Donald M. Baltz

Invasion Resistance to Introduced Species by a Native Assemblage of California Stream Fishes

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.

Microhabitat use by an assemblage of California stream fishes : developing criteria for instream flow determinations

Abstract 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 u...

Seasonal Changes in Microhabitat Selection by Rainbow Trout in a Small Stream

Abstract 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 diffe...

Influence of Temperature on Microhabitat Choice by Fishes in a California Stream

Abstract We used eight microhabitat variables to examine the assumption that the variables normally included in instream flow studies are adequate to discriminate among species microhabitats. When eight variables were available in stepwise-discriminant analysis models to distinguish among the microhabitats of four fish species in a northern California stream, the variance explained ranged from 52 to 77%, and 59 to 86% of the observational records were correctly classified to species. The variables measured were temperature, total depth, focal point elevation (distance of fish from the bottom), focal point velocity (water velocity at fishs snout), mean water column velocity, surface velocity, substrate, and cover. When the number of variables available was reduced to the three normally used in instream flow studies (i.e., total depth, mean water column velocity, and substrate), the variance explained ranged from 0 to 20%, and 46 to 55% of the observational records were correctly classified to species. Wh...

Distribution and microhabitat use by flatfishes in a Louisiana estuary

We used a 1 m beam trawl to characterize microhabitat use of flatfishes in monthly samples collected in Barataria Bay, Louisiana. Six strata were established along a salinity gradient from the nearshore zone along the Gulf of Mexico to approximately 30 km inland. Randomized sampling within strata characterized flatfish distributions and environmental conditions throughout the bay. Microhabitats were characterized by salinity, temperature, dissolved oxygen, depth, distance from shore, and substrate type. In 594 short-duration, beam-trawl samples collected from October 1992 through September 1994, we identified seven flatfish species from 7046 specimens, most of which were juveniles. Four species accounted for 98.3% of all flatfishes. The two most abundant, offshore tonguefish and bay whiff, were euryhaline and widely distributed over the salinity gradient; however, offshore tonguefish were concentrated in the lowermost stratum where bay whiff were uncommon. The third species, fringed flounder, was more abundant in middle and lower bay strata at higher salinities. The fourth species, blackcheek tonguefish, was most common in middle and upper bay strata and declined in abundance in coastal strata. A rotated factor analysis resolved six environmental variables into three major axes that explained 69 percent of the variance and were characterized as seasonal, depth-distance, and substrate-salinity axes. Separation of species and life history intervals in three-dimensional factor space reflected temporal and spatial segregation. Within the four common flatfishes, 14 of 24 variable comparisons showed clear ontogenetic trends in which at least two size-class means differed significantly. Among the common species, temporal differences were reflected by mean temperatures and dissolved oxygen concentrations and spatial differences were reflected by other environmental variables, including depth, distance, substrate, and salinity.

Segregation by species and size classes of rainbow trout, Salmo gairdneri, and Sacramento sucker, Catostomus occidentalis, in three California streams

SynopsisThe hypothesis that Sacramento suckers, Catostomus occidentalis, compete with rainbow trout, Salmo gairdneri, for space in streams was examined by measuring microhabitat utilization of both species in three California streams. Two streams were similar in most respects except one contained only trout and one contained trout and a large population of suckers. The third stream, formed by the union of the first two, contained trout and a small population of suckers. The species overlapped in five of the six microhabitat variables measured: maximum depth, mean water column velocity, focal point velocity, surface water velocity, and substrate type. However, the species had strong vertical segregation; there was little overlap between species in focal point depth. Mean focal point velocities were also significantly different. Suckers roamed over and generally remained in contact with the bottom while trout held position in the water column. Microhabitat utilization by trout in the stream without suckers was similar to in the stream with a higher sucker density. Differences in microhabitat utilization by trout between the third stream and the other two was attributed to the larger size of the third stream. Both sucker and trout showed a similar within-species segregation of size classes - fish under 50 mm in length sought shallow water. Size-specific trends indicated ontogenic shifts in resource utilization which reduced overlap within species. These results suggest that competition for space between trout and suckers was not a major factor regulating microhabitat utilization of trout, although the possibility that larger suckers may displace small trout needs further study.

