Riding the high seas are thousands of hitchhikers, criss-crossing the world courtesy of tankers, cargo ships, cruise ships, leisure craft and other floating hosts. Many are tiny for part of their life cycle, too small to be noticed. Some are firmly attached to hulls, others are caught in hollow spaces or water intakes, while many slosh about in ballast water. Invasive species can also arrive among fishing materials, and some even come as aquarium plants or animals and are released unwittingly.
Hideaways for stowaways
Most ships’ hulls have recesses below the water-line, known as sea chests, which take in sea water for ballast or engine cooling. Pumps draw water in, sucking in small organisms. Live marine creatures found in sea chests have included sponges, sea anemones, hydroids, worms, sea slugs, mussels, oysters, scallops, bryozoans, barnacles, crabs, sea spiders, sea stars, sea urchins, sea squirts and fish.
Introduced and invasive species
Introduced species are living organisms that are brought by humans to an area where they are not native, and where they survive and reproduce themselves. Introduced species that become pests in the new environment are described as invasive.
An isolated coastal ecosystem
Despite being connected to other regions by water, New Zealand’s coastal ecosystems have evolved in isolation, cut off by deep ocean basins, currents, and climate zones such as tropical or polar regions. Some coastal organisms that can overcome these barriers unassisted are found naturally all over the southern hemisphere and the globe. However, many more are unable to propel themselves over long distances or would not survive a slow, drifting journey away from their usual coastal environment.
Contact through trade
Growth in international trade means that New Zealand’s coastal ecosystems are increasingly exposed to contact with the outside world. There were 3,500 international shipping arrivals in New Zealand in 2003, compared with fewer than 1,000 in 1960. With new trading partners, there are links to different coastal regions – each a possible source of new and aggressive pests. In addition, the itineraries of cruise ships and yachts differ from trade routes, which increases the range of places of origin.