Research on the ecology and impacts of the zebra mussel (Dreissena polymorpha) has developed in parallel with the long history of their spread across Europe and to North America. There is less scientific knowledge however about the quagga mussel (Dreissena rostriformis bugensis). The recent spread of both zebra and quagga mussels to the Western states of the USA, has spurred on dreissenid research. The 17th International Conference on Aquatic Invasive Species (ICAIS), was held in San Diego, California in September 2010. This special edition of Aquatic Invasions includes topical research presented at ICAIS and also extra subsequent papers on ecology, risk assessment and control of various aquatic invasive species in different parts of the globe.

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The ballast water from ships carries marine organisms that have invasive potential. The International Maritime Organization Ballast Water Management Convention (2004) requires ballast water exchange or ballast water management (BWM) systems either onboard or ashore. Ships can be exempted on the basis of risk assessment, when exclusively sailing between specific ports or in an enclosed area. In reply to our questionnaire, the shipping sector argues that the North Sea is ecologically homogeneous and exemptions could therefore be granted. This paper proposes that the North Sea area is, in fact, not homogeneous in terms of hydrographical and biological conditions; therefore, ballast water is a relevant transport mechanism for organisms. Within the North Sea, the short shipping routes indicate a high risk for survival. We examined actual simulation models for ballast water risk assessment in the North Sea, and we have identified the major parameters that need to be included in such models. These models provided a basis; they further need to be combined and adapted for the purpose of evaluating the rationale for an exemption. We concluded that exemptions from BWM are not recommended for the North Sea area. Anticipating the Ballast Water Management Convention, ship owners might do well to study possibilities for installing BWM systems onboard.