Fishless Streams Research Articles

Modelling macroinvertebrate hydraulic preferences in alpine streams

AbstractAlpine streams face rapid hydrological changes due to the effects of global warming, glacier melting, and increased water uses such as hydropower production. Defining environmental flows (e‐flow) is crucial to mitigate the ecological impacts of flow alterations. Among e‐flow assessment methods, hydraulic habitat models predict changes in habitat suitability for aquatic species under different flow scenarios. They couple hydraulic models of stream reaches with biological models relating the abundance of taxa to microhabitat hydraulics. However, there is currently no suitable biological models for alpine, often fishless streams. In this study, we develop biological models for dominant macroinvertebrate taxa in alpine streams and compare their responses to hydraulics with those published in lowland streams. Using data collected in 150 microhabitats along a gradient of shear stress within five alpine streams, we performed generalized linear mixed models relating macroinvertebrate abundance to microhabitat hydraulics (shear stress, flow velocity, Froude number and water depth). We developed biological models for 41 taxa, and observed significant microhabitat selection for shear stress (18 taxa), velocity (20), Froude number (21), and depth (11). Most of them presented consistent responses across studied alpine streams, with shear stress and velocity as the main drivers. For common taxa, shapes of macroinvertebrate responses to hydraulics were comparable with those observed in lowland streams. Nevertheless, taxa preferred slightly lower shear stress in alpine streams compared to lowland streams, probably due to high‐fine sediment and oxygen concentrations, especially for taxa feeding on autochthonous organic matter. Many (23%) abundant taxa are rheophilic in alpine streams, thereby threatened by flow reduction, including the glacial stream specialists Diamesinae and Rhithrogena delphinensis, which will be also affected by glacier retreat. Combined with hydraulic models, our biological models will facilitate more robust e‐flow assessments, thereby reducing the impacts of flow alterations on alpine aquatic ecosystems.

Ontogenetic Dietary Shifts and Food Resource Partitioning in a Stream-Dwelling Urodela Community: Mechanisms to Allow Coexistence Across Seasons

Trophic interactions are important factors structuring animal communities. Although food resource partitioning among salamanders living in syntopy has often been explored in freshwater systems, the potential influence of ontogenetic shifts and seasonal patterns on resource partitioning is little known. The aim of this study was to explore the seasonal patterns in food resource partitioning among developmental stages of two Urodela species ( Calotriton asper and Salamandra salamandra ) in a fishless stream. We tested the hypothesis that seasonal patterns in intra- and interspecific food resource partitioning may occur at two different levels: (i) diet composition and (ii) prey size. For the purpose of this study, food resource partitioning was approached at three scales: (i) ontogenetic (larva, immature and adult), (ii) seasonal (winter, spring, summer and autumn) and (iii) species. Inter- and intraspecific competition for food was measured as dietary overlap according to Schoener’s index. The overlap in diet among developmental stages and species was high. However, prey size and the contribution of terrestrial invertebrates to the diets changed among developmental stages of species. This suggests that competition for food may be alleviated by ontogenetic dietary shifts in prey size and terrestrial invertebrates use.

Range expansion and ancestral niche reconstruction in the Mediterranean diving beetle genus Meladema (Coleoptera, Dytiscidae)

