The Effects of Low Flows and Rapid Changes of Flow on Wild Trout Populations
By *Robert A. Bachman, PhD.

One of the most harmful effects of rapid changes in flow on wild trout populations resulting from abrupt changes in releases from reservoirs, commonly referred to as the "YO-YO" effect, is the way that these changes unnaturally concentrate the trout in relatively small areas during period of low flow. It has been well documented that the factor that limits the number of adult trout in a stream is low summer flow. Trout compete for space in a flowing system and establish well-defined dominance hierarchies within specific areas of the stream. When trout are unnaturally concentrated during the time of year when they are aggressively feeding, they fight with each other. As a result, individuals within the hierarchy, or pecking order, are driven out, fail to find suitable feeding sites, and ultimately die through stress, starvation, or the combination of the two. Because not all of the trout affected die at the same time, "fish kills" are rarely observed, but the loss is nevertheless severe.

A more dramatic result of a rapid decrease in releases from a reservoir is the unnatural rapid increase in water temperature during hot weather. Although trout grow more rapidly if they can remain within their established summer home range, as temperatures rise and approach 68 F, the trout first feed less, and if the temperature continues to rise, they will move, usually upstream, in search of colder water. If such a temperature rise is too rapid and is accompanied by a sudden decrease in flow, the effects of the concentration of trout mentioned above is compounded. Rapid and large decreases in flow "trap" trout in areas that get too warm after the flow is decreased and the trout are unable to migrate as they would under normal circumstances. Although they may be able to find temporary refuge in cold spring "pockets", they are unable to feed adequately, lose weight, and eventually die.

Trout have evolved over millions of years to adapt to certain changes in their environment. Sudden increases in flow, for example, are common events in trout streams, typically associated with a sudden rain storm. Trout rarely have any problem adjusting to rapid increases in flow. They simply find refuge along the edge of the stream and behind boulders and because the suitable area for feeding usually increases with an increase in flow, the trout are little affected by such changes. On the other hand, a large, rapid DECREASE in flow is an unnatural event to which the trout are unable to adapt, for the reasons given above. The rapid decreases in releases from the Delaware River reservoirs along with low flows and high temperatures are the major causes of the decline in the trout population. Even short periods of low flows in spring and summer have a devastating and long term impact on the number of trout that the river can sustain. Consequently, any release strategy designed to protect trout populations must necessarily avoid rapid, unnatural decreases following abrupt increase. Natural flows decline in a trout stream over a period of days or weeks after a storm event, depending upon the severity of the storm. Low summer flows, even for just a few days, will inevitably result in much reduced numbers of trout, and must be eliminated if the trout population is to be sustained at its normal carrying capacity. If the amount of water becomes scarce, if the trout population is to be sustained, the low flow condition must be approached gradually in order for the trout to be able to adjust and adapt to this new condition.

*Commissioner Robert A. Bachman, PhD., Pennsylvania Fish and Boat Commission
Commissioner Bachman was appointed by Governor Rendell to serve as a member of the Board of Commissioners of the Pennsylvania Fish and Boat Commission in April of this year. He earned his PhD from Penn State doing a four-year behavioral study of free-ranging wild and hatchery trout in a Pennsylvania stream in 1982. He has over 20 years experience in managing trout and other fish, including Director of Maryland's Fishery Service and Director of Fish and Wildlife in Maryland. During his service in Maryland he gained considerable expertise in the management of trout in and downstream of major reservoirs, working with the US Army Corps of Engineers, the City of Baltimore, the Potomac River Basin Commission and local communities to find a balance between the needs of the fish and other water uses. He currently serves on the Sub-Committee on Ecological Flows (SEF) of the Delaware River Basin Commission.