Native Rainbow Trout Research Articles

Reduced Streamflow From Water Diversion Alters Stream Ecology and Fish Behavior

ABSTRACTUnderstanding invertebrate and fish responses to altered streamflows is important for water management that balances ecological function and societal needs. We studied the impacts of a significant water diversion on a stream food web, including benthic biofilms, benthic macroinvertebrates (BMI), and drifting macroinvertebrates. In the dry season, drift flux (number of aquatic individuals or biomass/s) and drift concentration (individual/m3) were lower below the diversion than above. Drift concentration (biomass/m3) was slightly higher below the diversion, due to the presence of large Trichopterans. All measures of drift dropped to nearly zero at the end of the dry season below the diversion, when streamflows were reduced by > 90% below the diversion relative to above, representing a significant loss of food resources for trout. Simultaneously, native Rainbow Trout (Oncorhynchus mykiss) switched from drift to benthic foraging 1 month earlier below the diversion as drift and water velocities declined. The ecology of the benthos was not as impacted by the diversion: BMI densities were higher below the diversion, while benthic biofilm densities were similar at the two sites. The taxonomic composition of BMI was also similar between the sites, but fewer Ephemeroptera entered the drift below the diversion. Overall, the late season decrease in drift below the diversion despite the similar or higher benthic production suggests that streamflows influence food availability for trout even in the absence of benthic responses. Reduced streamflows, due to water diversions as studied here, or due to climate change, will likely reduce food resources and growth opportunities for fishes.

Spawning interactions between hatchery‐reared and wild naturalized rainbow trout (Oncorhynchus mykiss, Walbaum, 1792) in high‐altitude tropical streams, Kenya

AbstractRainbow trout (Oncorhynchus mykiss) is among the most widely translocated fish species in the world. The current study evaluated the spawning interactions between naturalized and wild rainbow trout from two high‐altitude second‐order streams, in Kenya. Data on total length, weight, condition factor, fecundity, fertilization, egg diameter and fry survival were collected on spawning rainbow trout between March and December 2021. Length–weight relationship showed parabolic equations as W = 0.0144L2.900, W = 0.0069L3.0285 and W = 0.00027L3.175 for wild fish stock, hatchery‐reared and wild × hatchery‐reared rainbow trout, respectively. Total fecundity differed significantly among the hatchery‐reared, wild fish and the cross of the two (p < 0.05). The fertilization rate showed significant differences (p < 0.05), with no discernable difference observed between the hatchery‐reared and crossed (wild × hatchery‐reared). There was a positive correlation among the total fecundity to female egg weight, female body weight, fertilization rate and eyed egg survival in all the populations. Relative fecundity was significantly different among the three groups of fish (p < 0.05), but the differences between the hatchery and the crossed (wild × hatchery‐reared) fish showed no significant differences. We recommend the use of crossed (wild × hatchery‐reared) populations for fry production for use in aquaculture as they presented the highest fecundity and gave the best outcome of fry with high survival.

Reproductive performance assessed during three spawning seasons in a naturalized rainbow trout population from southern Chile