Species of the genus Meladema (Dytiscidae, Colymbetinae) are some of the largest macroinvertebrates in the western Palearctic region, being top predators in fishless streams. Two of the three described species, Meladema imbricata (Wollaston, 1871) and Meladema lanio (Fabricius, 1775) are Macaronesian endemics from the Canary Islands and Madeira, respectively, while the third, Meladema coriacea Laporte, 1835, is widely distributed from Morocco and the Iberian Peninsula to Turkey, including the Canary Islands. Previous phylogenetic analysis using only mitochondrial markers revealed the existence of two cryptic lineages within M. coriacea, one restricted to Corsica and the other including the rest of sampled populations. We reconstruct here the evolutionary history of the species of Meladema using a more comprehensive sampling covering its whole geographical range, adding nuclear markers and Bayesian molecular dating. Using environmental niche modelling, we test for possible differences in climatic preferences among lineages and reconstruct their ancestral climatic niche. Our results strongly supported the existence of four monophyletic lineages represented by the three recognized species plus a fourth cryptic lineage with populations of M. coriacea from the Tyrrhenian islands (Corsica, Sardinia and Montecristo). This pattern is not likely to be the result of mitochondrial artefacts due to Wolbachia infection, as all 11 tested individuals were negative for this parasite. Dating analysis placed the origin of Meladema in the Middle Miocene although diversification among extant Meladema lineages started in the early Pleistocene and took place in a relatively short time period. Phylogeographic analysis inferred a continental origin of Meladema, with an independent colonization of the Macaronesian and Mediterranean islands. From the south‐western Mediterranean region, the continental M. coriacea expanded its range up to Bulgaria and Turkey in the northern basin and to Tunisia in the southern. Results of niche modelling showed that seasonality is the critical factor in shaping the current distribution of Meladema. Island lineages (M. imbricata, M. lanio and the Tyrrhenian lineage of M. coriacea) occur in sites with low seasonality, within the range of the reconstructed ancestral climatic niche of the genus. On the contrary, continental M. coriacea expanded its range to localities outside the ancestral climatic range of the genus, with a higher seasonality and aridity.

Fish determine macroinvertebrate food webs and assemblage structure in Greenland subarctic streams

Summary Climate warming in (sub)arctic regions is expected to increase freshwater fish overwinter survival and dispersal, potentially with strong implications for macroinvertebrate assemblage composition and ecosystem processes. Several studies worldwide have shown large effects of top predators (usually fish) on macroinvertebrates in streams. However, the influence of top predators on trophic diversity, the range of food resources exploited and trophic niche redundancy is less well studied, particularly in cold regions. Using stable isotopes (13C and 15N) and fish gut content analysis, we investigated the effect of top predators on macroinvertebrate food webs in streams in Greenland by comparing adjacent sites with and without fish (Gasterosteus aculeatus). Food‐web metrics estimated from stable isotope data showed that the presence of fish reduced the diversity of food sources exploited by macroinvertebrates as well as their trophic diversity. In addition, fish presence increased packing and trophic redundancy of macroinvertebrate taxa in the food web, possibly due to behavioural changes in foraging activity. Furthermore, predatory macroinvertebrate taxa were unable to attain the trophic position of fish so that food webs were one trophic level longer in streams with fish. Focusing on macroinvertebrate food webs alone, predatory macroinvertebrates did not change their trophic position in the presence of fish. Filter feeders dominated the macroinvertebrate assemblage when fish were present, while the relative abundance of collector‐gatherers was marginally higher in the fishless streams. This pattern was consistent with stronger selectivity for collector‐gatherers by fish. Climate‐driven fish colonisation in currently fishless Greenland streams may induce a shift in macroinvertebrate food webs, with a reduction in both trophic diversity and the variety of food sources consumed. To some extent, this might counteract a change towards a broader use of food resources by macroinvertebrates that might otherwise be expected at higher temperatures. In addition, a shift in the macroinvertebrate assemblage towards dominance of filter feeders can be expected to promote an increase in periphyton biomass in streams with fish.

Trout reverse the effect of water temperature on the foraging of a mayfly.

Climate change is likely to increase the metabolisms of ectothermic animals living below their thermal optimum. While ectothermic top predators may compensate by increasing foraging, ectothermic prey may be unable to increase foraging because of increased predation risk from ectothermic predators. We examined how the diurnal drift behavior (i.e., the downstream movement associated with foraging) of the mayfly Baetis, an ectothermic herbivore, responds to changing temperature in the implied presence and absence of trout, an ectothermic predator. In an experiment replicated at the catchment scale, water temperature and trout presence strongly interacted to affect the diurnal drift of Baetis from artificial channels lacking periphyton over a water temperature range of 4.2-14.8 °C. In fishless streams, daytime drift increased with increasing water temperature, likely because of increased metabolic demand for food. However, in trout-bearing streams, daytime drift decreased with increasing water temperature. Our interpretation is that the perceived threat of trout rose with increasing water temperature, causing mayflies to reduce foraging despite heightened metabolic demand. These results suggest that anticipated increases in stream temperature due to climate change may further escalate divergence in structure and process between fishless and trout-bearing streams. Similar dynamics may occur in other ecosystems with ectothermic predators and prey living below their thermal optima.

