Rob Neumann, Associate Professor
Department of Natural Resources Management and Engineering
University of Connecticut

Annual variation in precipitation seems to be the rule rather than the exception in New England, and fish are generally adapted to living under such conditions. However, extended periods of drought, depending on the severity, can have several impacts on fishes in streams and lakes from both a water quantity and water quality perspective. During a normal spring season, high flows provide stream channel maintenance such as scouring and sediment transport. Fishes rely on regular channel maintenance for the creation of habitats, from spawning to adult life stages.

In the simplest case, the quantity of water in a stream can be substantially reduced during drought years, leading to direct reductions in living space for various life stages of fish species. During low-water years, important spawning areas might be left dry, and fishes inhabiting the stream may be forced into small pockets of water in pools. Cover, such as woody debris and vegetation, may not be accessible to juvenile fishes making them susceptible to predation. Fish that are crowded into smaller living spaces may also confront increased competition for food and other resources. Reduced stream flows can also lead to increases in water temperature. Under these conditions, coldwater fish species such as brook trout can become stressed resulting in decreased growth or even death. Coldwater springs feeding the stream may circumvent this problem, but even in a bad drought year, seepage from springs may be reduced to a minimum. Severely reduced stream flows might also hinder movements of migratory species, resulting in poor reproduction and survival.

Although not as apparent, droughts can also negatively impact fish populations in lakes, especially reservoirs. Low water levels during spring expose prime reproductive habitat for species such as black crappie and largemouth bass. Thus, during extreme drought years, low water levels may result in poor spawning success for these species. These year classes of fish will be “weak” for many years after. If water levels remain low during the summer, those juvenile fishes that successfully hatched and survived the spring may not have adequate shoreline cover to avoid predation. Also, low water levels during summer may lead to increased coverage of aquatic vegetation because of the shallow depth and increased sunlight penetration. Although aquatic vegetation is actually an important component of fish habitat, too much coverage can cause problems. Very dense aquatic vegetation may interfere with predator-prey relationships. In a worse case scenario, the combination of dense vegetation, shallow water, warm temperatures, and small lake size can lead to critically low levels of dissolved oxygen resulting in what’s known as a “summerkill.”