Naturalized rainbow trout (Oncorhynchus mykiss) populations are an important recreational fishing resource in the Araucanian lakes of southern Chile, but few reproductive studies have been undertaken to assess their sustainability. It is important to address this issue, since this analysis may provide information for improving management activities. In this study we carried out a biometric and reproductive analysis of naturalized rainbow trout breeders to describe their performance and to explore the variables that could determine fecundity, fertilization in addition to egg survival and size. We collected naturalized rainbow trout breeders from six tributaries of Calafquén Lake, during 2006, 2007 and 2012 to record their biometric and reproductive traits. Spawning and incubation were performed under controlled conditions in a hatchery facility to improve the recording process. In each spawning season, total fecundity (TF, No. of eggs/female), relative fecundity (RF, No. of eggs/kg female), fertilization rate (FR, %), egg diameter (ED, mm) and eyed egg survival (EES, %) were modeled as a function of biometric and reproductive parameters, using general linear mixed models. We used this statistical tool to explore the relationships between these reproductive traits and other breeder traits in order to identify all the potential combinations that could be linked to TF, RF, FR, ED and EES. The spawning period ranged from September to November (i.e., spring spawning season), with a clear spawning peak in October in all reproductive seasons. Mean TF (from 2275.9 to 3438.7) and RF (from 1730.5 to 1909.8) fell within the range of, or were close to, data reported for other Araucanian lakes, respectively. Mean values of FR and EES were relatively high, ranging from 88.8% to 93.3% and between 83.4% and 88.8%, respectively. Models indicated that TF is significantly affected by the interaction between female body weight and egg weight, while RF is affected by the combination of female body length and egg weight. In addition, a positive correlation was observed between female body weight and total fecundity, while an inverse association was recorded between relative fecundity and female body length. This trend is in accordance with the trade-off pattern between egg number and fish size described for naturalized populations of rainbow trout from the Northern Hemisphere. However, regression equations output suggests that the reproductive performance of the Calafquén Lake population has a more depleted fecundity in comparison either with naturalized populations from the Northern Hemisphere, or with cultured stocks of this species. Models for ED revealed that this variable is affected by egg weight and also by body length or body weight, all with positive correlations. This result concurs with observations reported for other naturalized populations of rainbow trout from North America, further supporting evidence that larger females of this species produce larger eggs than smaller females. Models for EES revealed the significant positive effect of fertilization rate and relative fecundity, being the latter an unexpected result, given that insignificant correlations between these variables have been found in cultured stocks of this species. Overall, the population analyzed exhibited a regular spawning period, and an optimal reproductive performance in terms of egg survival. To promote the sustainability of the naturalized rainbow trout of Calafquén Lake, we recommend the use of hatchery propagation of wild adults to support the production of fry to be used in subsequent restocking activities. This management effort can benefit because the broodstocks have a short spawning period during their spring run.

Intra- and inter-specific variability of IGF-I expression in farmed cold and warm-water species in Iran

Changes in the expression of liver insulin-like growth factor IGF-I mRNA in two growing cold-water fish species Oncorhynchusmykiss(one from locally produced and one from imported broodstock) and in two warm-water species, Hypophthalmichthysnobilis and Hypophthalmichthysmolitrix were analyzed. Gene expression analysis in liver tissues by using real-time PCR revealed a significant difference in expression level of IGF-I between native and non-native rainbow trout (P bighead carp> silver carp> native rainbow trout. Local strains of rainbow trout, bighead carp and silver carp exhibited stable IGF-I mRNA levels, at different stages of growth. The imported broodstock of rainbow trout exhibited increased IGF-I levels during the entire period studied, with a peak in the IGF-I mRNA levels of fingerlings. The results indicate intra- and inter-specific variability of IGF-I expression in farmed fish.

Genetic Differentiation and Origin of Naturalized Rainbow Trout Populations From Southern Chile, Revealed by the mtDNA Control Region Marker.

Numerous self-sustaining naturalized or introduced populations of rainbow trout (Oncorhynchus mykiss) are widely distributed throughout the freshwaters of southern Chile. In this study, analysis of the mitochondrial DNA control region (CR) marker was conducted to investigate the level of genetic divergence among populations and their phylogenetic relationships with respect to native lineages. This information provided a framework to interpret the genetic structure and origin that was shaped during historical trout introduction efforts. To this end, we analyzed eleven naturalized populations of lakes and rivers from five basins. The CR marker revealed five haplotypes. The overall haplotype (H) and nucleotide (Π) diversities were 0.684 ± 0.030 and 0.00460 ± 0.00012, respectively. Global F ST was 0.169, with several pairwise F ST estimates showing significant differences (P < 0.05). The exact test of population differentiation corroborated this result (P < 0.001). Significant geographic structure was found (P < 0.05), with variations explained primarily by differences within populations (61.65%) and among group basins (20.82%). Maximum likelihood phylogenetic analysis resolved two distinct clades with medium bootstrap support when naturalized populations were aligned in conjunction with reference native lineages. The haplotype network revealed a close association between naturalized populations and four main haplotypes representative of three native ecotypes or lineages from western North America (rainbow trout, steelhead trout and redband trout). These results indicate a genetic population structuring for naturalized rainbow trout from southern Chile and an origin probably represented by multiple lineages sources. Thus, mitochondrial DNA data strongly suggest that stocking of rainbow trout from different origins may have occurred during or after the initial introduction efforts.