Disturbance and productivity as codeterminants of stream food web complexity in the Arctic

Disturbance and productivity are potentially interacting factors often thought to underlie the complexity of aquatic food webs. We tested predictions about the effects of disturbance (bed movement, total water column freezing) and productivity (as indicated by primary consumer biomass) on the structure of 19 stream food webs in arctic Alaska. We predicted that food webs subject to high levels of disturbance would have low connectance, low linkage density, and short food chains, while food webs of streams with more benign disturbance regimes would be more complex (i.e., a test of the “dynamic stability hypothesis”). We further predicted that food chain length would increase with productivity (i.e., testing the “resource‐availability hypothesis”). Connectance and linkage density were negatively related to bed movement but not to freezing. Food chain length was related to freezing but not to bed movement or productivity. The relationships among food chain length, freezing, and productivity were driven by the occurrence of fishless streams, which all froze during winter. Streams containing fish had longer food chains than fishless streams, and food chain length was positively related to productivity only when fishless streams were removed from analyses. Our results show that disturbance and productivity are codeterminants of stream food web complexity in the Arctic. Our data further suggest that colonization dynamics are an important factor controlling food web structure, highlighting the importance of idiosyncratic habitat and life history related factors that are often overlooked in theoretical studies.

Nutrient limitation controls the strength of behavioral trophic cascades in high elevation streams

Indirect effects of predators on primary producers vary over space and time. Key components of the environmental context underlying that variability include the bottom‐up supply of resources and the defense, mobility and foraging efficiency of primary consumers. We manipulated key resources that limit primary producers and tested the effects of system enrichment on the strength of top‐down interactions between top predators (Salvelinus fontinalis), different types of grazing invertebrates and algae in high elevation streams of Colorado, USA. We added nitrogen and phosphorus to flow‐through microcosms powered by stream water and tested the direct effects of chemical cues from brook trout on the behavior of sedentary, predator‐resistant (caddisflies) and mobile, predator‐vulnerable grazers (mayflies), and the indirect cascading effects of predators on: (1) the biomass of algae mediated by grazer behavior and (2) the impact of grazers on algal biomass accrual. Recognizing the value of whole‐ecosystem experiments, we also tested the effects of supplementing nutrients to two headwater fishless streams on the impact of grazers on algae and the performance (growth rates) of four predominant grazer species compared to two adjacent reference streams with ambient (limited) nutrients. In both experiments, algal biomass increased with added nutrients where consumption by grazers was swamped by increased algal accrual due to enrichment. Results of the microcosm experiment were consistent with a context‐dependent behavioral trophic cascade whereby predator cues increased the biomass of algae only in treatments with enriched nutrients and mobile, predator‐vulnerable grazers (Baetis mayflies). Baetis showed risk‐sensitive behavioral responses to fish cues, which resulted in cascading effects on algae only under enrichment. In contrast, behavior of sedentary, predator‐ resistant caddisflies (Allomyia) was unaffected by predator cues and a cascade was never observed. The whole ecosystem experiment revealed no aggregative responses by grazers but instead, strong developmental responses of all grazer species to nutrient enrichment. Growth rates of grazers increased with mobility, with the highest rates occurring for mobile, predator‐vulnerable Baetis mayflies in enriched streams. Our results emphasize the importance of experimentally testing the context‐dependency of trophic cascades, and are consistent with the hypothesis that nutrient limitation may reduce the strength of trophic cascades.

