Gary L. Vinyard

Habitat Fragmentation and Extinction Risk of Lahontan Cutthroat Trout

Abstract We used survey data collected by the Nevada Division of Wildlife to analyze presence or absence of Lahontan cutthroat trout Oncorhynchus clarki henshawi in relation to habitat fragmentation (isolation), habitat size and shape, presence or absence of nonnative salmonids, elevation, latitude, longitude, and precipitation regime for 119 stream basins in the eastern Lahontan basin. Multiple logistic regression analysis revealed stream basin isolation to be the only significant correlate of Lahontan cutthroat trout occurrence. Eighty-nine percent of stream basins connected to another basin containing Lahontan cutthroat trout also supported Lahontan cutthroat trout, while only 3290 of isolated stream basins supported Lahontan cutthroat trout. This analysis highlights the potentially negative effect of habitat fragmentation on population persistence for this threatened species.

Effects of High Chronic Temperatures and Diel Temperature Cycles on the Survival and Growth of Lahontan Cutthroat Trout

Abstract Tests of chronic temperature tolerance indicated that the upper limit for survival and growth of Lahontan cutthroat trout Oncorhynchus clarki henshawi was between 22°C and 24°C. Lahontan cutthroat trout experienced complete mortality within 2 d at 28°C and approximately 60% mortality over 7 d at 26°C. Fish suffered no significant mortality at temperatures of 24°C and below. No significant differences in growth of fish held at 22°C relative to fish held at cooler temperatures were found, but significant reductions in growth were observed at 24°C. When fish were subjected to fluctuating temperatures similar to field conditions, which cycle daily from 20°C to 26°C, they still grew, though not as much as fish maintained at constant 13°C or 20°C. Exposure to diel temperature fluctuations did not affect tolerance of subsequent chronic elevated temperatures.

Local and Geographic Variability in the Distribution of Stream-Living Lahontan Cutthroat Trout

Abstract We investigated local and geographic variability in the up- and downstream distribution limits of threatened Lahontan cutthroat trout (Oncorhynchus clarki henshawi) in stream habitats of the eastern Lahontan basin in northern Nevada and southeastern Oregon. At a geographic scale, elevations of upstream distribution limits were significantly correlated with latitude and longitude, suggesting a potential influence of climatic gradients. Elevations of upstream distribution limits also were positively correlated with maximum basin elevation, which suggested topographic, rather than climatic constraints may be important. Upstream distribution limits were not significantly affected by local variation in stream size or presumptive dispersal barriers. Stream gradient was related to upstream distribution limits, but this was again confounded by maximum basin elevation. Stream gradients used by Lahontan cutthroat trout at upstream limits were considerably steeper than those observed for other subspecies of...

Incorporating Stream Level Variability into Analyses of Site Level Fish Habitat Relationships: Some Cautionary Examples

Abstract Spatial variation in stream fish populations and habitats can be partitioned into many hierarchical levels (e.g., among drainage basins, streams within basins, and sites within streams). Studies of site level habitat relationships in more than one stream are common in fisheries research. Analyses of such data typically involve multiple regression to relate site level habitat features and fish population characteristics (e.g., biomass). Because sites within streams may not be independent, multiple-regression models should also include qualitative stream effects. As we show here with hypothetical and real examples, ignoring stream effects can lead to erroneous conclusions about the significance of site level habitat variables. Site and stream level effects may function independently or interactively in relation to fish populations. Alternatively, site and stream level effects may be confounded. An example with data on trout populations revealed that highly significant site level effects were only m...

Particle ingestion by Tilapia galilaea is not affected by removal of gill rakers and microbranchiospines

Abstract Particle ingestion by filter-feeding Galilee Saint Peters fish Tilapia galilaea increased as a function of particle size, leveling off when particle diameter exceeded 20 μm. Ingestion rates by this cichlid also increased with particle concentration, asymptotically approaching maxima of 4,785 and 84,746 particles˙fish −1˙min−1 for small (3.9–6.0 cm standard length, SL) and large (12.6–14.3 cm SL) fish, respectively. Surgical removal of gill rakers and microbranchiospines did not affect particle ingestion rates or selectivity.