Evidence for lasting alterations to aquatic food webs with short-duration reservoir draining

Large dams and their respective reservoirs provide renewable energy and water security, but also can profoundly alter riverine ecosystems. Here, we present evidence of changing aquatic food web structure in the seasons following short-duration, extreme manipulation of water levels in a reservoir (i.e., draining of the reservoir to the original riverbed during fall to assist outmigration of juvenile Chinook Salmon). We find unintended and lagged consequences of transitioning from a lake to a river, even temporarily, that resulted in trophic shifts away from piscivory and towards feeding at lower trophic levels for two common piscivorous fishes in reservoirs. Using natural abundances of nitrogen stable isotopes, we observed lower trophic level of feeding for invasive Largemouth Bass (Micropterus salmoides) and native Rainbow Trout (Oncorhynchus mykiss) during the summers following reservoir refilling than in nearby reference reservoirs that were not temporarily drained during fall. Declines in trophic levels of aquatic top predators have been rarely documented outside of controlled laboratory conditions. While useful for assisting outmigration of juvenile salmonids, the temporary draining of a reservoir to riverbed can also result in novel shifts in foodweb dynamics including reduced piscivory. As large dams continue to be operated and constructed worldwide, increased understanding of the community and ecosystem-level effects of reservoir management will be critical to evaluating trade-offs between human water needs, conservation of high value species, and ecosystem services impacted by river fragmentation.

Environmental DNA for freshwater fish monitoring: insights for conservation within a protected area.

BackgroundMany fish species have been introduced in wild ecosystems around the world to provide food or leisure, deliberately or from farm escapes. Some of those introductions have had large ecological effects. The north American native rainbow trout (Oncorhynchus mykiss Walbaum, 1792) is one of the most widely farmed fish species in the world. It was first introduced in Spain in the late 19th century for sport fishing (Elvira 1995) and nowadays is used there for both fishing and aquaculture. On the other hand, the European native brown trout (Salmo trutta L.) is catalogued as vulnerable in Spain. Detecting native and invasive fish populations in ecosystem monitoring is crucial, but it may be difficult from conventional sampling methods such as electrofishing. These techniques encompass some mortality, thus are not adequate for some ecosystems as the case of protected areas. Environmental DNA (eDNA) analysis is a sensitive and non-invasive method that can be especially useful for rare and low-density species detection and inventory in water bodies.MethodsIn this study we employed two eDNA based methods (qPCR and nested PCR-RFLP) to detect salmonid species from mountain streams within a protected area, The Biosphere Reserve and Natural Park of Redes (Upper Nalón Basin, Asturias, Northern Spain), where brown trout is the only native salmonid. We also measured some habitat variables to see how appropriate for salmonids the area is. The sampling area is located upstream impassable dams and contains one rainbow trout fish farm.ResultsEmploying qPCR methodology, brown trout eDNA was detected in all the nine sampling sites surveyed, while nested PCR-RFLP method failed to detect it in two sampling points. Rainbow trout eDNA was detected with both techniques at all sites in the Nalón River’ (n1, n2 and n3). Salmonid habitat units and water quality were high from the area studied.DiscussionIn this study, a high quantity of rainbow trout eDNA was found upstream and downstream of a fish farm located inside a Biosphere Reserve. Unreported escapes from the fish farm are a likely explanation of these results. Since salmonid habitat is abundant and the water quality high, the establishment of rainbow trout populations would be favored should escapes occur. Environmental DNA has here proved to be a valuable tool for species detection in freshwater environments, and the probe-based qPCR highly sensitive technique for detection of scarce species. We would recommend this method for routine monitoring and early detection of introduced species within natural reserves.

