An Assessment of the Efficiencyof Ballast Water Exchange

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Ballast water exchange (BWE) has been used for a number of years as a method of reducing the risk of introducing non-native species via ballast water but there have been very few studies to assess the efficiency of this method. There is likely to be a continued reliance on ballast water exchange as the Ballast Water Convention agreed by the International Maritime Organisation (IMO) in 2004 outlines a phase-in period. During this time BWE will be used until alternative technologies are fitted to vessels in order to meet a treatment standard. In order to ensure that the environmental impact of introducing non-native species is reduced it is necessary to examine whether open ocean BWE is an efficient method at reducing the abundance and diversity of organisms within the ballast water. A study carried out by the FRS Marine Laboratory collected samples from a vessel before and after an exchange process on twelve different voyages. A generalized linear model (GLM) was used to assess which factors (e.g. method of exchange, season, and location of exchange) influence the outcome of exchange.