First evaluation of ballast water management systems on operational ships for minimizing introductions of nonindigenous zooplankton.

Ballast water is a leading pathway for the global introduction of aquatic nonindigenous species. Most international ships are expected to install ballast water management systems (BWMS) by 2024 to treat ballast water before release. This study examines if ballast water discharges managed by BWMS are meeting standards for organisms ≥50 μm in minimum dimension (i.e., <10 organisms per m³; typically zooplankton). Representative samples of ballast water were collected from 29 ships (using 14 different BWMS) arriving to Canada during 2017–2018. Fourteen samples (48 %) had zooplankton concentrations clearly exceeding the standard (ranging from 18 to 3822 organisms per m³). Nonetheless, compared to earlier management strategies, BWMS appear to reduce the frequency of high-risk introduction events. BWMS filter mesh size was an important predictor of zooplankton concentration following treatment. Greater rates of compliance may be achieved as ship crews gain experience with operation and maintenance of BWMS. • Ballast water is a main pathway for introduction of aquatic nonindigenous species. • Ships are using new ballast water management systems (BWMS) to treat ballast water. • Nearly 50 % of ballast water samples exceed treatment standards after using BWMS. • However, the frequency of high-risk introductions has been dramatically reduced. [ABSTRACT FROM AUTHOR]