Reservoir impairment

Understanding aging fisheries

Reservoirs — impounded rivers — in the southern United States provide much of the recreational fishing opportunities anglers enjoy.

Unfortunately, most of those reservoirs were built between the 1930s and the 1970s, and they are aging rapidly.

The impoundments in Mississippi are not exempt from accelerated senescence caused by many factors, including sedimentation, accumulation of nutrients and loss of cover. Habitat is the foundation of fisheries production, and habitat in many reservoirs is deteriorating.

Fisheries managers have worked hard to provide quality fishing opportunities by using progressive harvest regulations, yet fish production in some reservoirs has declined and many others will soon follow.

The simple fact is that no harvest regulation, stocking program or even catch and release can produce sustained quality fishing in an impaired environment.

Dr. Steve Miranda, fisheries scientist with the U.S. Geological Survey and professor in the Department of Wildlife, Fisheries and Aquaculture at Mississippi State University, and former graduate student Rebecca Krogman — now a fisheries biologist with Iowa Department of Natural Resources — took on the monumental task of assessing the impairment status of reservoirs nationwide.

The first step was identifying all possible causes of impairment. Drawing on previous research and their own extensive experience with reservoirs, the Krogman-Miranda team identified 52 different impairments.

Fishery biologists responsible for a selection of reservoirs in each state were then asked to provide a score for each impairment item using a 5-point scoring system in which 0 was no impairment and 5 was high impairment.

Statistical analyses of these responses allowed the scientists to assign each reservoir a rating score, with zero indicating no impairment and a maximum impairment score of 60.

Ratings ranged from 0 to 46 among almost 1,300 reservoirs, and most reservoirs had impairment ratings of 10 to 25.

Among 27 reservoirs in Mississippi for which impairment data were available, Ross Barnett, the four flood control reservoirs and three reservoirs on the Tenn-Tom Waterway had impairment ratings of 27 to 46.

Eight reservoirs in the southern half of Mississippi had impairment ratings of less than 12. The other 11 reservoirs scored 12 to 27 ratings.

Impairment factors that resulted in high impairment ratings were not available for individual states but were compiled by large ecological regions.

Mississippi is in the Coastal Plains ecoregion. Other states in the Coastal Plains includes eastern Texas, Louisiana, southern and eastern Arkansas, southern Alabama and Georgia, and eastern seaboard states from South Carolina to Virginia.

Sedimentation, excessive mud flats and shallowness were considered high impairments in about 25 percent of Coastal Plains reservoirs.

Other common impairments included excessive nutrients, limited connectivity to adjacent habitats such as backwaters and tributaries, limited shallow-water structure and large water fluctuations.

Point-source and non-point-source pollution were major impairments in only a few Coastal Plains reservoirs.

There is no single impairment rating score at which a fishery collapses, but high impairment scores suggest a reservoir where habitat conditions are likely to adversely affect fishing quality at present or in the future.

Most of the severe impairments are a consequence of activities outside the reservoir. Sedimentation and its consequences — excessive mud flats and shallow water, loss of connections to backwaters and tributaries — as well as excessive nutrients result from poor land management in the watershed.

Only the scarcity of shallow-water structure is something fisheries managers can affect by habitat improvement activities.

Fisheries managers are working hard to sustain productive and enjoyable fisheries in Mississippi’s reservoirs. Already, many reservoirs need some form of habitat restoration to achieve the fishing quality they provided anglers when they were new.

Restoration activities, such as planting vegetation in the water fluctuation zone in the flood control reservoirs, are underway.

But restoration activities will be short lived without changes in use and management of the larger landscape of which reservoirs and the recreation they provide are just one component.