Effect of osmotic shock as a management strategy to reduce transfers of non-indigenous species among low-salinity ports by ships

Open-ocean ballast-water exchange (BWE) is currently the most common treatment used to reduce the ballast transfer of organisms and the subsequent risk of invasions among coastal ecosystems. Freshwater or estuarine organisms remaining after BWE often experience high mortality, due to osmotic shock caused by high-salinity exposure. We conducted 70 salinity tolerance experiments on 54 different taxa to measure mortality rates of freshwater and estuarine organisms after exposure to oceanic seawater (34 psu), simulating both flow-through (F-T) and empty-refill (E-R) BWE methods. We focused especially on larval and adult crustaceans from freshwater and mesohaline habitats adjacent to ports of the Baltic Sea, North Sea, Great Lakes, Chesapeake Bay, and San Francisco Bay. Animals from oligohaline habitats (0-2 psu) experienced the highest mortality: all individuals died in 82% of the F-T treatments and 88% of the E-R treatments. The effectiveness of both treatment types decreased with animals from low-salinity (2-5 psu, 100% mortality in 27% of F-T and 46% of E-R treatments) and mesohaline habitats (5-18 psu, 100% mortality in 40% of F-T and 52% of E-R treatments). In 43% of cases among all salinity categories, empty-refill treatments required less exposure time to cause significant mortality than flow-through treatments. Invertebrates that exhibited significant survivorship were most often peracarid crustaceans including widely introduced species of mysid shrimps and amphipods. Although salinity shock does not completely prevent the transfer of all low-salinity biota, BWE provides a useful management tool to reduce species transfers, especially considering the combined effects of removal and mortality.