Non-native Brown Trout Salmo Trutta Research Articles

Low‐head dam fragmentation, habitat alteration, and invasive predators degrade a Western United States stream fish assemblage

AbstractDams fragment streams, alter hydrology and habitat, and facilitate establishment of nonnative species worldwide to the detriment of native biota. Understanding and mitigating these effects to conserve and restore stream fish assemblages has relied on short‐ and long‐term datasets to assess acute and chronic change through time, craft management strategies, and measure remediation success. We used sampling records collected over a 29‐year period (1993–2021) to examine likely causes of fish assemblage change in the Cache la Poudre River, Colorado, USA. Numerous low‐head dams have reduced connectivity and altered flow, temperature, and habitat in the transition zone, a reach that historically supported rare and sensitive taxa valuable to regional biodiversity. We found diversity, distribution, and abundance of native species declined since the early 1990s, with formerly rare taxa extirpated and some common species becoming rare. Native taxa remained numerically dominant in warmer downstream reaches most affected by streamflow diversion but were incrementally reduced in richness and abundance upstream of low‐head dams without fishways. Concurrently, nonnative Brown Trout Salmo trutta increased in distribution and abundance, dominating upstream reaches that receive cooler and more stable flows, and expanding into downstream reaches where they were formerly absent, with likely negative consequences for native fishes. In the absence of mitigation, these collective effects, plus recent wildfire disturbance and future water development, will continue to degrade stream fish assemblages in our study area, and worldwide, where resource managers face the often‐competing interests of conserving native species, providing recreational fisheries, and meeting increasing water demands.

Native Fish Loss in a Transition-Zone Stream Following Century-Long Habitat Alterations and Nonnative Species Introductions

Habitat destruction, hydrologic alteration, and nonnative species introductions have greatly altered the composition of North American stream fish assemblages. In the western United States, intermediate-elevation transition-zone stream reaches––those located between higher-elevation mountain and lower-elevation plains areas––historically supported native fish assemblages that included sensitive species and peripheral or isolated populations of more widespread taxa. We compiled historical records and conducted sampling to examine changes to the fish assemblage in the transition-zone reach of South Boulder Creek, Colorado, USA. Because human development of transition zones altered stream and riparian ecosystems prior to sampling, we developed a historical timeline of human activities to formulate hypotheses regarding factors that influenced fish assemblage change through time. Native fish species richness declined by 60% from the likely historical assemblage, and we hypothesize that widespread alteration of stream and riparian habitat in the mid-1800s caused a first wave of species losses, especially taxa with specific habitat or life history requirements. The remaining coolwater native fishes dominated through the mid-1990s, but more recently, the coldwater-tolerant, nonnative Brown Trout Salmo trutta has greatly expanded and is implicated in additional native species loss. Possible management actions to restore native fish assemblages in transition-zone streams include the following: increasing habitat diversity, particularly off-channel backwaters suited to lentic-adapted native fishes; providing refuges from predators or reducing predator abundance; restoring sinuosity to straightened reaches to improve channel function; improving stream connectivity via installation of fish passages; reintroducing beaver or installing beaver dam analogs; provisioning of suitable flow; and eventually, identifying native stocks for reintroduction. Management of other western United States transition-zone stream reaches may benefit from these insights, especially if conservation of the locally distinct native fish assemblages and their scarce habitat is a priority in the face of rapid human population growth and expanding invasive species.

When Nature Decides who Stays and who Goes:Priority effects Extirpating the Non-Native Brown Trout Salmo Trutta fario L. Population from a Himalayan River

Climate-induced stochastic events bring forth idiosyncratic changes in habitat and community assemblage patterns of an area. Nature governs its priorities in such incidents, where native taxa are preferred over the invasive ‘late-comers’, thus leading to extirpation of the latter, according to the concept of ‘priority effects’ 1 . This stronghold of nature strictly comes into play when an ecosystem is least altered from its natural state. River Asiganga in Uttarakhand, India was chosen as a case study to understand this theory, where cloudburst-induced flash floods are considered to be a reason for the complete wipe-out of non-native brown trout ( Salmo trutta fario ), leaving behind the native snow trout ( Schizothorax species).

