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Much debate about natural variability of stream fish assemblages may stem from investigating them at different spatial and temporal scales, and analyzing data of different resolution. We assessed variability of stream fish assemblages over an 11yr period in a southeastern Colorado plains watershed based on surveys of 12 river and five tributary sites during 1983/1984, 1987/1989, and 1993/1994, and surveys of 40-48 pools along five tributaries during the last two periods. We analyzed across space and time using species presence/absence and relative abundance to assess how conclusions depended on scale and resolution of data. Assemblage similarity through time based on both types of data was higher for whole river or tributary faunas than at individual sites, because rare species that occurred sporadically at sites were consistently present at larger spatial scales. Assemblage similarity among river sites based on species presence/absence was moderate during 1987 and 1993 but lower during 1983 because of differences in passive sampling gear used. It was also low among tributary pools because of spatial variability in both rare and common species. In contrast, assemblage similarity among river sites based on species relative abundance was high for each survey because the same species were dominant but was often low at individual sites among surveys because of an unexplained 92% decline in Cyprinella lutrensis abundance from 1983 to 1993. Assemblage similarity among tributary pools based on species relative abundance in 1993/1994 was high for two tributaries where Lepomis cyanellus was dominant but low in others where it was often absent. The greatest change through time in assemblages at most tributary sites was apparently caused by drought after 1987/1989, which eliminated small pools and limited opportunities for recolonization. Conclusions about the natural variability of these fish assemblages depended strongly on scale, emphasizing the importance of multiscale investigations.

We describe physical habitat and aquatic biota of a relatively undisturbed canyon reach of the Purgatoire River and its tributaries in southeastern Colorado. Flow regimes are highly variable due to unpredictable, brief, intense summer floods. River habitat consists of long, deep, silty pools with few large boulders separated by short cobble riffles, whereas tributaries contain isolated pools maintained by groundwater. Water chemistry and temperatures were within ranges tolerable by plains stream fishes at all sites. Benthic macroinvertebrate assemblages included representatives of four regional faunas. At lotic sites, a few taxa of two detritus-collecting functional groups predominated. The fish fauna is depauperate, consisting of only 11 native species which we divided into river, perennial stream, and generalist faunal associations. However, most fishes have generalized habitat, trophic, and reproductive requirements, which seem to adapt them to survive the harsh environmental conditions in this plains stream. Although the four species that made up 95% of individuals generally persisted at river sites over the 5to 7-year period sampled, the abundance of red shiners declined markedly from 1983 to 1987 despite favorable flow regimes. Little is known of the ecology and physical characteristics of streams of the western Great Plains (Matthews, 1988). However, fish and macroinvertebrate assemblages in such systems can be expected to differ from those in more mesic regions due to extremes in physical characteristics such as flow, turbidity, and temperature. Relatively pristine habitats in streams of the western plains are scarce due to a profusion of human activities, such as channelization, damming, dewatering, mining, overgrazing, use of agricultural chemicals, release of municipal and industrial wastes, and the introduction of nonnative species (Cross and Moss, 1987). Although the Purgatoire River is not immune to these perturbations, the reach we discuss here is one of few comparatively undisturbed remnants in the region and, therefore, offers an opportunity to study habitat and aquatic biota in a relatively natural stream of the western plains. The purpose of this paper is to describe the flow regime, physical habitat, and water chemistry of the Purgatoire River and its tributaries in Pifion Canyon and to document the species composition, distribution, and ecological characteristics of assemblages of benthic macroinvertebrates and fishes of these waters. Few reports of aquatic biota exist for this general area (Cope and Yarrow, 1875; Jordan and Evermann, 1891; D. Vana-Miller, in litt.), and none for the specific reach. We show that this reach harbors depauperate assemblages of entirely native aquatic biota which have generalized ecological requirements that adapt them for persistence in plains

