<em>Abstract.</em>—The Hudson River Estuary (defined here as the Hudson River drainage and New York Harbor) is home to a large and diverse ichthyofauna. Estimates of species richness reflect both their geographic boundaries and time periods. The most complete estimate is for the Hudson River drainage north of the southern tip of Manhattan, where, as of 2005, 212 fish species have been recorded. This includes 11 new forms not reported in the most recently published tally (1990). We categorize the fishes of the Hudson River drainage as derived from 12 zoogeographic or anthropogenic sources (including species for which we make no judgment [<em>n </em>= 26]), the largest contributions from which include temperate marine strays (<em>n </em>= 65), introduced species (<em>n </em>= 28), and freshwater species that survived Pleistocene glaciations in Atlantic coastal refugia (<em>n </em>= 21). Additional species appear to have invaded from the Mississippi refugia, some naturally (<em>n </em>= 11) and some later, via canals (<em>n </em>= 11). Only ten diadromous fishes occur in the estuary, but many of these are, or have been, commercially and recreationally important (e.g., Atlantic sturgeon <em>Acipenser oxyrinchus</em>, American shad <em>Alosa sapidissima</em>, and striped bass <em>Morone saxatilis</em>). Extremely high seasonal temperature changes in the main-channel Hudson River foster a seasonally dynamic ichthyofauna with relatively few species occurring year round. However, the small number of resident estuarine fishes (<em>n </em>= 8) often occur in high abundances. Species richness peaks between June and September and reaches a minimum in winter. Long-term data indicate that although species richness has increased with the additions of new species, diversity is decreasing because of the decrease in population size of certain species, especially native cyprinids. The Hudson estuary hosts a population of one federally endangered species, shortnose sturgeon <em>Acipenser brevirostrum</em>, which is flourishing. Only one species, the anadromous rainbow smelt <em>Osmerus mordax </em>appears to have become extirpated in the Hudson Estuary.
Salt-front movement in the Hudson River estuary, New York—Simulations by one-dimensional flow and solute-transport models