Evaluating the Trade‐Offs between Invasion and Isolation for Native Brook Trout and Nonnative Brown Trout in Pennsylvania Streams

AbstractA popular conservation strategy for native trout species in western North America is to prevent invasions by nonnative trout by installing barriers that isolate native trout populations into headwater streams. In eastern North America, native Brook Trout Salvelinus fontinalis are frequently replaced in coolwater habitats by nonnative Brown Trout Salmo trutta and relegated to small headwater streams. In this study, we compared the effects of isolation and invasion by nonnative Brown Trout on the distribution and demographic structure of Brook Trout populations from 78 trout streams in northwestern Pennsylvania. The Brook Trout and Brown Trout distributions varied in predictable ways along the stream size gradient, with Brown Trout becoming dominant in larger streams. However, there was a prominent barrier effect, with streams 12 times more likely to have Brook Trout than Brown Trout when a downstream barrier was present between the sample site and the nearest Brown Trout stocking location. In comparison, 91% of the streams with Brown Trout had no downstream barrier, suggesting that barriers are important in creating refugia for Brook Trout. Brown Trout also appeared to have a negative impact on Brook Trout population demographics, as Brook Trout populations in sympatry with Brown Trout had fewer age‐classes and lower population densities than allopatric Brook Trout populations. Isolating Brook Trout to small headwater streams with downstream barriers that prevent Brown Trout invasion could be a viable conservation strategy in regions where barriers would serve to reduce the negative impacts from Brown Trout. Since barriers could further fragment local Brook Trout populations, however, they would need to be strategically placed to allow for seasonal movements to maintain metapopulation structure and ensure population persistence.

Displacement of native white-spotted charr Salvelinus leucomaenis by non-native brown trout Salmo trutta after resolution of habitat fragmentation by a migration barrier.

After resolution of habitat fragmentation by an erosion-control dam, non-native brown trout Salmo trutta invaded the upstream side of the dam and displaced native white-spotted charr Salvelinus leucomaenis in Monbetsu stream, Hokkaido, northern Japan.

Density-dependent effects of non-native brown trout Salmo trutta on the species-area relationship in stream fish assemblages.

The spatial scale and density-dependent effects of non-native brown trout Salmo trutta on species richness of fish assemblages were examined at 48 study sites in Mamachi Stream, a tributary of Chitose River, Hokkaido, Japan. The density of age ≥1 year S. trutta was high in the upstream side of the main stem of Mamachi Stream. Fish species richness increased with increasing area of study sites (habitat size), but the increasing magnitude of the species richness with area decreased with increasing age of ≥1 year S. trutta density. The relationships between age ≥1 year S. trutta, however, and presence-absence of each species seemed to be different among species. Species richness was also determined by location and physical environmental variables, i.e. it was high on the downstream side and in structurally complex environments.

The density dependent interspecific competition between nonnative salmonids, rainbow trout and brown trout

Interspecific interactions play a key role in determining species assemblages when nonnative species are introduced. In the Chitose River system, Hokkaido, northern Japan, nonnative brown trout Salmo trutta may have replaced nonnative rainbow trout Oncorhynchus mykiss due to interspecific competition. The outcome of this competition is predicted to be influenced by the density of both species. Then, I conducted an enclosure experiment to test whether density-dependent interspecific competition between YOY (young-of-the-year) of these species may have explained the species replacement. Rainbow trout decreased their stomach contents in the high density treatment sympatry with brown trout, and growth rate of rainbow trout decreased in the high density treatments both allopatry and sympatry with brown trout. Stomach contents and growth of brown trout were not affected by competitor species (rainbow trout) or total fish density. These results imply that brown trout are competitively superior to rainbow trout. Therefore, there is the potential for the replacement of rainbow trout by brown trout through interspecific competition.

Evidence that an ontogenetic niche shift by native masu salmon facilitates invasion by nonnative brown trout

The mechanisms underlying successful invasions by a competitively subordinate species are poorly understood. In Japan, nonnative brown trout Salmo trutta L. have successfully invaded a number of streams that contain native masu salmon Oncorhynchus masou Brevoort, even though young of the year (YOY) brown trout are thought to be competitively subordinate to YOY masu salmon because of their smaller body size due to later emergence time. We conducted a laboratory experiment and field observations to evaluate whether ontogenetic habitat shifts could explain the success of brown trout. In a laboratory experiment, smaller YOY brown trout were competitively inferior to YOY masu salmon. Our field observations suggest that YOY masu salmon shift to deeper and faster velocity habitat before the emergence of brown trout fry. Thus, there is separation of habitat use between the two species. Taken together, our results suggest an ontogenetic niche shift by competitively dominant native masu salmon may reduce niche overlap and the opportunity for competitive interactions, and thus facilitate invasion by a nonnative brown trout.

