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.

Propagule pressure plays a key role in the successful establishment of introduced species. Explaining invasion patterns, predicting future invasions and reducing invasion rates are priority areas of research and management, especially in marine systems, which need more detailed correlates and invasion predictors. The commercial maritime shipping fleet is the most prolific long distance anthropogenic transfer mechanism (vector) of marine non‐indigenous species on a global scale, causing invasions of coasts by a wide diversity of organisms. Although most vessel arrivals provide an opportunity for organism introductions, there are often substantial differences among ship types—in both their “morphological traits” (structural design) and “behavioural ecology” (cargo delivery model and operational tempo)—that influence propagule delivery by ballast water and biofouling, the two dominant sources or sub‐vectors for ship‐mediated species transfers. We reviewed ship specialization and its implications for marine invasion and vector management. First, we identified factors that affect ship‐mediated propagule delivery characteristics (number, identity, diversity and quality/condition), classifying these as ship type independent or dependent factors. Second, we compared the relevance of these factors for both ballast water and biofouling. Third, we estimated and compared the magnitude of several key factors affecting propagule delivery among seven major ship types. Typical voyage speed varies by 74% and port residence time varies sixfold among ship types. Similarly, typical ballast water discharge varies by an order of magnitude among ship types. These and other ship type dependent factors affect propagule delivery characteristics, resulting in uneven magnitude of species transfer among ship types. Policy implications. Variation among commercial ship types is rarely integrated into analyses of marine bioinvasions and proxy measures of propagule delivery. Their inclusion may lead to more robust explanation, prediction and management of marine invasions. Risk analyses that account for differences among ship types and prevailing traffic directionality will likely offer greater insight than null models, which treat ships equally. Furthermore, ballast treatment technologies and hull husbandry may advance to reduce species transfers more effectively when tailored for different ship types, recognizing the variation and operational constraints (that affect propagule delivery) among the diverse range of ship types.

Aim The Panama Canal expansion, scheduled for completion in 2015, is expected to have major effects on commercial shipping and port operations throughout the world, with potential consequences for the transfer and establishment of non-indigenous species that remain largely unexplored. We developed a series of scenario-based models to examine how shipping traffic patterns may change after expansion and consider possible implications for species transfers and invasion dynamics in the USA. Location Coastal USA, excluding Alaska and HawaiiMethods Using a Monte Carlo simulation approach, we predicted changes in discharged ballast water, wetted surface area of ship hulls and frequency of ship arrivals modelled under scenarios that are based on (1) current shipping patterns from the western Pacific Rim to the USA, (2) estimates of fleet expansion and (3) diversion of traffic away from the US West Coast through the Panama Canal.Results During the 5-year period following canal expansion (2015-2019), our models estimated that the Gulf and East coasts would receive 78% and 99% median increases in total ballast discharge and 172% and 182% increases in total wetted surface area, respectively. For the West Coast, our models estimated 9.6% median decreases in both total ballast discharge and wetted surface area. We further predict that many ports in the Gulf and East coasts will receive up to three times the current number of arrivals and increased ballast water discharge, from this region after expansion.Main conclusions Our scenario-based analysis provides a first estimate for increases in frequency, magnitude and spatial distribution of exposure that the Gulf and East coasts will experience due to ships and ballast arriving from the western Pacific, following the canal expansion. If organisms transported via ballast water or ship hulls are able to survive transit of the canal, the predictions suggest increased likelihood of introduction along these coasts by species originating in the western Pacific.

Marine species arc in constant motion in the ballast water and on the hulls of the ships that ply the world's oceans; ships serve as a major vector for biological invasions. Despite federal and state regulations that require ballast water exchange (BWE), partictilar trade routes impose geographic and temporal constraints on comphance, limiting whether a ship can condtict BWE at the required distance (>200 nautical miles) from shore to minimize transfers of coastal organisms. Ships moving across the Americas are largely unable to conduct open-ocean BWE, but instead often conduct exchanges inside coastal waters. Overall, strong differences exist in volumes, geographic sources, and the use of BWE for ballast water discharge among the three major coasts of the contiguous United States. Such patterns suggest important geographic differences in invasion opportunities and also argue for more effective alternative ballast water treatments that can be apphed more evenly.

Ballast water is recognized globally as a major vector of aquatic nonindigenous species (NIS) introductions; domestic ballast water transfers, however, have generally been considered low risk in North America. We characterize ballast operations of domestic ships in the Great Lakes -St. Lawrence River system (Lakers) during 2005-2007 to examine the risk of primary and secondary introductions associated with ballast water transfers over short distances. Results indicate that Lakers transported at least 68 million tonnes of ballast water annually. Approximately 71% of ballast water transfers were interregional, with net movement being from lower to upper lakes. A small proportion of ballast water discharged in the Great Lakes (<1%) originated from ports in the St. Lawrence River that may serve as sources for new NIS. These results indicate that domestic ballast water transfers may contribute to NIS introductions and are likely the most important ballast-mediated pathway of secondary spread within the Great Lakes. Future efforts to reduce invasion impacts should consider both primary and secondary introduction mechanisms.