Recognising the potential effects on marine ecosystems from introductions of harmful non-native animals and plants from ballast waters, the IMO has taken action by developing guidelines which aim to minimise the risks of environmental damage, whilst maintaining ship safety. These guidelines were prepared to assist port and harbour authorities and ships’ masters, operators and owners in providing a precautionary approach to the management of ballast water in order to avoid and minimise the risk of introducing harmful non-native species and disease-causing micro-organisms. In the USA, guidelines on the control of the introduction of non-native species by ships' ballast water "stemming the tide" has been prepared by the Committee on Ships' ballast operations of the National Research Council (1996). This guide identifies the safety of the operation as being of paramount importance and provides detailed guidance on the three stages of control options which are:

• On or before departure control is based on preventing or minimising the intake of organisms during the loading of ballast water at the port of origin,
• During the voyage control is based on the removal of viable organisms prior to the discharge of ballast water at the destination port either by treatment or by open ocean ballast water change. Shipboard treatment could commence immediately upon departure and continue throughout the voyage.
• On arrival control at the port of arrival begins when the vessel's master intends to discharge all or some of the ballast water on board. Control strategies are aimed at preventing the discharge of unwanted organisms that could survive in the target environment.

The feasibility of using various control options varies depending on vessel size and type. Technology for the onboard treatment of ballast water is developing, although proven methods are not yet available. The IMO recommendations for action to be taken by ports and harbours today include the following:

• inform local agents and/or ships of areas and situations where uptake of ballast water should be avoided, such as near sewage outfalls, areas known to be contaminated with harmful organisms or in very shallow water where there is a risk of sediment being introduced to the ballast tanks,
• encourage the exchange of ballast water at sea (where it is safe to do so), and
• discourage unnecessary discharge of ballast water.

The arrangements for the control of ballast water transfer will eventually be supervised by classification societies and the MCA through the port state control mechanism, and not by ports. Ballast water management plans are proposed as the main way of implementing these measures in the future, and the discharge of ballast waters to port waste reception facilities has been suggested as a further option to minimise the potential risk of unwanted introductions. An emphasis has also been placed on the promotion of new technology used in ballast water exchange and the possible treatment of ballast waters using various methods, including ultraviolet light or heat to remove disease-causing micro-organisms where necessary. However, it is generally considered impractical and unnecessary for ports to undertake shore-side ballast water treatment at present, although in the future ports may have to provide reception facilities for materials filtered out of ballast waters.

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