Contrasting genetic metrics and patterns among naturalized rainbow trout (Oncorhynchus mykiss) in two Patagonian lakes differentially impacted by trout aquaculture.

Different pathways of propagation and dispersal of non‐native species into new environments may have contrasting demographic and genetic impacts on established populations. Repeated introductions of rainbow trout (Oncorhynchus mykiss) to Chile in South America, initially through stocking and later through aquaculture escapes, provide a unique setting to contrast these two pathways. Using a panel of single nucleotide polymorphisms, we found contrasting genetic metrics and patterns among naturalized trout in Lake Llanquihue, Chile's largest producer of salmonid smolts for nearly 50 years, and Lake Todos Los Santos (TLS), a reference lake where aquaculture has been prohibited by law. Trout from Lake Llanquihue showed higher genetic diversity, weaker genetic structure, and larger estimates for the effective number of breeders (N b) than trout from Lake TLS. Trout from Lake TLS were divergent from Lake Llanquihue and showed marked genetic structure and a significant isolation‐by‐distance pattern consistent with secondary contact between documented and undocumented stocking events in opposite shores of the lake. Multiple factors, including differences in propagule pressure, origin of donor populations, lake geomorphology, habitat quality or quantity, and life history, may help explain contrasting genetic metrics and patterns for trout between lakes. We contend that high propagule pressure from aquaculture may not only increase genetic diversity and N b via demographic effects and admixture, but also may impact the evolution of genetic structure and increase gene flow, consistent with findings from artificially propagated salmonid populations in their native and naturalized ranges.

Life history variation within and among naturalized rainbow trout populations of the Laurentian Great Lakes

Abstract We examined naturalized rainbow trout spawning stocks from across the Great Lakes to determine: i) how reproductive characteristics among adults within populations vary with sex, ontogeny and energetic status, and ii) if variations in growth and reproductive characteristics among populations reflect differences in the environments they inhabit. Females in spawning runs tended to be larger and older than males, but growth rates were similar between sexes. As predicted, females had larger and more lipid-rich gonads and livers, and lower somatic condition and muscle lipid concentrations than males. Within populations, gonad size, fecundity, and egg size were related primarily to body size, and secondarily to indices of adult condition. Gonadosomatic index (GSI) increased with body size in females and declined with body size in males. Egg size increased with female size and declined with liver size and/or lipid content, whereas relative fecundity showed opposite trends. In contrast, egg lipid content appeared to be more strongly related to liver size and lipid content (positive effects) than to female size. Among populations, growth rate and muscle lipid concentration declined from south to north, but contrary to predictions, none of the reproductive traits examined showed strong covariation with environmental gradients. Growth, energetic status, and reproductive investment of Great Lakes naturalized rainbow trout vary both within and among populations, but localized adaptation to environmental conditions appears to be subtle.

Temporal Genetic Variance and Propagule-Driven Genetic Structure Characterize Naturalized Rainbow Trout (Oncorhynchus mykiss) from a Patagonian Lake Impacted by Trout Farming.

Knowledge about the genetic underpinnings of invasions—a theme addressed by invasion genetics as a discipline—is still scarce amid well documented ecological impacts of non-native species on ecosystems of Patagonia in South America. One of the most invasive species in Patagonia’s freshwater systems and elsewhere is rainbow trout (Oncorhynchus mykiss). This species was introduced to Chile during the early twentieth century for stocking and promoting recreational fishing; during the late twentieth century was reintroduced for farming purposes and is now naturalized. We used population- and individual-based inference from single nucleotide polymorphisms (SNPs) to illuminate three objectives related to the establishment and naturalization of Rainbow Trout in Lake Llanquihue. This lake has been intensively used for trout farming during the last three decades. Our results emanate from samples collected from five inlet streams over two seasons, winter and spring. First, we found that significant intra- population (temporal) genetic variance was greater than inter-population (spatial) genetic variance, downplaying the importance of spatial divergence during the process of naturalization. Allele frequency differences between cohorts, consistent with variation in fish length between spring and winter collections, might explain temporal genetic differences. Second, individual-based Bayesian clustering suggested that genetic structure within Lake Llanquihue was largely driven by putative farm propagules found at one single stream during spring, but not in winter. This suggests that farm broodstock might migrate upstream to breed during spring at that particular stream. It is unclear whether interbreeding has occurred between “pure” naturalized and farm trout in this and other streams. Third, estimates of the annual number of breeders (N b) were below 73 in half of the collections, suggestive of genetically small and recently founded populations that might experience substantial genetic drift. Our results reinforce the notion that naturalized trout originated recently from a small yet genetically diverse source and that farm propagules might have played a significant role in the invasion of Rainbow Trout within a single lake with intensive trout farming. Our results also argue for proficient mitigation measures that include management of escapes and strategies to minimize unintentional releases from farm facilities.