Salamander diversity alters stream macroinvertebrate community structure

Summary Salamanders are abundant consumers in many fishless stream ecosystems, but few studies have explicitly examined their ecological role. Stream‐dwelling salamander larvae are generalist predators of aquatic macroinvertebrates and may play an important role in structuring macroinvertebrate communities. The potential for emergent effects of multiple predator species suggests that changes in salamander diversity could alter their effects on macroinvertebrate communities, but this has not been tested. We used in‐stream enclosures to manipulate the presence and diversity of the two most abundant salamanders, Eurycea wilderae (EWIL) and Desmognathus quadramaculatus (DQUA), in a southern Appalachian Mountain headwater stream to examine the impacts of salamander predation on macroinvertebrate communities. We were particularly interested in testing for potential diversity effects by comparing the effects of each species in monoculture to polycultures of both species (BOTH). Salamanders reduced macroinvertebrate abundance compared to the control treatment (CONTROL), but only when both species were present together. The general pattern of macroinvertebrate abundance among treatments was: CONTROL = EWIL = DQUA > BOTH, although no treatment significantly reduced the abundance of epibenthic taxa. The BOTH treatment also significantly altered macroinvertebrate community structure and reduced taxon richness by c. 57%. The effects of salamanders were particularly pronounced for chironomids (Diptera: Chironomidae), which comprised the majority of macroinvertebrate total abundance. Increased salamander diversity substantially altered macroinvertebrate communities, suggesting that niche complementarity or facilitation occurred despite the apparent functional similarity of these two species. Therefore, changes in salamander diversity may alter the effects of salamander communities on macroinvertebrates, which may cascade through food webs to affect stream ecosystem function.

Cascading effects of predatory fish on the composition of benthic algae in high‐altitude streams

Cascading effects of predators can affect ecosystem properties by changing plant biomass, distribution and assemblage composition. Using data from field surveys and whole‐stream experiments we tested the hypothesis that predatory trout change assemblage composition of benthic algae in high‐elevation streams mediated by grazer behavior. Field surveys revealed that the taxonomic composition of algal assemblages differed significantly between streams that contained trout and those that were fishless; but comparisons of palatable versus unpalatable algal taxa between fish and fishless streams were equivocal because of high natural variability. Therefore, we tested for a behavioral (non‐consumptive) trophic cascade experimentally by adding brook trout chemical cues to six naturally fishless streams for 25 days and compared responses of grazers and algae to six reference streams without fish cues added. Algal response variables included rates of change in the abundance of three physiognomic categories, from most palatable (attached erect and prostrate diatoms) to least palatable (non‐diatoms), as determined from food selectivity analyses of the most common grazers (mayflies and caddisflies). Fish cues did not affect the mean densities or changes in densities of total grazers or any individual grazer species. However, in streams where fish cues were added, rates of accrual of attached erect diatoms, which was the preferred algal type for the grazer most vulnerable to trout predation (Baetis), were higher and their densities increased significantly faster with increasing densities of this grazer species than in reference streams. Results of his experiment support the hypothesis that predator induced suppression of grazer foraging behavior, rather than cascading effects of top predators on grazer density, may contribute to variation in the composition of algal assemblages among streams by allowing proliferation of most palatable algal species.

Incorporating invertebrate predators into theory regarding the timing of invertebrate drift

Theory concerning the timing of lotic invertebrate drift suggests that daytime-feeding fish cause invertebrates to restrict their drift behavior to the nighttime. However, there is growing evidence that the nighttime foraging of invertebrate predators also contributes to the nocturnal timing of drift, though it is unclear whether the nocturnal behavior of invertebrate predators is innate or proximately caused by fish. In two experiments, one conducted in a fish-bearing stream and a second in a fishless stream, we compared the drift patterns of Baetidae (Ephemeroptera) from channels with and without benthic invertebrate predators. We tested whether invertebrate predators affect the timing of drift, either as a proximate cause of nocturnal drift in the fishless stream (diel periodicity) or as a proximate cause of a pre-dawn peak in drift in the fish-bearing stream (nocturnal periodicity). In the fish-bearing stream experiment, a pre-dawn increase of baetid drift occurred independently of invertebrate predators, indicating that invertebrate predators were not the proximate cause of nocturnal periodicity in the stream. In the fishless stream experiment, invertebrate predators caused more baetid drift at night than during the day, indicating that invertebrate predators caused the nocturnal drift pattern we observed in the stream, and that invertebrate predators can influence drift timing independently of fish. Therefore, we suggest that both visually feeding fish and nocturnally foraging benthic predators, when present, affect the timing of invertebrate drift; visually feeding fish by reducing daytime drift, and benthic predators by increasing nighttime drift.

