Most species carried in ships’ ballast water do not survive the voyage. Most of those that do, do not survive when discharged into the new environment. Under certain circumstances some species do survive to form viable populations, and may become serious pests.

Impacts can be divided into three main categories:

	Ecological: when the native biodiversity and/or ecological processes may be disrupted by the invading species. It is estimated that introduced marine species invade new environments somewhere in the world on a weekly to daily basis.
	Economical: when fisheries, coastal industry and other commercial activities and resources are disrupted by the invading species. It is estimated that the cost of all invasive species exceed US$138 billion per year in the USA alone!
	Human health: when toxic organisms, diseases and pathogens are introduced through ballast water, causing illness and even death in humans.

There are hundreds of examples of severe ecological, economical and human health impacts from invasive marine species around the world. Some outstanding examples include:

The European Zebra Mussel Dreissena polymorpha: Introduced to the North American Great Lakes. It has spread to infest more than 40% of US waterways, fouls the cooling-water intakes of industry. It may have cost between US$750 million and US$1 billion in control measures from 1989 to 2000.

Zebra Mussels...

Once established, it is virtually impossible to control an invasive marine species.
Impacts are usually irreversible.

The North Pacific Seastar Asterias amurensis: Introduced to southern Australia. This voracious predator threatens commercial stocks of shellfish species such as oysters and scallops. This large Seastar is a prolific breeder and in one estuary alone in Tasmania reached an estimated total population of nearly 30 million individuals, at densities greater than any recorded in it’s native range.

Toxic dinoflagellates: Spread to several locations around the world through ships’ ballast water. Under favourable conditions these micro algae may bloom to form ‘red tides’. If absorbed into filter-feeding shellfish such as oysters and scallops, they may release toxins. This can cause paralysis or death in humans who eat the contaminated shellfish.

Toxic dinoflagellates - Gustaaf Hallegraeff