Patterns of Hybridization of Nonnative Cutthroat Trout and Hatchery Rainbow Trout with Native Redband Trout in the Boise River, Idaho

AbstractHybridization is one of the greatest threats to native fishes. Threats from hybridization are particularly important for native trout species as stocking of nonnative trout has been widespread within the ranges of native species, thus increasing the potential for hybridization. While many studies have documented hybridization between native cutthroat trout Oncorhynchus clarkii and nonnative rainbow trout O. mykiss, fewer have focused on this issue in native rainbow trout despite widespread threats from introductions of both nonnative cutthroat trout and hatchery rainbow trout. Here, we describe the current genetic (i.e., hybridization) status of native redband trout O. mykiss gairdneri populations in the upper Boise River, Idaho. Interspecific hybridization was widespread (detected at 14 of the 41 sampled locations), but high levels of hybridization between nonnative cutthroat trout and redband trout were detected in only a few streams. Intraspecific hybridization was considerably more widespread (almost 40% of sampled locations), and several local populations of native redband trout have been almost completely replaced with hatchery coastal rainbow trout O. mykiss irideus; other populations exist as hybrid swarms, some are in the process of being actively invaded, and some are maintaining genetic characteristics of native populations. The persistence of some redband trout populations with high genetic integrity provides some opportunity to conserve native genomes, but our findings also highlight the complex decisions facing managers today. Effective management strategies in this system may include analysis of the specific attributes of each site and population to evaluate the relative risks posed by isolation versus maintaining connectivity, identifying potential sites for control or eradication of nonnative trout, and long‐term monitoring of the genetic integrity of remaining redband trout populations to track changes in their status.Received February 24, 2011; accepted August 4, 2011

Spawning-related movement patterns of a unique rainbow trout (Oncorhynchus mykiss) population in a South Dakota headwater stream

The objectives of this study were to determine the spawning locations and movement patterns, and provide insight to the success of a naturalized rainbow trout (Oncorhynchus mykiss) population that occurs in a 3 km section of Spearfish Creek, South Dakota. Twelve rainbow trout from Spearfish Creek, South Dakota were implanted with radio transmitters and tracked from 6 December 2006 to 19 June 2007. Initiation of rainbow trout spawning-related movement was correlated with increasing discharge (r = 0.65, p = 0.0009). Mean gross movement was significantly lower during the pre-spawn period (mean = 236 m, SE = 59) compared with the spawning period (mean = 1661 m, SE = 313) (Kruskal–Wallis, F 1,30 = 56.3, p < 0.0001). Mean net movement was 25 m during the pre-spawn period and −9 m (downstream) during spawning. All rainbow trout used in this study moved into Cleopatra Creek (a tributary of Spearfish Creek) during the spawning period even though other nearby tributaries was available. The presence of brown trout (Salmo trutta) in the other tributaries is a possible reason why the rainbow trout did not use those areas for spawning.