The influence of recruitment on within-generation population dynamics of a mayfly

The relative contributions of recruitment and post-recruitment processes are pivotal to understanding the mechanisms influencing the population dynamics of organisms. We estimated recruitment by oviposition to populations of the mayfly Baetis bicaudatus in multiple streams of one drainage basin in western Colorado, USA, and subsequent changes in larval population densities of one cohort over one generation. Our goal was to compare the potential for recruitment limitation and post-recruitment processes to explain variation in population dynamics of one cohort in streams with predatory fish (brook trout) and fishless streams. We censused Baetis egg masses oviposited in 14 streams during summer 2001, and estimated larval densities of this cohort during September 2001, April and June 2002. There was substantial variability among streams in densities of eggs and subsequent larval densities, with qualitatively different larval dynamics in fish and fishless streams. In fishless streams egg densities were generally lower than in fish streams due to hydrogeomorphic habitat constraints; and increasing egg densities resulted in higher densities of early stage larvae. This pattern was consistent with recruitment limitation of population size of early stages, which persisted until April. In contrast, increasing egg densities in trout streams resulted in lower densities of early stage larvae, consistent with strong density-dependent losses due to post-recruitment processes. In both fish and fishless streams the loss rates of early stage larvae during winter were density-independent; but from April to June the pattern suggests strong density-dependent losses of larvae in fish streams, and weaker, although significant density-dependent losses in fishless streams as larvae developed to late instars from April to June. We conclude that recruitment can limit the population size of early larval stages of Baetis related to the availability of preferred oviposition sites for females; but post-recruitment processes eventually (in lower risk environments) or almost immediately swamp the effects of recruitment on population dynamics especially in fish streams. This study illustrates that the relative influences of recruitment limitation and post-recruitment processes on mayfly population dynamics varies across life stages and among environments with different habitat constraints and predation risks.

Translocation of stream-dwelling salmonids in headwaters: insights from a 15-year reintroduction experience

Translocation programs are a common strategy to increase the number of viable populations of threatened freshwater fishes. Yet, only in a minority of cases the success or failure of translocations has been assessed through a quantitative analysis of demographic traits, compensatory responses, life-histories and population dynamics of the threatened species. A paradigmatic case a translocation program combining both management- and research-oriented activities is represented by the Marble Trout Conservation Program, which started in 1993 in the upper reaches of the Soca, Idirjca and Baca river basins (Slovenia) for the conservation of stream-dwelling marble trout Salmo marmoratus. In order to enhance the viability of the species, two new populations were created in 1996 by stocking 500 marble trout aged 1+ in previously fishless streams (Gorska and Zakojska) within the core habitat of the species. The new populations have been systematically monitored for 15 years by individually tagging and sampling marble trout. Our analyses show that deterministic extinction of marble trout populations are unlikely and that high-magnitude environmental stochasticity (i.e., severe floods) is the only main cause of local population extinction, despite the high resilience to flood-induced massive mortalities exhibited by marble trout through compensatory mechanisms (e.g., relaxation of density-dependent body growth and survival at low densities). Fishless headwaters, probably characterized by a history of recurrent severe floods, should not be considered as candidate sites for the creation of new populations. Fewer individuals than originally reintroduced (i.e., 500 fish aged 1+ in each stream) might be sufficient to establish viable populations, since compensatory mechanisms are likely to regulate population size around stream carrying capacity in a few years. Besides enhancing the species viability, translocation programs can provide an excellent framework for the estimation of ecological traits (e.g., life-histories, demography, population dynamics etc.), identify potential vulnerabilities and thus guide well-formed management actions for the threatened species.