Evaluating the Reintroduction Potential of Lahontan Cutthroat Trout in Fallen Leaf Lake, California

AbstractWe evaluated the potential for reintroducing Lahontan cutthroat trout Oncorhynchus clarkii henshawi, a species listed under the Endangered Species Act, into a lacustrine system where the biotic community has changed dramatically since the species' extirpation there. Since 2002, 76,547 Lahontan cutthroat trout have been reintroduced into Fallen Leaf Lake, California; we used creel surveys, diet data, mark–recapture methods, bioenergetics modeling, and netting data across seasons to evaluate the habitat use, growth, and relative abundance of Lahontan cutthroat trout and the abundance, diet, habitat use, and predation by nonnative species. Sampling totals (n = 2) and survey observations (n = 3) indicate low survival and abundance of reintroduced fish and together with creel data indicate the importance of epilimnetic habitats across size‐classes. Despite substantial growth, Lahontan cutthroat trout exhibited low condition factor values (average = 0.69). We found substantial predation pressure from a large population of lake trout Salvelinus namaycush (Ṋ = 8,799 fish; 95% confidence interval [CI] = 4,990–16,530 fish); analysis of lake trout diets showed an increase in piscivory and in the percentage of stomachs containing Lahontan cutthroat trout anchor tags with increasing predator size. Overall, we estimated that lake trout consumed over 38% of reintroduced Lahontan cutthroat trout (mean number consumed = 7,736 fish; 95% CI = 4,388–14,534 fish). With bioenergetics modeling, however, we estimated that lake trout consumed considerable amounts of salmonid biomass during this period (mean biomass = 3,137 kg; 95% CI = 1,779–5,893 kg), which greatly exceeded the biomass of Lahontan cutthroat trout reintroduced in 2006. During the stratification period, there was little overlap in habitat use between lake trout and Lahontan cutthroat trout, but overlap was high during the spring and autumn. We found moderate‐sized populations of nonnative brown trout Salmo trutta, kokanee O. nerka (lacustrine sockeye salmon), and rainbow trout O. mykiss. Together, our results suggest that Lahontan cutthroat trout have few refugia from direct and indirect negative interactions with nonnative species and that alternative approaches are needed.

Evidence for interspecific hybridization between native white‐spotted charr Salvelinus leucomaenis and non‐native brown trout Salmo trutta on Hokkaido Island, Japan

Hybrids between native white-spotted charr Salvelinus leucomaenis and non-native brown trout Salmo trutta were identified in streams of Hokkaido, Japan, using both appearance and genetic characters. The DNA analyses indicated that the specimens were hybrids between female S. leucomaenis and male S. trutta. Occurrence of such hybrids implies increased mating opportunities between these species in wild streams.

Interactions between Slimy Sculpin and Trout: Slimy Sculpin Growth and Diet in Relation to Native and Nonnative Trout

AbstractTo investigate whether introductions of nonnative trout affect growth and diet of nongame fish in small streams, we designed a field experiment to examine interactions between slimy sculpin Cottus cognatus and native brook trout Salvelinus fontinalis or nonnative brown trout Salmo trutta. We hypothesized that brown trout would compete with and reduce growth of slimy sculpin. We expected no change in slimy sculpin growth in treatments with brook trout because the two species co‐occur in their native range and thus may have evolved methods to partition resources and decrease competitive interactions. Enclosures (1 m2) were stocked with (1) juvenile brown trout and slimy sculpin, (2) juvenile brook trout and slimy sculpin, or (3) slimy sculpin alone (control). Fish were stocked at three densities to examine intraspecific versus interspecific competition. Replicates of each treatment were placed in riffles in Valley Creek, Minnesota, and six experimental trials were conducted over three summers (2002‐2004). Brown trout presence was associated with reduced growth of large slimy sculpin in enclosures, whereas brook trout presence produced no change in slimy sculpin growth; these effects did not depend on fish density. Brown trout or brook trout presence was not associated with shifts in the diets of slimy sculpin, indicating that reduced slimy sculpin growth in the presence of brown trout was not due to prey selection or prey availability changes. Our research suggests that effects on growth of slimy sculpin in Valley Creek differ between introduced brown trout and native brook trout; however, the mechanisms underlying changes in slimy sculpin growth are unclear. Although brook trout and brown trout appear to fill similar ecological roles in small, coldwater streams, brown trout may negatively impact growth of nongame fish.