Local genetic differentiation in the Papago. Amer. J. Hum. Genet. 22:24-49. WRIGHT, S. 1978. Evolution and genetics of populations, Vol. 4. Variability within and among natural populations. University of Chicago Press, Chicago, Illinois. ZUG, D. A., AND W. A. DUNSON. 1979. Salinity preference in freshwater and estuarine snakes (Nerodia sipedon and N. fasciata). Florida Scientist 42:1-8. (RL) OSHER FOUNDATION LABORATORY FOR MOLECULAR SYSTEMATICS AND DEPARTMENT OF HERPETOLOGY, CALIFORNIA ACADEMY OF SCIENCES, GOLDEN GATE PARK, SAN FRANCISCO, CALIFORNIA 94118; (AJM) DEPARTMENT OF ZOOLOGY, UNIVERSITY OF GEORGIA, ATHENS, GEORGIA 30602; (PGF) 5779 EAST MCMULLEN, FLORAL CITY, FLORIDA 32636; AND (PEM) FLORIDA GAME AND FRESH WATER FISH COMMISSION, WILDLIFE RESEARCH LABORATORY, 4005 SOUTH MAIN STREET, GAINESVILLE, FLORIDA 32601. Accepted 19 July 1990.

A model is developed to predict potential net energy gain for salmonids in streams from characteristics of water velocity and invertebrate drift. Potential net energy gain, or potential profit, is calculated for individuals of three species of juvenile salmonids in a laboratory stream aquarium, based on the energy available from drift minus the cost of swimming to maintain position. The Michaelis–Menten or Monod model is used to describe the relationship between potential profit and specific growth rate. Potential profit was a better predictor of specific growth rate for coho salmon (Oncorhynchus kisutch) than for brook trout (Salvelinus fontinalis) or brown trout (Salmo trutta). Coho salmon always achieved higher specific growth rates than either brook trout or brown trout in concurrent experiments, and maintained growth to lower resource thresholds. In each experiment, fish established intraspecific hierarchies and dominant fish held positions affording maximum potential profit. The use of potential profit as an optimal foraging model was tested by predicting the potential for net energy gain at coho salmon positions from the overall pattern of flow and invertebrate drift in the stream aquarium, and ranking these positions from highest to lowest potential profit. This predicted ranking was nearly identical to the rank observed in the linear dominance hierarchy. The results of experiments confirm ideas of other investigators about mechanisms of microhabitat selection by stream salmonids.

(1983). Sinuous Stream Aquarium to Simulate Lotic Fish Habitat. The Progressive Fish-Culturist: Vol. 45, No. 2, pp. 113-116.

The recently proposed index of biotic integrity (IBI) was evaluated for several watersheds throughout the midwestern United States. Five of the community metrics comprising the IBI depend on the number of fish species present and must be adjusted for changes in expected species richness with stream size or zoogeography. We use basic relationships of fish species richness versus stream size, calculated from historical fish community data for seven watersheds, to define lines of maximum species richness. These lines are used to predict attributes of “excellent” fish communities, the basis of comparison for assigning scores to 5 of 12 IBI metrics. When zoogeographic and stream-size factors were accounted for in assigning scores, the IBI accurately reflected watershed and stream conditions. As partial tests of the IBI, we found that the index conforms to knowledge of biologists familiar with several watersheds, is independent of stream order in homogeneous watersheds, and is not biased upwards in gen...

Salmonids have been introduced worldwide to regions where they were not indigenous, usually without considering the effects on native species. This has led to many combinations of species in unnatural sympatry, some of which are suspected to interact strongly, but we currently have inadequate knowledge to allow us to predict the outcome of most such introductions. For example, despite current controversy about the potential effects of Pacific salmonids now stocked in Atlantic waters, we have only empirical evidence with which to judge whether they will be detrimental to Atlantic salmon (Salmo salar). Moreover, most experimental tests of interspecific competition between other pairs of stream salmonids have not been rigorous. A review of literature about the suspected competition between brook trout (Salvelinus fontinalis) and rainbow trout (Salmo gairdneri) in streams reveals that despite empirical evidence that water temperature and velocity play a critical interacting role in determining the outcome, most experiments have failed to elucidate the important mechanisms involved. Carefully designed experiments that compare the strength of interspecific versus intraspecific competition for different size classes will allow us to improve predictions about the effects of introduced salmonids on native species in streams.

