Historical Changes in Large River Fish Assemblages of the Americas
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<em>Abstract.</em>—The Virgin–Moapa River system supports nine native fish species or subspecies, of which five are endemic. Woundfin <em>Plagopterus argentissimus </em>and Virgin River chub <em>Gila seminuda </em>are endemic to the main-stem Virgin River, whereas cooler and clearer tributaries are home to the Virgin spinedace <em>Lepidomeda mollispinis</em>. Moapa dace <em>Moapa coriacea </em>and Moapa White River springfish <em>Crenichthys baileyi moapae </em>are found in thermal springs that form the Moapa River, and Moapa speckled dace <em>Rhinichthys osculus moapae </em>is generally found below the springs in cooler waters. The agricultural heritage of the Virgin–Moapa River system resulted in numerous diversions that increased as municipal demands rose in recent years. In the early 1900s, trout were introduced into some of the cooler tributary streams, adversely affecting Virgin spinedace and other native species. The creation of Lake Mead in 1935 inundated the lower 80 km of the Virgin River and the lower 8 km of the Moapa River. Shortly thereafter, nonnative fishes invaded upstream from Lake Mead, and these species have continued to proliferate. Growing communities continue to compete for Virgin River water. These anthropogenic changes have reduced distribution and abundance of the native Virgin–Moapa River system fish fauna. The woundfin, Virgin River chub, and Moapa dace are listed as endangered, and the Virgin spinedace has been proposed for listing. In this paper we document how the abundance of these species has declined since the Endangered Species Act of 1973. Currently, there is no strong main-stem refugium for the Virgin River native fishes, tributary refugia continue to be shortened, and the Moapa River native fishes continue to be jeopardized. Recovery efforts for the listed and other native fishes, especially in the Virgin River, have monitored the declines, but have not implemented recovery actions effective in reversing them.


<em>Abstract.</em>—In this paper, we review information regarding the status of the native fishes of the combined Sacramento River and San Joaquin River drainages (hereinafter the “Sacramento–San Joaquin drainage”) and the factors associated with their declines. The Sacramento–San Joaquin drainage is the center of fish evolution in California, giving rise to 17 endemic species of a total native fish fauna of 28 species. Rapid changes in land use and water use beginning with the Gold Rush in the 1850s and continuing to the present have resulted in the extinction, extirpation, and reduction in range and abundance of the native fishes. Multiple factors are associated with the declines of native fishes, including habitat alteration and loss, water storage and diversion, flow alteration, water quality, and invasions of alien species. Although native fishes can be quite tolerant of stressful physical conditions, in some rivers of the drainage the physical habitat has been altered to the extent that it is now more suited for alien species. This interaction of environmental changes and invasions of alien species makes it difficult to predict the benefits of restoration efforts to native fishes. Possible effects of climate change on California’s aquatic habitats add additional complexity to restoration of native fishes. Unless protection and restoration of native fishes is explicitly considered in future water management decisions, declines are likely to continue.


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<em>Abstract.</em>—The Willamette River is Oregon’s largest river, with a basin area of 29,800 km<sup>2</sup> and a mean annual discharge of 680 m<sup>3</sup>/s. Beginning in the 1890s, the channel was greatly simplified for navigation. By the 1940s, it was polluted by organic wastes, which resulted in low dissolved oxygen concentrations and floating and benthic sludge deposits that hindered salmon migration and boating. Following basin-wide secondary waste treatment and low-flow augmentation, water quality markedly improved, salmon runs returned, and recreational uses increased. However, water pollution remains a problem as do physical habitat alterations, flow modification, and alien species. Fish assemblages in the main-stem Willamette River were sampled systematically, but with different gear, in the summers of 1945, 1983, and 1999. In the past 53 years, tolerant species occurrences decreased and intolerant species occurrences increased. In the past 20 years, alien fishes have expanded their ranges in the river, and four native fish species have been listed as threatened or endangered. We associate these changes with improved water quality between 1945 and 1983, fish migrations, altered flow regimes and physical habitat structure, and more extensive sampling.


<em>Abstract.</em>—The Rio das Velhas is a tributary of the Rio São Francisco, one of Brazil’s largest rivers. It is the Rio São Francisco’s second most important tributary in water volume (mean annual discharge of 631 m<sup>3</sup>/s), with a drainage area of 27,867 km<sup>2</sup>, length of 761 km, and mean width of 38 m. Like many other rivers around the world, it became heavily polluted in the 1900s. The Rio das Velhas is the most polluted river of Minas Gerais state because the basin contains approximately 4.5 million people. Unlike other Brazilian rivers, its fish fauna was studied from 1850 to 1856. Fifty-five fish species were recorded; 20 of them were first described at that time, when there were previously no more than 40 known species in the entire São Francisco basin. Recent fish collections, approximately 150 years later, indicate 107 fish species, but some may be locally extinct. There are good prospects of rehabilitating this fauna because of the connectivity of the Rio das Velhas with the São Francisco main stem, its well-preserved tributaries, and increased investments in sewage treatment.


<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.


