<em>Abstract.</em>—The main channel of the Hudson River is a tidal estuary from its mouth in New York Harbor to Troy, New York, 247 km upstream. It drains about 35,000 km<sup>2</sup> and is an important navigational, commercial, and recreational system. Since the arrival of European settlers over 400 years ago, it has undergone numerous environmental changes. These changes have included channel maintenance by dredging, wholesale dumping of industrial and domestic wastes, scattered in-basin urbanization and shoreline development, deforestation of the watershed and an increase in agriculture, and water removal for commercial, industrial, and agricultural needs. In addition, the biota of the river has supported commercial and recreational harvesting, exotic species have become established, and habitats have become fragmented, replaced, changed in extent, or isolated. The tidal portion of the Hudson River is among the most-studied water bodies on Earth. We use data from surveys conducted in 1936, the 1970s, the 1980s, and the 1990s to examine changes in fish assemblages and from other sources dating back to 1842. The surveys are synoptic but use a variety of gears and techniques and were conducted by different researchers with different study goals. The scale of our assessment is necessarily coarse. Over 200 species of fish are reported from the drainage, including freshwater and diadromous species, estuarine forms, certain life history stages of primarily marine species, and marine strays. The tidal Hudson River fish assemblages have responded to the environmental changes of the last century in several ways. Several important native species appear to be in decline (e.g., rainbow smelt <em>Osmerus mordax </em>and Atlantic tomcod <em>Microgadus tomcod</em>), others, once in decline, have rebounded (e.g., striped bass <em>Morone saxatilis</em>), and populations of some species seem stable (e.g., spottail shiner <em>Notropis hudsonius</em>). No native species is extirpated from the system, and only one, shortnose sturgeon <em>Acipenser brevirostrum</em>, is listed as endangered. The recent establishment of the exotic zebra mussel <em>Dreissena polymorpha </em>may be shifting the fish assemblage away from openwater fishes (e.g., <em>Alosa</em>) and toward species associated with vegetation (e.g., centrarchids). In general, the Hudson River has seen an increase in the number and importance of alien species and a change in dominant species.
Changes in phytoplankton community structure during the zebra mussel (Dreissena polymorpha) invasion of the Hudson River (New York)