The index of biotic integrity (IBI) integrates 12 measures of stream fish assemblages for assessing water resource quality. Initially developed and tested in the Midwest, the IBI recently was adapted for use in western Oregon, northeastern Colorado, New England, the Appalachians of West Virginia and Virginia, and northern California. The concept also was extended to Louisiana estuaries. In regions of low species richness, the IBI proved difficult to apply and often required extensive modification. Adapting the 1BI to those regions required that metrics be replaced, deleted, or added to accommodate regional differences in fish distribution and assemblage structure and function. Frequently replaced metrics include: proportion of individuals as green sunfish (Lepomis cyanellus), proportion of individuals as insectivorous cyprinids, proportion of individuals as hybrids, and number and identity of sunfish and darter species. The proportion of individuals as top carnivore metric was often deleted. Metr...

Competition between brook trout (Salvelinus fontinalis) and brown trout (Salmo truttta) was studied by measuring characteristics of daytime positions held by brook trout before and after removal of the brown trout from 1800 m of a stream. We used four criteria as indices of position quality: "water velocity difference" (the difference between velocity at the focal point and in the fastest current within 60 cm of the fish), water depth, distance to stream bed, and lighting. After brown trout removal, brook trout larger than 15 cm chose resting positions with more favorable water velocity characteristics and more often in shade. The position shift was greatest for the largest brook trout, those of 20–30 cm. Feeding positions of brook trout changed little upon brown trout removal according to our criteria. The shift in resting positions of brook trout after release from competition with brown trout indicates that brown trout excluded brook trout from preferred resting positions, a critical and scarce resource. The combined effects of such interspecific competition, differential susceptibility to angling, differential response to environmental factors, and predation of brown trout on juvenile brook trout may account for declines of brook trout populations while brown trout populations expand in many streams of the northeastern United States where the two species are sympatric.Key words: brook trout, brown trout, competition, ecological release, microhabitat use, resting positions, feeding positions, stream, Michigan

The index of biotic integrity (IBI) has been used to assess the biological quality of flowing water systems in areas throughout the United States. Yet, only rarely has biotic integrity been related to independent measures of water or habitat quality. We show that the IBI ranks sites similarly in two Illinois watersheds where conditions remained relatively stable during 3 years of sampling. Further, rankings among sites conform to prior assessments of site quality based on habitat and water quality. Neither a species diversity index (H') nor any of the individual metrics that constitute IBI performed as consistently at ranking sites as did the IBI. Sampling should be conducted during early summer to reduce variation due to seasonal fish migration and fall recruitment of young-of-the-year fish. In an Indiana watershed subject to extensive conservation planning, the IBI reflects known habitat and water quality perturbations of both natural and anthropogenic origin. Little or no improvement in biotic...

Among the three species of Pacific salmon Oncorhynchus established in the Laurentian Great Lakes, juveniles of coho salmon O. kisutch are most likely to compete with brook trout Salvelinus fontinalis and brown trout Salmo trutta for food and space, because their juvenile life histories and ecologies are similar where they rear together in tributary streams. Coho salmon emerged 2–3 weeks earlier and were 6–8 mm longer than brook or brown trout at emergence during 1979 in Lake Michigan tributaries where the species were sympatric. During the first summer of life, coho salmon generally were 6–21 mm (7–54%) longer and weighed 0.3–4.1 g more than brook and brown trout. Size at emergence and first-year growth were similar among brook trout and brown trout. In laboratory stream experiments with pairs of the three species, coho salmon dominated brook or brown trout of equal size, and brook trout dominated equal-size brown trout. Competitive superiority of dominant species was based on the ability of fish...