Spatial, temporal and host factors structure the Ceratomyxa shasta (Myxozoa) population in the Klamath River basin

The myxozoan parasite Ceratomyxa shasta is a virulent pathogen of salmonid fish in the Klamath River, Oregon/California, USA. We previously defined four principal genotypes of the parasite (O, I, II, III) based on a trinucleotide repeat (ATC) 0–3 in Internal Transcribed Spacer region 1 sequences. Genotypes occur in sympatry and show marked host preference: I in Chinook salmon ( Oncorhynchus tschawytscha) and II in non-native rainbow trout ( O. mykiss). In the present study, we sequenced the parasite from river water samples collected in May, June and September at three localities below, above and between the Klamath's five dams. We also sampled adult and juvenile coho salmon ( O. kisutch), steelhead trout ( O. mykiss, anadromous form) and native redband rainbow trout ( O. mykiss, freshwater form) and additional Chinook salmon and non-native rainbow trout. We found that the C. shasta population was highly structured spatially, temporally and with respect to fish host species. Genotype O was present in water throughout the basin but detected almost exclusively in steelhead and native rainbow trout. Genotype I was in water only below the dams and detected only in Chinook salmon. Genotype II was detected in coho salmon below the dams, and in non-native rainbow trout exposed both above and below the dams. The same genotypes were detected in adult and juvenile fish of the same species. These findings have major implications for the design of effective surveillance and control programs for this economically and ecologically important fish parasite.

Ghrelin decreases food intake in juvenile rainbow trout ( Oncorhynchus mykiss) through the central anorexigenic corticotropin-releasing factor system

Ghrelin stimulates pituitary growth hormone (GH) release, and has a key role in the regulation of food intake and adiposity in vertebrates. To investigate the central effect of native rainbow trout ghrelin (rtghrelin) on food intake in rainbow trout, as well as its possible mode of action, four groups of fish received a single injection into the third brain ventricle (i.c.v. injection): (1) control group (physiological saline) (2) ghrelin-treated group (2.0 ng rtghrelin g bwt −1), (3) group given the corticotropin-releasing hormone receptor antagonist α-helical CRF 9–41 (ahCRF) (4.0 ng g bwt −1) and (4) group receiving the same dose of both ghrelin and ahCRF. Food intake was assessed 1 h after treatment. In addition, the presence of the GHS-R (the ghrelin receptor) in the rainbow trout CNS was examined with Western blot. To investigate peripheral effects of ghrelin, rainbow trout received an intraperitoneal cholesterol-based implant with or without rtghrelin, and daily food intake was measured during 14 days. Weight and length were measured at the start and termination of the experiment and specific growth rates were calculated. Mesenteric fat stores, muscle and liver lipid content were analysed after the treatment period. Central ghrelin injections decreased food intake compared with controls, and treatment with ahCRF abolished the ghrelin-effect. Western blot analysis of the GHS-R revealed a single band at around 60 kDa in pituitary, hypothalamus, brain and stomach. Long-term peripheral ghrelin treatment decreased daily food intake compared with controls. This was reflected in a ghrelin-induced decrease in weight growth rate ( p < 0.06). There was no effect of ghrelin on plasma GH levels or tissue fat stores. The conclusion from this study is that the GHS-R is indicated in the CNS in rainbow trout and that ghrelin may act there as an anorexigenic hormone, through a CRF-mediated pathway. Elevated peripheral ghrelin levels also seem to lead to decreased feed intake in the longer term.

Influences of Wildfire, Habitat Size, and Connectivity on Trout in Headwater Streams Revealed by Patterns of Genetic Diversity

