Reducing propagule supply and coastal invasions via ships: Effects of emerging strategies

Ships' ballast water is a leading mechanism for the transport and introduction of nonindigenous species to ports worldwide. Two management strategies are being advanced to reduce propagule supply and invasions from overseas shipping. Ballast water exchange (BWE) is now required by several nations and is expected to be replaced by discharge standards (maximum organismal concentrations), negotiated as a treaty within the International Maritime Organization (IMO). Here, we provide the first forecast and comparison of changes to propagule supply at a national scale, resulting from these alternate management strategies. For unmanaged ballast water, sampled ships (n = 354) arriving to the US typically contained zooplankton concentrations < 3000 organisms m^-3, but some ships (1.1%) contained > 50 000 organisms m^.3. Only 3.8% of these arrivals meet the IMO standards. BWE substantially reduces zooplankton concentrations, but we estimate that ≤ 17.2% of BWE ships will meet IMO standards. Although most overseas arrivals discharged < 1500 m³ of ballast water, discharges are reported as high as 103 000 m³, and total inocula ≥ 10⁶ remain possible, even under the more stringent IMO strategy.