Conservation Biology

Alien species transport via ship ballast water

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Live Zebra mussels (Dreissena polymorpha) with open siphons

Alien species transport via ship ballast water ia a significant mechanism for assisting invasion of non-native species into a new environment. Alien species are organisms that are foreign to a given natural environment; moreover, many alien species have have migrated or been transported from their native habitats for centuries. However, because of globalization and technological advances, goods and people now move at a much faster rate and to far more distant locations than in earlier times. As a result of human activities, plants, animals and other organisms are transported to new habitats with a speed and efficiency that thousands of years of evolution have not facilitated (IMO 2006a). Alien species are not necessarily intrinsically harmful; intentionally introduced non-indigenous species have sometimes appeared to produce beneficial effects to certain agricultural and livestock industries. Crops such as potatoes, wheat, rye, rice, corn, soybeans and peanuts were spread throughout the world to feed the growing human population. Similarly, animals such as cows, chickens, ducks, horses, sheep, goats and buffalo were dispersed to provide the world's meat, milk and egg products (Pimentel 2002: 3-4).

Scope of the problem

There are numerous instances in which alien species are invasive, e.g. reproducing in large numbers and crowding out native species. Such alien organisms pose a serious threat to existing ecosystems, especially if the natural predators of an introduced species are not present to control the population. Invasive species thrive in their new environment, competing with the habitat's naturally-occurring species, and may replace keystone species or potentially cause the decline or extinction of one or more indigenous species, severely disrupting the ecosystem. The impact of introducing alien species can be particularly severe for endangered biota; consequences of invasive species introductions are considered either partially or directly responsible for the placement of 35 to 46 percent of the species on the endangered species list. Currently, alien invasive species are the second greatest cause of biodiversity loss, behind human population growth and its related activities .

Additionally, invasive species are financially detrimental; Billions of dollars per annum are expended worldwide to prevent, control and eradicate these alien organisms. The United States Environmental Protection Agency estimates that alien species are responsible annually for $100 billion in damages in the USA alone.

Alien invasive species can have catastrophic effects, threatening biodiversity and causing economic harm. The introduction of alien species often precipitates environmental repercussions which may well be irreversible. Their unpredictable expansion can interrupt or devastate plans for sustainable development in communities without the resources to combat them and may also decimate the local flora or fauna upon which local inhabitants depend. It has only been in the last two decades or so that the negative effects of alien invasive species have been recognized by the international community (see Text Box 1). Their global expansion will continue to increase unless the pathways and mechanisms facilitating their spread are identified and adequate methods of combating them are implemented.

Ballast water as a mode of introduction

The introduction of alien species to a new environment often occurs via "hitchhiking", the unintentional transport of species via airplanes, ships, trucks, packing materials and shipping containers. The ballast water of ships is one principal pathway of this type of introduction, a direct consequence of the rapid worldwide expansion of global trade and shipping. Ballast is any material whose weight is utilized to balance or stabilize an object. A ship takes in water as ballast when its hold is empty for balance and stability and discharges it when it loads new cargo, maintaining equilibrium. It usually picks up ballast water at port, where water is shallow and living organisms, particularly larvae and eggs, abound. Under favorable conditions, these hitchhikers survive their migration to a new destination. When the water is discharged, these species have the potential to become invasive species in their new environment, although decades may pass before they expand and become invasive.

caption Cross section of a ship showing ballast tanks and ballast water cycle. Source: Global Ballast Water Management Programme

The shipping industry is principally responsible for transporting goods internationally, overseeing the trans-shipment of an estimated 80 percent of the world's commodities. Shipping vessels transport approximately ten billion tons of ballast water globally per year. This traffic increases the potential for the accidental transfer of unwanted species; an estimated 3000 species are transferred to new environments each day in ballast water. The International Maritime Organization, a specialized United Nations agency entrusted with decreasing ship pollution and improving maritime safety, contends that the introduction of invasive species is one of the greatest threats to Earth's oceans, alongside marine pollution, overexploitation of marine resources and the physical alteration/destruction of marine habitats. The shipping industry and the ballast water of its vessels clearly play a critical role in the conservation or destruction of global ecosystems.