AbstractWildfire is an important natural process in many stream ecosystems, but the ability of fish to respond to wildfire‐related disturbances is increasingly constrained by human activities that fragment and degrade stream habitats. In this study, we used molecular genetic markers (nuclear microsatellites) to examine the effects of wildfire and related disturbances along with habitat fragmentation on native rainbow trout in the Boise and Payette River basins, Idaho. We surveyed the genetic diversity of fish in 55 tributary streams to compare the level of diversity in samples without the recent influence of wildfire with that of those influenced by stand‐replacing wildfire and those influenced by both wildfire and a severe channel‐reorganizing disturbance. Stream habitats also varied substantially in size (catchment basin area) and isolation caused by road culverts. Based on prior work in our study streams, we expected that both wildfire and channel reorganization would reduce local population sizes significantly. Accordingly, we expected that wildfire‐related disturbances would reduce genetic diversity via founder effects or population bottlenecks. Our results, however, showed little evidence of these influences. In contrast, the level of genetic diversity was lower in fish collected upstream of culvert barriers, probably because of restricted gene flow. We also observed the expected positive correlation between habitat size and genetic diversity, which suggested the importance of larger local population sizes and habitat diversity in maintaining genetic diversity. An unexpected finding was that 15 of the 55 samples showed genetic evidence of hybridization between rainbow trout Oncorhynchus mykiss and nonnative cutthroat trout O. clarkii. The results of this study suggest that human influences such as barriers to dispersal and introductions of nonnative fish may pose greater threats to populations of native trout than wildfire itself.

Invasion by Nonnative Brook Trout in Panther Creek, Idaho: Roles of Local Habitat Quality, Biotic Resistance, and Connectivity to Source Habitats

AbstractTheoretical 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 (&lt;150‐mm [fork length]) and large (≥150‐mm) brook trout separately, assuming that the former represents an established invasion while accounting for the higher potential mobility of the latter. The occurrence of small brook trout was strongly associated with the proximity of sites to large, unconstrained valley bottoms, providing evidence that such habitats may serve as sources for the spread of brook trout invasion. Within sites, winter degree‐days and maximum summer temperature were positively associated with the occurrence of small brook trout. The occurrence of large brook trout was not related to any of the variables considered, perhaps due to the difficulty of linking site‐specific habitat factors to larger and more mobile individuals. The abundance of rainbow trout was not conclusively associated with the occurrence of either small or large brook trout, providing little support for the role of biotic resistance. Overall, our results suggest that source connectivity and local habitat characteristics, but not biotic resistance, influence the establishment and spread of nonnative brook trout populations. Further work is needed to confirm that the patterns observed here are relevant to other localities where brook trout have invaded and to understand the mechanisms contributing to the invasion process.

Influences of Wildfire and Channel Reorganization on Spatial and Temporal Variation in Stream Temperature and the Distribution of Fish and Amphibians

Wildfire can influence a variety of stream ecosystem properties. We studied stream temperatures in relation to wildfire in small streams in the Boise River Basin, located in central Idaho, USA. To examine the spatio-temporal aspects of temperature in relation to wildfire, we employed three approaches: a pre‐post fire comparison of temperatures between two sites (one from a burned stream and one unburned) over 13 years, a shortterm (3 year) pre‐post fire comparison of a burned and unburned stream with spatially extensive data, and a short-term (1 year) comparative study of spatial variability in temperatures using a ‘‘space for time‘‘ substitutive design across 90 sites in nine streams (retrospective comparative study). The latter design included streams with a history of stand-replacing wildfire and streams with severe post-fire reorganization of channels due to debris flows and flooding. Results from these three studies indicated that summer maximum water temperatures can remain significantly elevated for at least a decade following wildfire, particularly in streams with severe channel reorganization. In the retrospective comparative study we investigated occurrence of native rainbow trout (Oncorhynchus mykiss) and tailed frog larvae (Ascaphus montanus) in relation to maximum stream temperatures during summer. Both occurred in nearly every site sampled, but tailed frog larvae were found in much warmer water than previously reported in the field (26.6� C maximum summer temperature). Our results show that physical stream habitats can remain altered (for example, increased temperature) for many years following wildfire, but that native aquatic vertebrates can be resilient. In a management context, this suggests wildfire may be less of a threat to native species than human influences that alter the capacity of stream-living vertebrates to persist in the face of natural disturbance.

