Jason B. Dunham

Effects of fire on fish populations: landscape perspectives on persistence of native fishes and nonnative fish invasions

Our limited understanding of the short and long-term effects of fire on fish contributes to considerable uncertainty in assessments of the risks and benefits of fire management alternatives. A primary concern among the many potential effects of fire is the effects of fire and fire management on persistence of native fish populations. Limited evidence suggests vulnerability of fish to fire is contingent upon the quality of affected habitats, the amount and distribution of habitat (habitat fragmentation), and habitat specificity of the species in question. Species with narrow habitat requirements in highly degraded and fragmented systems are likely to be most vulnerable to fire and fire-related disturbance. In addition to effects of fire on native fish, there are growing concerns about the effects of fire on nonnative fish invasions. The role of fire in facilitating invasions by nonnative fishes is unknown, but experience with other species suggests some forms of disturbance associated with fire may facilitate invasion. Management efforts to promote persistence of fishes in fire-prone landscapes can take the form of four basic alternatives: (1) pre-fire management; (2) post-fire management; (3) managing fire itself (e.g. fire fighting); and (4) monitoring and adaptive management. Among these alternatives, pre-fire management is likely to be most effective. Effective pre-fire management activities will address factors that may render fish populations more vulnerable to the effects of fire (e.g. habitat degradation, fragmentation, and nonnative species). Post-fire management is also potentially important, but suffers from being a reactive approach that may not address threats in time to avert them. Managing fire itself can be important in some contexts, but negative consequences for fish populations are possible (e.g. toxicity of fire fighting chemicals to fish). Monitoring and adaptive management can provide important new information for evaluating alternatives, but proper implementation is often hampered by inadequate study designs and inconsistent financial and institutional support. The challenge for providing better management guidelines will be to add solid empirical data and models to assess the relevance of emerging concepts and theories, and provide a sense of where and when fires pose significant risks and/or benefits to fishes.

Validation of Abundance Estimates from Mark–Recapture and Removal Techniques for Rainbow Trout Captured by Electrofishing in Small Streams

Abstract Estimation of fish abundance in streams using the removal model or the Lincoln– Peterson mark–recapture model is a common practice in fisheries. These models produce misleading results if their assumptions are violated. We evaluated the assumptions of these two models via electrofishing of rainbow trout Oncorhynchus mykiss in central Idaho streams. For one-, two-, three-, and four-pass sampling effort in closed sites, we evaluated the influences of fish size and habitat characteristics on sampling efficiency and the accuracy of removal abundance estimates. We also examined the use of models to generate unbiased estimates of fish abundance through adjustment of total catch or biased removal estimates. Our results suggested that the assumptions of the mark– recapture model were satisfied and that abundance estimates based on this approach were unbiased. In contrast, the removal model assumptions were not met. Decreasing sampling efficiencies over removal passes resulted in underestimated population...

Sources and Magnitude of Sampling Error in Redd Counts for Bull Trout

Abstract Monitoring of salmonid populations often involves annual redd counts, but the validity of this method has seldom been evaluated. We conducted redd counts of bull trout Salvelinus confluentus in two streams in northern Idaho to address four issues: (1) relationships between adult escapements and redd counts; (2) interobserver variability in redd counts; (3) sources of interobserver variability; and (4) temporal and spatial variation in spawning activity. We found that estimated adult escapements and redd counts were strongly correlated on a logarithmic scale, but both sources of data probably contained large estimation or observation errors. In particular, redd counts varied significantly among observers in replicate counting trials. Observer counts ranged between 28% and 254% of the best estimates of actual redd numbers. Counting errors included both omissions and false identifications. Correlations between counting errors and redd and habitat characteristics were highly variable and provided lim...

CHINOOK SALMON USE OF SPAWNING PATCHES: RELATIVE ROLES OF HABITAT QUALITY, SIZE, AND CONNECTIVITY

Declines in many native fish populations have led to reassessments of management goals and shifted priorities from consumptive uses to species preservation. As management has shifted, relevant environmental characteristics have evolved from traditional metrics that described local habitat quality to characterizations of habitat size and connectivity. Despite the implications this shift has for how habitats may be prioritized for conservation, it has been rare to assess the relative importance of these habitat components. We used an information-theoretic approach to select the best models from sets of logistic regressions that linked habitat quality, size, and connectivity to the occurrence of chinook salmon (Oncorhynchus tshawytscha) nests. Spawning distributions were censused annually from 1995 to 2004, and data were complemented with field measurements that described habitat quality in 43 suitable spawning patches across a stream network that drained 1150 km2 in central Idaho. Results indicated that the most plausible models were dominated by measures of habitat size and connectivity, whereas habitat quality was of minor importance. Connectivity was the strongest predictor of nest occurrence, but connectivity interacted with habitat size, which became relatively more important when populations were reduced. Comparison of observed nest distributions to null model predictions confirmed that the habitat size association was driven by a biological mechanism when populations were small, but this association may have been an area-related sampling artifact at higher abundances. The implications for habitat management are that the size and connectivity of existing habitat networks should be maintained whenever possible. In situations where habitat restoration is occurring, expansion of existing areas or creation of new habitats in key areas that increase connectivity may be beneficial. Information about habitat size and connectivity also could be used to strategically prioritize areas for improvement of local habitat quality, with areas not meeting minimum thresholds being deemed inappropriate for pursuit of restoration activities.

