The U.S. Geological Survey Mississippi Cooperative Fish and Wildlife Research Unit surveyed habitat impairment in U.S. reservoirs and developed a tool for rating reservoirs relative to fish habitat status. The habitat ratings provide a measure of the overall habitat impairment status of a reservoir and can be used for rapid assessment and for comparing reservoirs. The ratings provide a method to quickly identify high-quality reservoirs for protection and degraded reservoirs for rehabilitation. Comparison of ratings among reservoirs facilitates prioritization of restoration activities.

As the median age of U.S. reservoirs surpasses 70 years, degradation of fish habitat has become a serious concern. Habitat issues such as sedimentation, excessive nutrient loadings, and lack of submerged structure may emerge and worsen over time, and are accompanied by undesirable shifts in reservoir fish assemblages and fisheries. These shifts can have important conservation, recreation and economic implications. Some reaches of reservoirs can support distinct and speciose fish assemblages that help conserve fish in upstream river reaches. According to a 2011 survey, exclusive of the Great Lakes, 84 percent of freshwater recreational anglers (23 million) fish in reservoirs and lakes generating $22.8 billion in retail sales. Nevertheless, the extent, distribution, and characteristics of reservoir habitat decay, and their effect on fisheries, were previously undocumented at large geographical scales.

The ratings developed in this new research captured a variety of fish habitat impairments in reservoirs nationwide, with relative importance of specific impairment components changing spatially across wide geographic areas. Impairments such as sedimentation and nonpoint source pollution were widespread, affecting all or nearly all regions to different degrees, and were often associated with impairments generally attributed to inputs from upstream watersheds. Impairments such as point-source pollution and limited nutrients were identified in relatively few reservoirs.

Impairment due to large water level fluctuation was most common in the dryer areas of the contiguous U.S., including the West and Great Plains. Water is scarcer in these areas and is typically impounded for irrigation ? water levels may fluctuate widely as incoming water is stored (a) during the rainy season and released throughout the growing season, or (b) in wet years and released in dry years. The water storage and allocation required to optimize water availability for irrigation can often conflict with the needs of fish in a reservoir by altering environmental cues or seasonal habitat availability. Large water level fluctuation was also the most important impairment in the Northeast; however, the extent of this impairment was relatively lower than in other regions.

Unlike the West, most habitat impairments in the Midwest and South emphasized factors related to incoming water quality and land management in the reservoir's watershed, rather than water storage. Excessive nutrient input was the most important impairment in the Midwest, followed by siltation and nonpoint source pollution. Runoff from agricultural land contributes to all of these impairments. In the South, siltation and mudflats/shallowness were the most important impairments, while an excessive nutrient input was less important. This coincides with a lesser land coverage by traditional agricultural and greater land coverage by timber.

The ratings reflected a wide range of fish habitat issues that transcend those that can be readily measured during onsite quantitative surveys. Many of the habitat descriptors included in the survey measured constructs that are not typically quantified during onsite surveys, providing new perspective on reservoir fish habitat. Elements such as sediment and nutrient loading, resultant habitat diversity loss, loss of connectivity to adjacent habitats such as backwaters, water storage patterns, algal blooms and nuisance species may not be captured by most time-limited onsite water quality and quantitative habitat surveys. The Mississippi Co-op Unit's rating tool tapped into observational experience accumulated by field biologists. While there are limitations associated with relying on this type of knowledge, improvement in assessment accuracy by upgrading to objective onsite quantitative habitat surveys would require a substantial rise in cost, potentially without parallel increases in evaluation accuracy.

Regional differences in habitat impairment sources can inform effective allocation of funding for fish habitat protection and rejuvenation. For example, a nonprofit organization like the Reservoir Fish Habitat Partnership, which provides funding for fish habitat improvement for reservoirs across the U.S., has been using these ratings to develop region-specific priority impairments to allocate their restoration dollars. A habitat improvement project that addresses priority impairments specific to a region is given funding precedence over projects that do not address priority impairments. The rating tool as a whole provides a national snapshot of fish habitat in large reservoirs, enabling objective comparison of a wide variety of reservoirs for decision-making about national, regional and local habitat management strategy.

Although most large reservoirs were not constructed to benefit fish or fishing, reservoir fish habitat conservation is essential for maintaining the health of river systems. Most reservoirs are impounded rivers. Native riverine species may persist in the upstream portion of a reservoir where they rely on riverine conditions and on floodplains of the original river now periodically inundated by reservoir water level fluctuations. Fragmentation of riverine habitat by sedimentation processes changes fish community structure in the fragmented portion relative to the rest of the reservoir, and loss of access to tributaries and related spawning and nursery habitats can negatively affect the reservoir fish community. Connectivity to tributaries allows persistence of riverine species in a reservoir, including potamodromous fish that migrate upstream to spawn (e.g., walleye, paddlefish, pallid sturgeon). The long-term integrity and resilience of impounded river ecosystems depend on fish habitat conservation in impounded sections of the river.

Funding for this research was provided by the Multistate Conservation Grant Program through the Reservoir Fish Habitat Partnership. The report that outlines the rating system for reservoirs relative to fish habitat condition is in press; an abstract is available for an article that appeared in the March 2015 journal Environmental Monitoring and Assessment on the reservoir classification system.

The ONB features articles from Cooperative Fish and Wildlife Research Units across the country. Working with key cooperators, including WMI, Units are leading exciting, new fish and wildlife research projects that we believe our readers will appreciate reading about. This article was written by Leandro (Steve) Miranda, Assistant Unit Leader at the USGS Mississippi Cooperative Fish and Wildlife Research Unit at Mississippi State University.