Managing risk of non-indigenous species establishment associated with ballast water discharges from ships with bypassed or inoperable ballast water management systems

Ballast water is recognized as a leading pathway for the introduction of aquatic non-indigenous species which have caused substantial ecological damage globally. Following international regulations, most international ships will install a ballast water management system (BWMS) by 2024 to limit the concentration of aquatic organisms in ballast water discharges; however, these new technologies may not operate as expected at global ports having variable water quality or may periodically malfunction. Using simulations informed by empirical data, we investigated the risk of non-indigenous species establishment associated with BWMS inoperability and evaluated potential mitigation strategies. Scenarios considered included bypassed or inoperable BWMS achieving no reduction in organisms, and partially functioning BWMS with discharged organism concentrations exceeding permissible limits. These scenarios were contrasted to outcomes with fully functioning BWMS and to voyages where ballast water exchange (BWE) was used to mitigate risk. Partially functioning BWMSs were nonetheless beneficial, reducing organism concentrations in ballast and thus establishment risk. When a BWMS is bypassed or partially functioning, BWE is a useful emergency mitigation measure, reducing establishment risks more than partial BMWS. However, the greatest risk reduction was achieved when partial BWMS and BWE were combined. Voyage-specific characteristics such as concentration of organisms at uptake and destination port salinity can affect the optimal management strategy for voyages when the BWMS does not achieve compliant discharges. Synthesis and applications. The risk of aquatic invasions and their associated ecological damages can be substantially reduced by using a ballast water management system (BWMS) and/or ballast water exchange (BWE). When a BWMS is inoperable, appropriate mitigation measures should be decided on a trip-by-trip basis considering voyage route and reason for BWMS inoperability (when known). BWE is a useful strategy for reducing invasion risk, except when uptake concentrations are very low. Combining BWE and partial BWMS always reduced risk compared with BWE alone, but did not greatly reduce risk when uptake concentrations were high.