7405

• UCREFRP Catalog Number: 7405
• Author: Bain, M. B., ed. 1990.
• Title: Workshop Synopsis,
• USFW Year: Ecolog
• USFW - Doc Type: U.S. Department of the Interior,
• Copyright Material: NO

14 BIOLOGICAL REPORT 90(5)

A Pluralistic View of Factors Influencing Warmwater Stream
Fishes: Implications for Environmental Management

by

Isaac J. Schlosser

University of North Dakota
Department of Biology
Box 8238, University Station
Grand Forks, North Dakota 58202

Fish mortality is usually greatest during early life
stages. Therefore, establishing what determines
growth and mortality of fishes during early life
stages will be essential for understanding fish
population dynamics and for predicting the effect
of environmental modifications on fishery resources.
Unfortunately, our understanding of the influence
of abiotic and biotic factors on early life stages of
fishes in warmwater streams is poorly developed.
Conducting controlled experiments is difficult and
we lack a conceptual framework for identifying
critical ecological processes.
In a previous descriptive study (Schlosser 1982a),
I examined temporal and spatial variation in fish
community structure along a pronounced habitat
gradient. Based on these results I proposed a con-
ceptual framework that attempted to integrate the
relative effect of abiotic and biotic factors on fishes
in small warmwater streams (Schlosser 1987a).
From this conceptual framework it was evident that
three factors potentially have major effects on
growth and survival of early life stages of stream
fishes: (1) harsh winter conditions, (2) direct effects
of flow regime on invertebrate resource availabil-
ity and fish survival, and (3) trophic interactions, in-
cluding predation and competition.
My recent research used both descriptive and
experimental approaches to examine the effects of
trophic interactions and flow regimes on juvenile
fishes. Results of this research indicated that
(1) spring floods during spawning periods are asso-
ciated with a major decrease in the density of juve-
nile cyprinid and centrarchid fishes (Schlosser 1985;
see also Harvey 1987; Bain et al. 1988); (2) elevated
(nonscouring) flow results in a dramatic increase in
invertebrate abundance during spring and summer,
when most growth of juvenile fishes is likely to occur
(Schlosser and EbeI1989); (3) in the absence ofpred-
ators, small fishes depress invertebrate abundance

in pools but not in riffles (Schlosser and EbeI1989);
and (4) centrarchid predators restrict juvenile fishes
to shallow refugia while engaging in both species-
and size-selective predation (Scholosser 1987b,
1988b).
These results indicate that multiple abiotic (e.g.,
channel morphology, flow regime) and biotic (e.g.,
resource depression, predation) factors interact to
influence growth and survival of early life stages of
stream fishes. Because many environmental mod-
ifications simultaneously alter several of these
variables, such modifications are likely to have con-
siderable effects on the fishery resources in modified
streams. For example, agricultural land use alters
flow regime, channel morphology, seasonal timing of
resource availability, and abundance of piscivorous
fishes (Karr and Schlosser 1978; Schlosser 1982b).
These modifications, in turn, have major effects on
temporal variability and juvenile recruitment in
headwater and downstream areas (Schlosser 1982b).
These results also indicate that the development
of a predictive paradigm for environmental and
fisheries management in streams will require refine-
ment in our understanding of how abiotic and biotic
factors interact to affect early life stages of stream
fishes. This refinement will be achieved through a
combination of (1) long-term monitoring to establish
the influence of natural environmental variability on
growth and survival of early life stages and (2) ex-
perimental studies to assess the effect of variability
in flow and temperature regime on trophic interac-
tions. Lastly, fishes are highly migratory during
reproductive activities, and stream systems are
heterogeneous and interconnected. Hence, it will be
necessary to explore the effects environmental
modification in one stream have on population and
community dynamics of fishes in adjacent parts of
the stream system (landscape approach of Forman
and Godron 1986).

~