Assemblages of native stream fishes in California show a remarkable ability to resist invasion by introduced fishes as long as the streams are relatively undisturbed by human activity. Previous studies had indicated a high degree of spatial (microhabitat) segregation among the native fishes, which was confirmed by a principal components analysis of microhabitat use data from Deer Creek, a tributary of the Sacramento River. A null modelling study using the same data set was performed to see if competition was a major force structuring the assemblage, because theoretical studies had indicated that a competitively structured assemblage should be most able to resist invasions. The null models indicated that competition was not the major structuring force, so it is likely the assemblages are structured through a combination of morphological specialization (reflecting evolutionary history), predation, and some competition. The assemblages resist invasion through both environmental and biotic factors. Predation seems to be an especially important biotic factor.

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SynopsisWe used a drop sampler to characterize use of the marsh-edge ecotone by small fishes along two transects running inland from the Gulf of Mexico for ca. 25 km in Louisiana's Barataria-Caminada Bay System. Monthly sampling was stratified among upper, middle, and lower reaches and within reaches to characterize fish responses to salinity, depth, distance from shore, substrate, dissolved oxygen concentration, temperature, turbidity, velocity, and emergent stem density. In 681 quantitative samples, covering 658 m2, collected between October 1987 and October 1989, we collected 57 fish species and 16 864 individuals, primarily larvae and juveniles. The 15 most abundant fishes, comprising 97.7% of all individuals, were concentrated near the marsh edge (i.e., 0 to 1.25 m distance). Some significant differences within species for seasonal variables (e.g., temperature and dissolved oxygen concentration) reflected the ephemeral duration of early life history stages. Other differences reflected ontogenetic microhabitat shifts (e.g., depth and distance from shore). Within ecological groups, characterized as demersal residents, nektonic transients, and demersal transients, spatial and temporal segregation reflected the particular habitat requirements of each species. In a principal component analysis of microhabitat use, the first three components were interpreted as seasonal, depth-anddistance, and salinity axes, respectively. The array of species and size classes in principal component space reflected the complex dimensionality of microhabitat use. The high density of fish larvae and juveniles near the marsh edge confirmed the importance of the marsh-edge ecotone as a nursery for many estuarinedependent fishes.

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Microhabitat requirements were determined for eight species of native California stream fishes: Rainbow trout Salmo gairdneri; Sacramento sucker Catostomus occidentalis; Sacramento squawfish Ptychocheilus grandis; hardhead Mylopharodon conocephalus; California roach Hesperoleucus symmetricus; speckled dace Rhinichthys osculus; tule perch Hysterocarpus traski; and riffle sculpin Cottus gulosus. Two or three size classes were evaluated for each species. Each species had a preferred microhabitat (defined on the basis of depth, velocity, substrate), as did each size class within each species, but there was much similarity in microhabitat use within and among species. The amount of microhabitat available to each species differed in three stream reaches in which availability was quantified, but the differences were not enough to explain the differences in composition of the fish assemblage found at each site. This study indicates that recommendations for instream flows should be based on microhabitat use data collected on site together with habitat availability data. Even on‐site data should be used cautiously because intraspecific interactions and changes in a streamˈs physical characteristics, especially in its temperature regime, may cause unexpected shifts in microhabitat use.

SynopsisSpawning site selection by spotted seatrout and black drum was studied by locating drumming aggregations through the use of a hydrophone. From March 1987 to October 1990,315 sound observations were made to identify and characterize spawning seasons and environmental requirements of both species in the Barataria, Caminada, and eastern Timbalier Bay systems of Louisiana. The sounds produced by the spawning aggregations were identified and verified against known recordings. Spawning was verified on several occasions by capturing and rearing zygotes (eggs) into identifiable larvae. Spotted seatrout formed drumming aggregations from late May to early October at salinities ranging from 7.0 to 25.8 ppt and temperatures from 24.5 to 33S°C. Black drum formed drumming aggregations between January and April in salinities from 10.0 to 27.0ppt and temperatures from 15.0 to 24.0°C. Large drumming aggregations of spotted seatrout were located from 1800 to 2400h and from 1800 to 2200h for black drum. Spotted seatrout aggregation size was highly correlated with water temperature and for black drum with dissolved oxygen concentrations. Spawning sites for both species were frequently located in deep moving water between barrier islands as well as in channels in open water where water depth ranged from 3 to 50 m. Spawning site selection depended on a particular range of environmental conditions and spawning locations varied seasonally and yearly depending upon hydrological variation.

Two morphologically dissimilar stream fishes occupied the same microhabitat in different riffles of Deer Creek, Tehama County, California. In a 12.5 km reach of the creek, speckled dace (Rhinichthys osculus) dominated riffles at the lower end while riffle sculpin (Cottus gulosus) dominated the riffles at the upper end, where dace were confined to slower water. Between these sets of riffles, relative abundances of the two species were negatively correlated. Routine metabolic rates of sculpin increased rapidly with temperature and indicated that sculpins were unable to tolerate temperatures in the low elevation riffles in the summer. Dace swimming performance appeared to be adequate at low temperatures to permit them to occupy the sculpin-dominated riffles. However, stream tank experiments indicated that at low temperatures sculpin were able to displace dace from cover in high velocity water. Our analysis thus demonstrated that competitive interactions between dace and sculpin for preferred microhabitat were mediated by temperature.Key words: competition, critical swimming velocity, experimental stream, microhabitat utilization, routine metabolism

Shifts in microhabitat selection by rainbow trout Oncorhynchus mykiss were related to seasonal and ontogenetic factors in a small stream characterized by short riffles, small pools, and boulder substrate. Resource availability did not differ significantly between summer and November sampling dates for most variables related to water velocity, substrate, and cover, although depths were greater and temperatures were significantly lower in November. Ontogenetic shifts were found for total depth, focal elevation, mean water column velocity, focal velocity, surface velocity, and substrate, but not for relative depth or temperature. When microhabitat selection was adjusted for fish size, selection was significantly different between seasons, most notably for velocity. Ontogenetic shifts in microhabitat use by young‐of‐year rainbow trout were interrupted by cooling winter temperatures. These changes resulted in substantially different microhabitat requirements for all rainbow trout size‐classes in different seasons.