<em>Abstract.</em>—The Patoka River drainage is a lowland-gradient watershed of the Wabash River lowlands in southwestern Indiana. During the late 18th century, the river was part of an extensive riparian floodplain wetland that connected the White River with the lower Wabash River. Through anthropogenic changes as a result of ditching, channelization, levee creation, coal extraction, and oil and gas exploration, the Patoka River drainage has been highly altered. These changes have resulted in a loss of sitespecific biological diversity and integrity, causing drainage-wide biological diversity decline. Extirpations in the watershed have resulted in the local loss of 12.7% of the fish fauna during the last century. The local extirpations of six species included central mudminnow <em>Umbra limi</em>, black redhorse <em>Moxostoma duquesnei</em>, brindled madtom <em>Noturus miurus</em>, bluebreast darter <em>Etheostoma camurum</em>, slenderhead darter <em>Percina phoxocephala</em>, and saddleback darter <em>P. vigil</em>. Black redhorse, bluebreast darter, slenderhead darter, and saddleback darter were only known from pre-1900, while brindled madtom and central mudminnow were known until the early 1940s. These species may have been rare to begin with in the Patoka River drainage, but since they are widespread elsewhere, it seems more probable that they disappeared as a result of the land-use changes. Sensitive species of darters and minnows have declined in abundance, but recent sampling has shown that they remain in the watershed at low abundance. Based on a probability sample, less than 12% of the channels represented reference least-disturbed conditions, while 61% exhibited degraded conditions.


<em>Abstract.</em>—The Mackenzie River is the second longest river in North America and drains 1.8 × 10<sup>6</sup> km<sup>2</sup>. of Arctic and sub-Arctic Canada. Thirty-eight fish species have been recorded in the lower Mackenzie River. These species represent a unique mixture of fishes from the Beringian and Agassisian refugia. Many of the species important for subsistence and commercial fisheries in the lower Mackenzie River have complex life cycles and undertake long migrations to spawn, rear, and overwinter. The lower Mackenzie River is a relatively pristine environment with no dams or major industry, a low human population, and species only lightly harvested. This explains why the species composition is relatively stable. However, recently, the effects of climate change may be starting to influence the species composition in terms of greater frequency of rare species such as Pacific salmon. Moreover, a major gas pipeline proposed for the lower Mackenzie River region will probably disturb the fish assemblage structure.


<em>Abstract.</em>—An investigation of historical fisheries information for pools 4–13 of the upper Mississippi River (UMR) was conducted to 1) determine the pre-1938 relative abundance and distribution of bluegill <em>Lepomis macrochirus </em>and largemouth bass <em>Micropterus salmoides, </em>2) determine the composition and relative abundance of the preimpoundment fish assemblage, and 3) determine if a shift in frequency of occurrence and relative abundance has occurred due to impoundment.


<em>Abstract.</em>—From its headwaters in the Rocky Mountains, the Platte River drains 230,362 km<sup>2</sup> in Colorado, Wyoming, and Nebraska. The Platte River is formed by the confluence of the North Platte and South Platte near the city of North Platte, Nebraska, and receives additional flow from the Loup and Elkhorn rivers that drain the Sand Hills region of Nebraska. Water diversions for mining and irrigation began in the 1840s in Colorado and Wyoming, and irrigation diversions in Nebraska began in the 1850s. Construction of dams for control of river flows commenced on the North Platte River in Wyoming in 1904. Additional dams and diversions in the North Platte, South Platte, and Platte rivers have extensively modified natural flow patterns and caused interruptions of flows. Pollution, from mining, industrial, municipal, and agricultural sources, and introductions of 24 nonnative species have also taken their toll. Fishes of the basin were little studied before changes in land use, pollution, and introduction of exotic species began. The current fish fauna totals approximately 100 species from 20 families. Native species richness declines westward, but some species find refugia in western headwaters streams. Declines in 26 native species has led to their being listing as species of concern by one or more basin states.


<em>Abstract.</em>—The interior Río Nazas basin is located in arid north-central México. It is an interior drainage, subject to dewatering since the early 20th century, and sustains wide fluctuations in runoff. It drains 85,530 km<sup>2</sup> and has a major dam in the middle reaches, producing a highly controlled river, with 100% consumption for agriculture and urban use. Hydrologic gauge reports at Torreón from the Comisión Nacional del Agua indicate a 10-year average runoff of 581.9 million m3 from 1936 to 1945, and only 66.4 million m3 in 1972, the last year of recorded runoff. Its 13 known native fish species are of Rio Grande/Rio Bravo origin. Eleven are endemic to the basin complex (only one absent from the study area), seven species have been listed by the Mexican federal government as threatened or endangered, and three are undescribed. The basin has 13 invasive alien species. An index of biological integrity (IBI), based on historical data, was applied to the current fish assemblage at 10 localities in the lower basin, below El Palmito reservoir. The IBI ranged from 50 to 57 at sites in the northern branch, to 39–61 in the southern branches, and to 0–57 from below their junction to the lower reaches, and averaged 37 or very poor. The overall biotic integrity is very low, especially near reservoirs and in the lower reaches of the river, where human activities consume all available water. The main causes of fish loss from this interesting fish fauna are alien invasive species, habitat disruption, pollution, and dewatering.


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