Life history variation among female surfperches (Perciformes: Embiotocidae)

SynopsisLife history variation within the family Embiotocidae is extensive and involves differences in age of first reproduction, fecundity schedules, growth rates, longevity and size of young. Based on maximum reported body lengths, there are three distinct size groups among the familys 23 species. Small species do not exceed 215 mm TL, medium-size species attain 275 to 335 mm TL, and the large species attain 380 to 470 mm TL. The longevity oh surfperches varies from two to ten years, growth is indeterminate, and females of the medium-and large-size groups may delay first reproduction beyond age one. With one exception, all species show increasing length-specific fecundities. The life history characteristics of females differ among the three size groups. Relative to smaller species, the largest species have moderately high fecundity, delayed maturity and long life. Medium-size species have low fecundity, may delay maturity for 1 to 3 years and have intermediate life spans. Small species have generally higher, but variable, fecundity, do not delay maturity, and are short lived. Among the small North American species, the trend in fecundity varies inversely with environmental predictability. Fecundity is highest in the species which occupies highly seasonal freshwater environments. Coastal species produce moderately large broods and species which occupy stable deep water environments produce the smallest broods.

Life history characteristics of tule perch (Hysterocarpus traski) populations in contrasting environments

SynopsisThe life history characteristics of tule perch were compared within and among populations in three drainages with substantially different environmental conditions. Within populations there were, in general, substantial increases in brood size, size of young, brood weight, and gonadal-somatic index (GSI) with age and with size of female. In at least one population there was a trade-off between the number and size of young produced. However, in a lake where individual growth was very slow, brood weight, GSI, and other life history characters showed decreasing trends with age. Among populations in isolated drainages, female length at first reproduction and longevity varied directly and mean brood size varied inversely with environmental predictability. Morphological and geological evidence and some comparative litter characteristics suggest that life history differences among populations in isolated drainages are in part genetic.

Spatial, seasonal and diel distribution of fishes in a California reservoir dominated by native fishes

Abstract During 21 months of sampling with various techniques, we captured 24 species of fish in Britton Reservoir. Nine species comprised over 96% of the number of fish captured and approximately 88% of the biomass. Five native non-game species accounted for over 77% of the catches. The native non-game fishes have maintained large populations in the reservoir despite continued introductions of non-native species. Two sources of non-native species exist. The first is the introduction of exotic species directly into the reservoir during fish-stocking programs. The second is the continuous movement of non-native fishes into the reservoir from large populations which reside in a major tributary of the reservoir. Factors responsible for the large number of native fishes are: management of the reservoir for hydroelectric generation; temperature regime; reservoir morphology. The fish community structure is stratified along two axes: upper basin/lower basin and inshore/offshore. Most of the 24 species were found inshore: 14 species were found offshore. Four of the native non-game fishes were most abundant in the upper basin: three introduced non-native fishes were most abundant in the lower basin of the reservoir. The offshore community was dynamic on a daily and seasonal basis.

Morphometric Analysis of Tule Perch (Hysterocarpus traski) Populations in Three Isolated Drainages

Morphometric analysis of tule perch from six sites in three isolated drainages was conducted to assess the validity of previous systematic work. Nine variables were subjected to principal component and discriminant analysis. Principal component analysis indicated that groupings by drainage were real. Classification by discriminant functions among drainages was 96% accurate. Classification among collection sites was less accurate (88% overall), but most misclassified cases were assigned to another site in the same drainage. Ecophenotypic variation was apparent but the degree of overlap between drainages was minimal and indicated that the within-drainage differences were less than the among-drainage differences. Our analysis supports the view that populations in separate drainages are sufficiently distinct to warrant subspecific designation. T HE tule perch (Hysterocarpus traski) is the only exclusively freshwater member of the surfperch family (Embiotocidae) and is confined to three drainages in central California. It is unique among the freshwater fauna of California in that it is the only native viviparous fish. One population is largely confined to two lakes in the Clear Lake drainage basin, another is confined to the Russian River, a large coastal stream, and the third is found in a wide variety of habitats in the Sacramento-San Joaquin drainage including small tributary streams, main river channels, impoundments, and sloughs in the Sacramento-San Joaquin Estuary. Hopkirk (1962, 1973) recognized the populations in each drainage as morphological subspecies but Hubbs (1974) questioned Hopkirks findings, mostly on the basis of his methodol