7933

• UCREFRP Catalog Number: 7933
• Author: Orth, D. J.
• Title: Ecological Considerations In The Development And Application Of Instream Flow-Habitat
• USFW Year: 1987
• USFW - Doc Type: Regulated Rivers
• Copyright Material: YES

INSTREAM FLOW-HABITAT MODELS

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of impacts. First-order impacts occur immediately after a dam is built and include effects on energy sources
and transport, flow regime, water quality and temperature, and sediment transport. Second-order impacts
occur as a result of frrst-order impacts and include changes in habitat structure (channel form and
substrate composition), riparian vegetation and sources of organic matter, and periphyton and
macrophyte communities. Assessment of impacts after regulation is complicated because the
second-order impacts may require 1-100 years to achieve a new equilibrium (Petts, 1984). Changes in
fish and invertebrate communities, which are the most critical concerns for resource management
agencies, are third-order impacts which arise as a consequence of first-and second-order impacts. For
example, Lehmkuhl (1972) demonstrated that changes in the thermal regime following flow regulation
eliminated several species of mayflies. Also, Smith (1976) reported that steel head trout (Salmo gairdneri)
runs were significantly reduced after flow regulation because of sand accumulation in the riffles. The time
scale for third-order impacts to be manifested is extremely variable depending on the time required for
second-order impacts to become significant and the generation time of the organisms of concern.

POPULA nON DYNAMICS

The effects of first- or second-order impacts on fish population abundance can be understood by
considering the conceptual model which includes the six population parameters that determine numerical
changes in fish stock biomass - growth of individual fish, recruitment, natural mortality, fishing
mortality, immigration, and emigration. Any of these population parameters may be influenced by first-,
second-, or third-order impacts. For example, temperature is a major determinant of growth in fishes and
this relationship has been well studied (Brett and Groves, 1979). The amount of suitable winter cover
(habitat structure) influences numbers of stream salmonids that will emigrate and numbers that can
survive over winter (Bjornn, 1971). Extreme low flows. ca_n peGiIn~Je..warmwater stream fish and
""'- _ __..' . 4.
invertebrates and higher flows may permit rapid recolonization -(Larimore et al., 1959). Recruitment
success is strongly influenced by the flow regime during and after the breeding season (Schlosser, 1985).
This conceptual model of population dynamics permits the development of hypotheses concerning
population regulation. The hypothesis that miCrohabitat availability limits population abundance is a
basic assumption in recommending flow regimes using PHABSIM. However, recent critiques document
that WUA is not a consistent predictor of abundance (Shit:vell, 1986; Scott and Shirvell, in press).
Weighted usable area is an index that'represents the amount of acceptable (or suitable) habitat for a given
species and life stage (see Bovee, 1982 for formula). Suitability can be defined based on microhabitat
(depth, velocity, substrate, and cover) and macighabitat (temperature, water quality) variables.
However, most studies cited by Scott and Shirvell (in press) calculated WUA on the basis of only three
microhabitat variables (depth, velocity, and substrate). At least three' hypotheses for stream fish
population regulation can be offered to explain why WVA docs not predict popul:ltion size. These three
hypotheses assume that habitat availability (WUA) and population abundance can be adequately
quantified. .
First, many validation studies have be~~. on exploited game f:ish populations; all but one of eleven
studies reviewed by Scott and Shirvell (in press) focused ~.'Fslusively on game fish. Adult game fish
populations may be limited by exploitation rather than available microhabitat. Consequently, many of
these studies were inadequate tests because the effects of exploitation were not measured.
A second hypothesis is that microhabitat availability does not regulate population abundance but only
determines where stream fishes will be. Therefore, food availability, or biotic interactions (competition or
predation) may be the major determinants of population size within suitable habitat patch~s. For
example, high availability of food can reduce emigration and thereby increase densities of salmonids
(Mason and Chapman, 1965; Wilzbach, 1.985). Also, predators (rock bass Ambloplites rupestris) can
reduce the densities of prey fish (sculpins Cottus spp.) even when suitable microhabitat is available
(Anderson, 1985).
A third hypothesis to explain population dynamics of stream fishes is that microhabitat availability
. limits populations only during short periods due to extreme variations in stream flow (Horwitz, 1978;

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