Feeding behavior of the cichlid, Sarotherodon galilaeum: selective predation on Lake Kinneret zooplankton

In laboratory feeding trials, we analyzed the feeding behavior and selectivity of the cichlid, Sarotherodon galilaeum, for zooplankton prey from Lake Kinneret, Israel. The feeding behavior was dependent on fish size. Fish less than 20 mm SL fed on zooplankton as obligate particulate feeders. Fish from 20 to 42 mm SL fed either as particulate feeders or as filter feeders. Fish larger than 62 mm SL fed as obligate filter feeders. Particulate-feeding fish were size selective and had highest feeding electivities for large-sized zooplankton species. Filter-feeding fish had highest feeding electivities for zooplankton species with poor escape ability. In general, S. galilaeum predation pressure would be greatest on Ceriodaphnia reticulata, a large-bodied and easily captured species which is selected by both particulate-feeding and filter-feeding fish.

Assessing Extinction Risk: Integrating Genetic Information

Risks of population extinction have been estimated using a variety of methods incorporating information from different spatial and temporal scales. We briefly consider how several broad classes of extinction risk assessments, including population viability analysis, incidence functions, and ranking methods integrate information on different temporal and spatial scales. In many circumstances, data from surveys of neutral genetic variability within, and among, populations can provide information useful for assessing extinction risk. Patterns of genetic variability resulting from past and present ecological and demographic events, can indicate risks of extinction that are otherwise difficult to infer from ecological and demographic analyses alone. We provide examples of how patterns of neutral genetic variability, both within, and among populations, can be used to corroborate and complement extinction risk assessments.

Effects of High Levels of Total Dissolved Solids in Walker Lake, Nevada, on Survival and Growth of Lahontan Cutthroat Trout

Abstract Walker Lake, Nevada, is an endorheic terminal lake experiencing significant increases in total dissolved solids (TDS) because of culturally derived reductions in inflow and continued evaporative water losses. Maintenance of the Walker Lake fishery for native Lahontan cutthroat trout Oncorhynchus clarki henshawi requires artificial propagation because fish no longer have access to suitable spawning sites upstream in the Walker River. In recent years, survival of the stocked fish has been very low. We examined survival and growth of Lahontan cutthroat trout in response to present and anticipated total dissolved solids (TDS) concentrations in Walker Lake. Laboratory experiments indicated that current TDS levels in Walker Lake adversely affected both survival and growth of Lahontan cutthroat trout, and any increase in TDS is likely to further decrease survival. Larger fish are more tolerant of high concentrations of TDS. Acclimation to diluted Walker Lake water improves subsequent survival when fish ...

Drift ecology of western catostomid larvae with emphasis on Warner suckers, Catostomus warnerensis (Teleostei)

We studied drift ecology of Warner sucker, Catostomus warnerensis, larvae in streams of Warner Valley, Oregon in 1992 and 1993 in part to determine whether downstream transport of larvae limits juvenile recruitment. Variation in runoff for the two years was extreme, where peak discharge in 1992 was two orders of magnitude lower than peak discharge in 1993. We deployed drift nets in low-gradient stream sections that are modified by diversions. Low catches of drifting sucker larvae (zero in 1992 and two in 1993) indicate that losses into diversions were probably minimal. In 1992, we conducted in situ experiments to determine how sucker larvae behaved when exposed to high velocity current by placing individuals of various sizes in mid-channel and tracking drift responses by snorkeling. Even the smallest larvae used did not drift far (2.6 m) or for very long (0.97 min), as they resisted downstream transport by exploiting cover provided by aquatic plants and debris. We also recorded locations of larvae during a 24-hour cycle to determine whether positions occupied during the day changed at night. We found that groups were positioned closer to shore at night in 1993 than in 1992, suggesting that larvae reduced the likelihood of becoming entrained into swift current by moving away from mid-channel at night when visual orientation is impaired. These patterns of drift avoidance are different from behaviors of other western catostomid larvae, potentially because terminal lake habitat in Pleistocene basins such as the Warner Valley has been unreliable.

Gyrinid searching tactics: empirical observations and a tactical model

SummaryData on Gyrinus picipes were collected to examine the influence of prey distribution and capture on search path configuration. All paths were analyzed before and after prey capture. Parameters measured were turning angles, step lengths, and step direction. Three indices of search efficiency were calculated: linear displacement, thoroughness, and straightness. Prey distribution had little impact on search configuration. However, prey capture caused significant reductions in step length, linear displacement, and straightness. Thoroughnnes and turning angle increased significantly after prey capture.A simulation model was developed to analyze influences of search parameters on search efficiency. Search paths, simulated using empirically derived parameters, did not differ from actual search paths indicating that the model accurately describes gyrinid search paths and may be used in these analyses. Actual search paths were compared to a simulated Brownian (random) search. Search paths before prey capture were significantly different from random, but Brownian search can describe search paths after prey capture. Comparisons of simulations using various input parameters indicate that step length has the greatest effect on search paths. Turning angles and direction are important before prey capture but not after.