Anthropogenic influences on population sizes, age and growth of naturalized rainbow trout, Oncorhynchus mykiss, in Kenya

AbstractRiverine fishery in Kenya has witnessed profound changes since the 1950s, especially after independence (in 1963) when watersheds on the south‐eastern slopes of Mt Kenya were opened up for settlement. In particular, rainbow trout populations have declined essentially resulting from anthropogenic changes through over‐exploitation and/or degraded habitats. While there still are self‐sustaining rainbow trout populations in this stream among others they are in decline an indication that changes that have occurred on the watershed have altered their sizes, age and growth. The study was carried out in the Sagana, a third‐order stream, which rises at about 4000 m altitude on the south‐eastern slope of Mt Kenya, to obtain and document information on population sizes, age and growth of rainbow trout populations and to relate them with those recorded in the 1950s when trout streams were pristine (with little human influence). Stations were fished bi‐monthly from 1996 to 1998 and later for 6 months in 2002. Information on age and growth was obtained by use of annual marks, tagging fish of known age, and by validating their age using captive fish of known age. The minimum mean back‐calculated length at age for age one rainbow trout was 13.09 cm in upstream station and 15.10 cm for downstream stations. However, there was no significant difference in mean back‐calculated lengths at age for all years between fish in upstream and downstream stations (t‐test, t = −0.01, P = 0.99). Although female fish showed higher mean annual back‐calculated length increments than males, there was no significant difference in mean back‐calculated lengths at age between sexes (t‐test, t = −0.27, P = 0.80). The rate of growth in length was rapid for 1‐year‐old fish and declined in the second and third years. This study observed that most of the fish were small with only a few reaching more than 2 years of age because of overfishing. There are good reasons for optimism about the future of trout populations in this stream but concerted efforts are required to rehabilitate them. If trout populations are to increase, a management strategy is required to reduce fishing pressure and to maintain stream fishery against competing needs for resources in the catchments.

Ecotypic differentiation of native rainbow trout (Oncorhynchus mykiss) populations from British Columbia

We sampled 34 native rainbow trout (Oncorhynchus mykiss) populations over a wide geographic area of British Columbia to determine whether variation in morphology is related to differences in habitat or fish community characteristics experienced by a population. After correcting for differences in body size, the most dramatic differences occurred in external characteristics between stream- and lake-dwelling populations. Rainbow trout from streams had more robust bodies with larger caudal peduncles and heads and longer paired fins than rainbow trout from lakes. Unlike other lake populations, piscivorous rainbow trout populations had relatively large heads and mouths that were more similar in size to those of stream-dwelling populations. We found fewer differences in the size of internal organs across ecotypes, producing little predictive ability of these characteristics to differentiate populations of rainbow trout. We also sampled 27 additional rainbow trout populations with unknown fish communities to determine how well large-scale patterns predict morphological differentiation over a smaller spatial scale. Of the five ecotypes that we detected at a smaller scale, the largest differences again occurred between stream and lake populations. Our data suggest that external morphological variation among populations of rainbow trout covaries with ecological conditions at a variety of spatial scales, particularly between lakes and rivers.

Temperature-dependent Effects of Rainbow Trout on Growth of Atlantic Salmon Parr

Temperature may influence interactions between species by regulating energy balances of individuals. We conducted a laboratory study to determine whether temperature influenced the effects exerted by large rainbow trout on the growth of Atlantic salmon parr. Bioenergetic models were used to predict maintenance rations so that food resources were limiting over a range of temperatures; equal biomasses of rainbow trout were substituted for Atlantic salmon to evaluate the relative effect of interspecific interactions on Atlantic salmon growth. In the presence of rainbow trout, salmon growth increased as temperatures increased from 15°C to 25°C; no such temperature effect occurred for salmon maintained alone. Growth differences between salmon maintained with and without trout were highly significant at 25°C but not at 15°C. We conclude that the presence of trout depressed salmon growth at 15°C but not at higher temperatures, most likely a result of differences in thermal optima between these two species. Field data show that the proportion of stocked Atlantic salmon to wild rainbow trout coexisting in natural streams is a function of mean summer temperature. As stream temperatures increased, Atlantic salmon became increasingly favored over rainbow trout, but with a concomitant decrease in total salmonine biomass. We suggest that Atlantic salmon restoration programs focus more attention on relatively warm streams in watersheds where interactions with naturalized rainbow trout may occur.