zealanders are far more aware of these exotic plants and animals on land and in our freshwater lakes and streams than they are about similar introductions in the marine realm. This is quite natural, as on land we can readily observe the arrival of strange organisms and monitor changes, but in the sea it is more difficult to notice new arrivals until they have become well established and have greatly multiplied in numbers. Thus it is particularly difficult to determine which of the widespread cosmopolitan marine species living around New Zealand today came here naturally and which were introduced with the assistance of human activities, such as shipping and have subsequently become well established. HOW FOREIGN MARINE ORGANISMS REACH NEW ZEALAND Natural transport methods 1. Currents In some years, the natural current patterns around northern New Zealand become favourable to carry juveniles and larval stages of a number of shallow warmer water fish, sea eggs, molluscs and other organisms from Norfolk or the Kermadec Islands to New Zealand (e.g. Powell 1976). These are carried southward along the east coast of northern New Zealand in the East Auckland Current, with new arrivals settling into suitable habitats they encounter. These are mostly around the offshore islands, such as the Poor Knights, Mokohinaus, Aldermen and White islands. Eddies in the current sometimes touch the mainland coast and warm water immigrants are known to have become established in places such as Cape Karikari, Cape Brett and Parengarenga (e.g. Powell 1976). Undoubtedly the vast majority of these warm water migrants do not survive the journey or do not encounter suitable habitats in which to settle. Of those that do find a suitable new homeland, a number, such as the swimming crab Scylla 198 serrata (Dell 1964), grow to maturity but find the conditions too cool to breed here and disappear after a few years. Only a few of these natural immigrants appear to find conditions suitable to establish viable breeding populations. The New Zealand fossil record documents thousands of natural immigration events of tropical and subtropical marine species reaching New Zealand and becoming established during warm periods since we split from Gondwana, 80-55 million years ago. It also documents the extinction of many of these warm water taxa during periods of cooler sea climate (Beu 1990). In the ten thousand years since the Last Ice Age, several thousand species may have been added to the modern New Zealand marine fauna by this method of introduction. Most are still restricted in their distribution to the northeast coast of the North Island. These range extensions and contractions occur continually in the marine environment. If climate warming eventuates we can expect many more species to naturally extend their ranges into northern New Zealand waters. Current transport will successfully introduce only those taxa that can survive the journey suspended in oceanic water for perhaps one to several weeks, ie. organisms that have long-lived, planktonic larval stages such as many sea eggs, crustaceans and some molluscs, or organisms that are free-swimming such as reef fish. 2. Attached to logs or organisms Barnacles, tube worms, algae and some nestling molluscs and crustaceans may be attach to free-swimming organisms, such as turtles or whales, or more frequently to floating logs which occasionally cross the oceans. Their chance of survival and successful establishment in New Zealand would be similar to organisms carried in by currents. Fossils document many successful introductions in the distant past and undoubtedly there have been a number since the Last Ice Age that are now part of our native marine biota. Australia is the most likely source of most successful immigrant species introduced attached to logs because of the current and storm patterns, availability of logs and similar climatic factors. Most tropical fouling species would find it too cold to become established in New Zealand. Temperate northern hemisphere fouling organisms are less likely to survive passage through the tropics during their long journey to New Zealand. 3. On feet of migrating seabirds Migrating seabirds are another mode of trans-oceanic transport that has probably been responsible for the natural introduction of a number of terrestrial and tidal flat animals and plants to New Zealand. This is particularly true for many microscopic organisms that live in brackish intertidal mud flats. They could 199 easily be transported in mud attached to the feathers or feet of one of the millions of wading birds that migrate to New Zealand along the east Asia flyway each season. This is the most plausible explanation for the observation that 19 of the 20 brackish species of foraminifera (Protozoa) living in New Zealand have a cosmopolitan temperate-subtropical distribution and also occur along north Atlantic coasts (Hayward & Hollis 1994). Human-assisted transport methods Despite the above-mentioned methods of natural transport and introduction of marine organisms to New Zealand, there are vast numbers of shallow water marine species living in other parts of the world that have not been able to cross the oceanic and climatic barriers. Recognition of introductions of marine species facilitated by some form of human assistance is often difficult. Strong suspicions are raised when coastal species that come from temperate and subtropical seas in the northern hemisphere are recorded suddenly appearing in New Zealand waters or are found living here in limited areas around one or more ports. There are several forms of deliberate and accidental human-assisted transport methods. 1. Deliberate introductions There have been numerous attempts to introduce foreign marine organisms to New Zealand waters especially prior to the 1920s. These have included Atlantic, sockeye and Quinnat salmon, European lobsters, Australian prawns, herrings, turbot and edible crabs. Most have failed, except for the salmon which are commercially farmed (Hine 1995). Three species of the intertidal cord grass, Spartina, were deliberately introduced from the UK and USA and planted here between 1913 and 1960 (Partridge 1987). They were introduced as a biological aid to reclamation of sheltered tidal flats, which in those days were not valued as ecosystems and important nurseries for marine life as they are today (Chapman & Ronaldson 1958). Spartina is now recognised as an environmental weed and thousands of dollars are spent each year around the country trying to control and eradicate it. 2. Aquarium releases Although freshwater is the preferred medium for aquaria, saltwater is reasonably common especially for colourful, tropical coral reef community imitations. A number of overseas marine organisms are imported live into New Zealand specifically to supply this market. There is every possibility that one or more of these species may be released by accident or on purpose into the New 200 Zealand marine environment.