Competition between Hatchery‐Raised Rio Grande Cutthroat Trout and Wild Brown Trout

AbstractAn important problem for conservation of cutthroat trout Oncorhynchus clarkii is the presence of nonnative trout. We used eight 20‐m instream enclosures on the Rio Cebolla, New Mexico, to evaluate competition between hatchery‐raised Rio Grande cutthroat trout O. clarkii virginalis and similar‐sized wild, nonnative brown trout Salmo trutta. Four control enclosures each contained 20 fish of the same species; four treatment enclosures each contained equal numbers of like‐sized fish of the two species. Results indicated that wild brown trout were superior competitors to hatchery‐raised Rio Grande cutthroat trout. Significant interspecific competition was inferred from comparisons of treatment fish with their controls. Among Rio Grande cutthroat trout, control fish grew in length and body weight, whereas growth of fish confined in enclosures with brown trout was significantly lower than that of controls (P < 0.001). In contrast, brown trout enclosed with Rio Grande cutthroat trout experienced significantly greater growth than did control brown trout (P < 0.005). Rio Grande cutthroat trout in enclosures with brown trout had lower postmortem fat content and an elevated incidence of caudal fin damage than did control fish. Gut contents showed that control fish of both species fed most frequently on small larval caddisflies Lepidostoma spp. However, when the two species were confined together, more brown trout had food in the gut and the food items consumed by Rio Grande cutthroat trout shifted from Lepidostoma spp. to surface‐occurring invertebrates (terrestrial infall, water striders, water mites, and phaerate adult aquatic insects) and benthic‐associated invertebrates (oligochaetes, caddisflies (Limnephilidae and Helicopsyche spp.), and snails). Collectively, the results indicated interspecific competition that was mediated through elevated aggression by brown trout that altered foraging activity by both species. The results underscore the importance of minimizing contact with nonnative brown trout when using hatchery fish to restore cutthroat trout to suitable habitats.

Experimental Effects of Nonnative Brown Trout on the Individual‐ and Population‐Level Performance of Native Bonneville Cutthroat Trout

AbstractIn western North America, nonnative trout invasions threaten the persistence of native cutthroat trout Oncorhynchus clarkii subspp. through competition, predation, and hybridization. While small‐scale individual‐level studies on nonnative–native fish interactions have helped elucidate mechanisms of impact, very few investigations have occurred at a scale consistent with that of species replacement. Thus, large‐scale population‐level studies are needed to understand the effects of nonnative fish as well as to more effectively conserve native fishes in the presence of nonnative fish. For this reason, we conducted an evaluation of the effects of nonnative brown trout Salmo trutta on the individual‐ and population‐level performance of native cutthroat trout using a field‐experimental approach. We studied local populations of cutthroat trout in the presence and absence of brown trout in long stream reaches and subsequently compared growth, condition, stable‐isotope‐based measures of dietary habits, movement, and survival between treatment groups. We found clear evidence for reduced cutthroat trout growth owing to the presence of brown trout. Additionally, we observed differences in cutthroat trout dietary habits based on δ13C signatures resulting from our experimental manipulation of brown trout abundance. Finally, brown trout suppressed cutthroat trout movement but had no apparent effect on their survival. Considering previous studies, we concluded that the individual‐level effects of brown trout on cutthroat trout are scale invariant. Higher‐level impacts (i.e., on movement and survival), in contrast, appear to be more sensitive to the scale at which the investigation is conducted. Overall, our results indicate that brown trout can have strong negative effects on cutthroat trout performance and should therefore be more explicitly considered in native fish conservation plans.