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UCREFRP
UCREFRP Catalog Number
7809
Author
Wydoski, R. S.
Title
Responses of Trout Populations to Alterations in Aquatic Environments
USFW Year
1978.
USFW - Doc Type
A Review, reprinted from John R. Moring, ed., Proceedings of the Wild Trout - Catchable Trout Symposium.
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<br />ities that fish or other aquatic organisms <br />may have available to them. Therefore, it <br />is not often possible to predict the re~ <br />sponses of fish to changes in streamflow <br />with accuracy because adequate data on <br />preferences on these ecological requirements <br />are lacking. <br /> <br />The behavior, ecology, and basic biol- <br />ogy of stream-dwelling salmonids has been <br />instensively studied when compared to other <br />groups of fish (Allen 1969; Giger 1973; <br />Hooper 1973) and the streamflow requirements <br />of these fish are closely related to their <br />ecology and behavior (Chapman 1966; Edmund- <br />son et al. 1968; Smith 1973). These studies <br />and others have demonstrated that the carry- <br />ing capacity of a stream is greatly influ- <br />enced by water velocity that is, in turn, <br />a function of streamflow. Lewis (1969) <br />concluded that water velocity accounted <br />for 66% of the variation in numbers of <br />brown trout occupying pool environment of <br />a stream. Lewis suggested that greater <br />food drift, accompanied by increased water <br />velocities, might support greater densities <br />of trout. This suggestion is supported <br />by other investigators (e.g. Chapman 1966; <br />Champman and Bjornn 1969; Waters 1969) who <br />also concluded that food and invertebrate <br />drift can limit the carrying capacity of <br />streams for territorial fish such as trout. <br />Trout will move when water velocities change <br />and select those velocities that are best <br />suited to them (Baldes and Vincent 1969; <br />Edmundson et al. 1968). <br /> <br />Although the lack of water is usually <br />believed to be most critical to aquatic <br />organisms, too much water can also be im- <br />portant to fish populations. For example, <br />catostr:ophic floods in various parts of <br />the United States have been directly relat- <br />ed to extreme reductions and, in fact, al- <br />most complete destruction of young trout <br />(Hansen and Waters 1974; Hoopes 1975; See- <br />grist and Gard 1972). <br /> <br />In Sagehen Creek, California, Seegrist <br />and Gard (1972) reported that the abundance <br />of brook trout (Salvelinus fontinalis] and <br />rainbow trout (Salmo gairdneri] fry was <br />correlated to flooding. Survival of brook <br />trout was low in years when fall floods oc- <br />curred in Sagehen Creek but was high in <br />years following spring floods; this pattern <br />was reversed for spring-spawning rainbow <br />trout. Seegrist and Gard pointed out that <br />extreme floods may cause the loss of a <br />year class in streams containing only one <br />trout species, but not in streams contain- <br />ing spring and fall - spawning species <br />since two extreme floods in one year is <br />probably less likely to occur. <br /> <br />In contrast to these detrimental ef- <br />fects, scouring streamflows may be necessary <br />to maintain or to create new holding areas <br />in streams with soft substrates such as <br />those found in valley bottoms where meand- <br />ers in the stream create pools and back- <br />water habitats (Tennant 1976). The over- <br /> <br />winter survival of trout has been improved <br />when the pool habitat of streams in the <br />Intermountain West was increased either <br />through the use of instream devices or <br />through beaver activity (Wydoski and Duff <br />1978). Fish populations in streams with <br />steep gradients that fluctuate widely in <br />streamflows and water velocities are limit- <br />ed by poor invertebrate production that <br />serves as the food supply, and by lack of <br />suitable habitat that is related to the <br />carrying capacity for trout. <br /> <br />Trout populations in regions where <br />many tributary streams are intermittent <br />may use these streams at times when water <br />velocities, water depth, and size of the <br />substrate are suitable for spawning. The <br />eggs develop in such areas and the emerging <br />fry move downstream into the main stems <br />as the tributaries become low or completely <br />dry. <br /> <br />Land use practices can greatly influ- <br />ence the runoff in a watershed and thereby <br />govern the streamflow characteristics for <br />a particular basin. Many such practices, <br />regardless if caused by grazing, logging, <br />mining, or some other man-made alteration, <br />may result in the removal of vegetation <br />and resulting litter that aids in the re- <br />tention of water in the soil. The water <br />retention of soils (0-20 cm deep) in natural <br />areas along a Montana stream was about 9 <br />times greater than for grazed areas along <br />the same stream (Fig. 6; Marcuson 1977). <br />The overall effect on the stream flow can <br />be tremendous, since soils that hold more <br />water can buffer large fluctuations in <br />streamflow and, therefore,' be better suited <br />to trout. This can be easily observed if <br />one compares the runoff characteristics of <br />an urbanized area where much of the land <br />is covered by homes, asphalt, and concrete <br />with a nearby forested area. The runoff <br />occurs quickly in the urbanized area with <br />wide fluctuations in streamflow in contrast <br />to the forested watershed where the stream <br />does not rise as quiCkly and is also slower <br />to drop after a major storm. A grazed <br />watershed that was studied for 10 years <br />near Grand Junction, Colorado, had 30% more <br />runoff than an ungrazed watershed (Lusby <br />1970). Of course, the runoff and resulting <br />streamflows in a watershed would be direct- <br />ly related to the degree of man-made altera- <br />tions to the watershed. <br /> <br />Instream changes in gradient and water <br />velocity result from the channelization of <br />streams, dams, dredging, and mining. The <br />basic problem is that little consideration <br />is given to the hydraulic characteristics <br />of a stream and changes that occur upstream <br />can have effects much farther downstream <br />(Leopold et al. 1964; Morisawa 196B). The <br />reservoirs produced by dams change the <br />ecology of the stream and favor some fish <br />species such as the northern squawfish <br />(Ptychocheilus oregonensis] in the Pacific <br />Northwest or the Utah chub (Gila straria] <br />in the Great Basin. Changes in the fish <br /> <br />64 <br />
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