The introduction of invasive marine species into new environments by ships’ ballast water, attached to ships’ hulls and via other vectors has been identified as one of the four greatest threats to the world’s oceans. The other three are land-based sources of marine pollution, overexploitation of living marine resources and physical alteration/destruction of marine habitat.
Shipping moves over 80% of the world’s commodities and transfers approximately 3 to 5 billion tonnes of ballast water internationally each year. A similar volume may also be transferred domestically within countries and regions each year. Ballast water is absolutely essential to the safe and efficient operation of modern shipping, providing balance and stability to un-laden ships. However, it may also pose a serious ecological, economic and health threat.
A potentially serious environmental problem arises when this ballast water contains marine life.
There are thousands of marine species that may be carried in ships’ ballast water. These include bacteria and other microbes, small invertebrates and the eggs, cysts and larvae of various species. The problem is compounded by the fact that virtually all marine species have life cycles that include a planktonic stage or stages. Even species in which the adults are unlikely to be taken on in ballast water, because they are too large or live attached to the seabed, may be transferred in ballast during their planktonic phase. Over the past millennia, marine species have dispersed throughout the oceans by natural means, carried on currents and attached to floating logs and debris. Natural barriers, such as temperature and land masses, have prevented many species from dispersing into certain areas. This has resulted in the natural patterns of biogeography observed in the oceans today. In particular, the pan-global tropical zone has separated the northern and southern temperate and cold water zones. This has allowed many species to evolve quite independently in these latter zones, resulting in quite different marine biodiversity between the north and the south. In tropical areas species have not faced the same barriers. This is exemplified by the relatively homogenous marine biodiversity spanning the huge area of the Indo-Pacific, from the east coast of Africa to the west coast of South America.
Humans have aided this process for as long as they have sailed, mainly by dispersing marine species that have attached to the hulls of vessels. The commencement of the use of water as ballast, and the development of larger, faster ships completing their voyages in ever shorter times, combined with rapidly increasing world trade, means that the natural barriers to the dispersal of species across the oceans are being reduced. In particular, ships provide a way for temperate marine species to pierce the tropical zones, and some of the most spectacular introductions have involved northern temperate species invading southern temperate waters, and vice versa.
It is estimated that at least 7,000 different species are being carried in ships’ ballast tanks around the world. The vast majority of marine species carried in ballast water do not survive the journey, as the ballasting and deballasting cycle and the environment inside ballast tanks can be quite hostile to organism survival. Even for those that do survive a voyage and are discharged, the chances of surviving in the new environmental conditions, including predation by and/or competition from native species, are further reduced. However, when all factors are favourable, an introduced species survive to establish a reproductive population in the host environment, it may become invasive, out-competing native species and multiplying into pest proportions.
As a result, whole ecosystems are being changed. In the USA, the European Zebra Mussel has infested over 40% of internal waterways and may have required between US$750 million and US$1 billion in expenditure on control measures between 1989 and 2000. In southern Australia, the Asian kelp is invading new areas rapidly, displacing the native seabed communities. In the Black Sea, the filter-feeding North American jellyfish Mnemiopsis leidyi has on occasion reached densities of 1kg of biomass per m2. It has depleted native plankton stocks to such an extent that it has contributed to the collapse of entire Black Sea commercial fisheries. In several countries, introduced, microscopic, ‘red-tide’ algae (toxic dinoflagellates) have been absorbed by filter-feeding shellfish, such as oysters. When eaten by humans, these contaminated shellfish can cause paralysis and even death.
There are hundreds of other examples of catastrophic introductions around the world, causing severe human health, economic and/or ecological impacts in their host environments.
Invasive marine species are one of the four greatest threats to the world’s oceans! Unlike other forms of marine pollution, such as oil spills, where ameliorative action can be taken and from which the environment will eventually recover, the impacts of invasive marine species are most often irreversible!
The UN considers the introduction of non-indigenous oceanic species to be one of the top four serious threats to the global environment. At any given time, 35 000 ships are en route on the water of the Earth and more than 3000 species are being transported in their ballast tanks. The UN’s International Maritime Organization estimates that ten billion tons of ballast water is transported around the world every year. The past decade has seen a marked increase in the spread of species to areas where they do not naturally belong. This creates an imbalance in ecosystems and is a serious environmental threat. Many times the invader has no natural predator and the original species become extinct and the entire marine ecosystem is disrupted. This has dramatic consequences for biodiversity and for industries such as fishing and aquaculture.