Fine-scale natal homing and localized movement as shaped by sex and spawning habitat in Chinook salmon: Insights from spatial autocorrelation analysis of individual genotypes

Natal homing is a hallmark of the life history of salmonid fishes, but the spatial scale of homing within local, naturally reproducing salmon populations is still poorly understood. Accurate homing (paired with restricted movement) should lead to the existence of fine‐scale genetic structuring due to the spatial clustering of related individuals on spawning grounds. Thus, we explored the spatial resolution of natal homing using genetic associations among individual Chinook salmon (Oncorhynchus tshawytscha) in an interconnected stream network. We also investigated the relationship between genetic patterns and two factors hypothesized to influence natal homing and localized movements at finer scales in this species, localized patterns in the distribution of spawning gravels and sex. Spatial autocorrelation analyses showed that spawning locations in both sub‐basins of our study site were spatially clumped, but the upper sub‐basin generally had a larger spatial extent and continuity of redd locations than the lower sub‐basin, where the distribution of redds and associated habitat conditions were more patchy. Male genotypes were not autocorrelated at any spatial scale in either sub‐basin. Female genotypes showed significant spatial autocorrelation and genetic patterns for females varied in the direction predicted between the two sub‐basins, with much stronger autocorrelation in the sub‐basin with less continuity in spawning gravels. The patterns observed here support predictions about differential constraints and breeding tactics between the two sexes and the potential for fine‐scale habitat structure to influence the precision of natal homing and localized movements of individual Chinook salmon on their breeding grounds.

Assessing the Consequences of Nonnative Trout in Headwater Ecosystems in Western North America

Abstract Intentional introductions of nonnative trout into headwater lakes and streams can have numerous effects on the receiving ecosystems, potentially threatening native species and disrupting key ecological processes. In this perspective, we focus on seven key issues for assessing the biological and economic consequences of nonnative trout in headwater ecosystems: (1) effects of nonnative trout can span multiple biological domains, (2) effects of nonnative trout can extend beyond waters where they are introduced, (3) nonnative trout do not travel alone, (4) not all habitats are equal, (5) ecosystems vary in their resistance and resilience to nonnative trout, (6) prioritization can improve management of nonnative trout, and (7) economic costs of recreational fisheries in headwater ecosystems can be substantial. Assessments that address these issues could provide more effective guidance for determining where recreational fisheries for nonnative trout are justified in headwater ecosystems and where they ...

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...

Invasion by nonnative brook trout in Panther Creek, Idaho: Roles of local habitat quality, biotic resistance, and connectivity to source habitats

Abstract Theoretical models and empirical evidence suggest that the invasion of nonnative species in freshwaters is facilitated through the interaction of three factors: habitat quality, biotic resistance, and connectivity. We measured variables that represented each factor to determine which were associated with the occurrence of nonnative brook trout Salvelinus fontinalis in Panther Creek, a tributary to the Salmon River, Idaho. Habitat variables included measures of summer and winter temperature, instream cover, and channel size. The abundance of native rainbow trout Oncorhynchus mykiss within sampled sites was used as a measure of biotic resistance. We also considered the connectivity of sample sites to unconfined valley bottoms, which were considered habitats that may serve as sources for the spread of established populations of brook trout. We analyzed the occurrence of small (<150-mm [fork length]) and large (≥150-mm) brook trout separately, assuming that the former represents an established invasi...

Influences of Body Size and Environmental Factors on Autumn Downstream Migration of Bull Trout in the Boise River, Idaho

Abstract Many fishes migrate extensively through stream networks, yet patterns are commonly described only in terms of the origin and destination of migration (e.g., between natal and feeding habitats). To better understand patterns of migration in bull trout, Salvelinus confluentus we studied the influences of body size (total length (TL)) and environmental factors (stream temperature and discharge) on migrations in the Boise River basin, Idaho. During the autumns of 2001–2003, we tracked the downstream migrations of 174 radio-tagged bull trout ranging in size from 21 to 73 cm TL. The results indicated that large bull trout (>30 cm) were more likely than small fish to migrate rapidly downstream after spawning in headwater streams in early autumn. Large bull trout also had a higher probability of arriving at the current terminus of migration in the system, Arrowrock Reservoir. The rate of migration by small bull trout was more variable and individuals were less likely to move into Arrowrock Reservoir. The...