Zebra mussel and Chinese mitten crab

Zebra mussels (Dreissena polymorpha), native to the Black Sea, Caspian Sea and Azov Seas, were introduced into the Great Lakes in the mid-1980s through the ballast water of vessels from Europe, and have subsequently become one of the most injurious invasive species to affect the USA. In only two decades these mussels have spread from a handful of states adjacent to the Great Lakes to as far as Louisiana. They have even been found as far west as Washington; it is believed that zebra mussels traveled over land via the hulls of ships. The map below depicts the mussels spread from the Great Lakes to the streams and estuaries of neighboring states and Canada.

caption Zebra mussel sightings in the USA. Source: USGS

Zebra mussels have altered the ecosystems of the Great Lakes by reducing phytoplankton, the foundation of the local food chain. They are particularly destructive to unionid mussels, a freshwater mussel native to the Great Lakes and a vital component of North American biodiversity. Zebra mussels compete with the indigenous unionid mussels for food and resources and also prevent unionid mussel reproduction by attaching to them . These impacts have caused an overall decline in unionid mussel abundance and North American biodiversity. Moreover, zebra mussels have negative economic impacts; they attach to and damage the hulls of ships, docks and other structures and clog water intake pipes and irrigation ditches. The introduction of zebra mussels has been referred to as the "poster child" of biological invasions , producing some positive effects. It has served as impetus for U.S. action to combat alien invasive species and brought their destruction of biodiversity and ecosystems to the attention of the international community.

The zebra mussel is only one of many alien invasive species that have negatively affected global ecosystems. Although the zebra mussel features prominently on the IMO "Ten Most Wanted" list of invasive species transported via the ballast water of ships, another of the most wanted, the Chinese mitten crab, has had extremely negative impacts on biodiversity in Europe. The Chinese mitten crab (Eriocheir sinensis), originally a native of east Asia, has spread via the ballast water of ships to Continental Europe, the United Kingdom, and even to the USA, where it has become a classic example of invasive species. Its negative impacts on biodiversity and local industry are many: interference with commercial fishing, soil erosion caused by burrowing or digging, and competition with indigenous fish and invertebrates. Additionally, the Chinese mitten crab directly harms fishing communities and livelihoods by damaging nets and catches. Chinese mitten crabs are also the intermediate hosts for parasitic lung flukes, which can infect humans.

Technological efforts

Although the present technique of ballast water exchange is the safest and most efficient way to discharge a ship's ballast, it is not completely effective (U.S. House 2004). New technologies for handling ballast water are being developed (see Table below), including heat treatment, hydrocyclones, biodegradable chemicals and electrochemical control. While these are all innovative, concerns persist about their effectiveness and economic cost.

Technical methods used to reduce the transfer of harmful aquatic organisms. Source: Adapted from The International Association of Independent Tank Owners, Environmental Issues: Ballast Ballast Water and Invasive Species

Method Description
Ballast Water Exchange This is deemed the most practical method at present and is recommended in most ballast water legislation. The two methods are the sequential method and the flow-through method.
1. Sequential Exchange The ballast tanks are purged of their original ballast and refilled with different ballast water, which hopefully does not contain the unwanted aquatic organisms.
2. Flow Through Exchange Water is pumped into the bottom of the ballast tank via the suction head and overflow water exits through the air pipes and access hatches. Flushing with three tank volumes in this way will displace approximately 95 percent of the original ballast water.
Freshwater Ballast The use of freshwater rather than seawater ballast could provide an inexpensive source of freshwater for irrigation and industrial usage in several major oil-exporting countries. This method has received no further attention.
Heat Treatment Australia has developed this method in which heated salt water from the main engine cooling circuit is rerouted to the ballast water tank. A full-scale test on a bulk carrier proved successful, but INTERTANKO is concerned about the damage that could be caused to ballast tank coatings by the heated water.
Filtration The method utilizes a filter, placed over the ballast water pipe inlet, as a means of controlling organism transferal. During tests using different size filter meshes, the build-up of organisms and sediment on the filtration screen was minimized by using a backwash procedure, but ballasting is inevitably slowed. A secondary treatment, such as the use of ultraviolet light or heat, will probably be necessary to eliminate unwanted organisms like bacteria and viruses. One benefit of filtration is that the amount of sediment taken onboard is reduced.
Hydrocyclones A joint Norwegian/Canadian project uses hydrocyclones, (i.e., centrifugal separators), backed by a secondary ultraviolet light treatment. The former process eliminates the sediments, which are pumped out, while the latter kills the unwanted organism. Such systems, which would cost approximately US$2.5 million per ship, have been used onboard offshore installations for a number of years.
Biodegradable Chemicals Germany is developing a biodegradable ballast water treatment chemical. Approximately 50 litres of the chemical, costing US$150, would be needed to treat 1,000 tonnes of ballast water.
Electrochemical Control Japan is considering an electrochemical control concept in which low potential electric power is applied to ballast water flowing through porous graphite electrodes in order to kill any micro-organisms present. Fullscale tests are planned.

