Immediately following the passage of a tropical cyclone, aquatic organisms may experience localized mortalities in association with high nutrient loading and subsequent periods of hypoxia (low oxygen levels). Long-term storm effects often include widespread alterations in the distribution, abundance, and movement of mobile aquatic species. Freshwater species may move upstream as storm surge brings high salinity water into the area; species that tolerate low salinities may vacate areas impacted by heavy rainfall (which would decrease salinity), and move downstream into more saline waters. Rising waters may also flood areas that usually do not contain water. After water levels recede, fish and other aquatic animals may become trapped and/or stranded, and die. Recovery following tropical cyclones is usually rapid, however, with a return to pre-hurricane numbers and distribution occurring within a matter of months. Providing that there has not been a loss of critical habitat, animals are able to rapidly repopulate the areas from which they had been displaced.