The International Convention for the Control and Management of Ship Ballast Water and Sediments

Over the past decade, the IMO has developed legislative mechanisms to further control the introduction of alien invasive species into coastal waters and elsewhere and to regulate species already introduced. The work of the IMO with the international community has culminated in the "International Convention for the Control and Management of Ships' Ballast Water and Sediments", a multilateral, binding document to regulate ballast water and combat marine alien invasive species. The Convention will enter into force 12 months after 30 nations that represent greater than 35 percent of the world's shipping tonnage have ratified it (BWC Article 18). As of March 2006, there are six Contracting Parties that represent 0.62% of the world's shipping tonnage. They are Maldives, Saint Kitts and Nevis, Spain, Nigeria, Tuvalu and Syria. The IMO website indicates that as of July 2005, Argentina, Australia, Brazil, Finland and the Netherlands had signed but not ratified the Convention.

The Ballast Water Convention creates an integrated regimen for contracting parties to "give full and complete effect to the provisions of this Convention and the Annex" to prevent and eventually eradicate aquatic alien species. States must pass national legislation to implement the Convention's ballast water plan (Article 4(2)) but are encouraged to take more stringent measures (BWC  Article 2(3)). The Convention prescribes ballast water standards, dates of compliance, and ballast exchange procedures, and provides that the standards will be reviewed as technologies improve. These standards must be implemented in all vessels constructed after 2009; vessels built prior to 2009 have until 2014 to 2016 to comply, depending on their size (BWC Regulation B3). Ballast water exchange must take place at 200 nautical miles and not any closer than 50 nautical miles from shore (BWC Regulation B4), minimizing the intake of species.

The new Convention ensures stability, consistency, clarity and uniformity. It fills the gap left by piecemeal domestic legislation (see text box above) and removes conflict between the respective requirements of the States. However, technology has yet to catch up with the Convention's objectives. If effective ballast water removal methods are not created, many States will be in violation of the Convention. Reliance on the development of new technologies may therefore render the Convention ineffective. Some even contend that the measures adopted by the Convention are not stringent enough to be protective. In addition, the fixed dates are set far in advance, and Parties have a decade or more to achieve the standards. If the rate of transfer of aquatic invasive species continues to rapidly increase, the efforts of the Convention may be futile.


The world's biodiversity and ecosystems are vulnerable to acute disturbances. Alien invasive species can disrupt the food chain, with disastrous effects for sustainable development in affected communities. The establishment of the Ballast Water Convention is a promising collective step to combat incursions of alien invasive species. This document sets clear and consistent standards for the international community, and furthermore, provides aspirational goals. However, the Convention will be ineffectual if it never enters into force. Prevention is clearly the best course of action, but if these invasions cannot be completely prevented, the minimization of their impacts is imperative. International cooperation is imperative if alien invasive species are to be reduced or eradicated. The preservation of world ecosystems will safeguard their diversity and the lives of people who depend upon them.

caption Image Source: Wikimedia Commons


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Institute, W. (2012). Alien species transport via ship ballast water. Retrieved from


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