Invertebrate mesopredators are larger in streams with fish

Species with complex life cycles (e.g., aquatic larvae, terrestrial adults) are expected to shorten the time spent in the larval stage if mortality risks are high, a trade-off that lowers predation risk at the cost of reduced time for growth and thus smaller adult size. We tested these predictions by comparing the timing of and size at emergence for two relatively large and common invertebrate mesopredator species (Isoperla montana and Rhyacophila vibox) that inhabit small coastal streams, with and without predatory fish, in eastern Canada. Contrary to expectations based on predation risk–foraging trade-off theory, individuals of both invertebrate species tended to be larger rather than smaller in streams with fish than in fishless streams. The patterns were consistent, however, with the expected ecological effects of top predators on food webs, where fish lower abundances of invertebrate mesopredators, increasing resource availability and thus growth rates for the remaining individuals. We conclude that variation among streams in size at emergence is better explained by the impact of fish on resource availability than to behavioural or life history trade-offs occurring under risk of predation.

The management of small, isolated salmonid populations: do we have to fix it if it ain't broken?

In their recent paper, Sato H Gharrett et al., 1999; McGinnity et al., 2003; Tallmon et al., 2004), but claimed the close geographical proximity, similar habitat and small genetic differences among remnant populations might considerably reduce this risk. In a recent review, Edmands (2007) stated that concerns over outbreeding should be taken seriously, as the effects of outbreeding can be, in some cases, as damaging as severe inbreeding. Here, we raise concerns about mixing of populations as a strategy to increase genetic variation, and we bring evidence of likely outbreeding depression observed in the rehabilitation project of the endangered freshwater salmonid marble trout Salmo marmoratus, started in Slovenia in 1993 (Crivelli et al., 2000). Since the beginning of the rehabilitation project, four marble trout populations went extinct due to landlsides (Predelica) and severe flood events (Gorska, Sventarska and Zakojska). The seven remnant populations of marble trout living in Slovenian streams are genetically distant (FST=0.66, Fumagalli et al., 2002), present a very low effective population size (for the Huda Grapa population a total population size ranging from 29 to 60 individuals was estimated between 2000 and 2007) and very low genetic variability estimated at 14 microsatellite loci (for Huda Grapa Ho=0.046, Fumagalli et al., 2002). In order to expand the genetic variability of marble trout living in the watershed, a new population (Gatsnik) was created in 1998 in a previously fishless stream by mixing the progeny of marble trout from two remnant populations from isolated streams with similar habitat (Trebuscica and Lipovesck). After two generations, the population of Gatsnik presented the typical features associated with maladaptation and outbreeding depression, that is lower annual survival rate (0.39 0.02 for trout born in the stream, higher than 0.55 for other marble trout populations living in the study area, Vincenzi et al., 2008), reproductive underperformance at the F2 generation and unusual morphological abnormalities (Fig. 1). Moreover, preliminary results from a egg-to-fry experiment performed in the fish farm for the F1 generation of Gatsnik showed 80.4% of mortality from fertilization to hatching (35.6 29.2% for the remnant populations) (D. Jesensek, pers. comm.). The translocation experiment in Gatsnik is still ongoing and may provide unequivocal evidence of outbreeding effects in 3–4 years. Nevertheless, despite the striking contrast of salmonid species in lifehistory traits may hinder generalizations, what we observed in the marble trout population of Gatsnik urges now further considerations on the opportunity of mixing populations of salmonids to increase their genetic variability. As recently reviewed by Willi et al. (2006), small populations are predicted to have low potential to adapt to environmental changes, as genetic variation and potential response to selection should be positively related to population size. Moreover, individuals in small populations may have lower fitness and be inbred, which may decrease an adaptive response to unpredictable environmental changes. Apart from the obvious cases of overexploitation, pollution and human-induced fragmentation, the small population size and low heterozygosity frequently observed in endemic

The influence of introduced trout on the benthic communities of paired headwater streams in the Sierra Nevada of California

Summary1. Non‐native trout have been stocked in streams and lakes worldwide largely without knowledge of the consequences for native ecosystems. Although trout have been introduced widely throughout the Sierra Nevada of California, U.S.A., fishless streams and their communities of native invertebrates persist in some high elevation areas, providing an opportunity to study the effects of trout introductions on natural fishless stream communities.2. We compared algal biomass and cover, organic matter levels and invertebrate assemblages in 21 natural fishless headwater streams with 21 paired nearby streams containing stocked trout in Yosemite National Park.3. Although environmental conditions and particulate organic matter levels did not differ between the fishless and trout streams, algal biomass (as chlorophyllaconcentration) and macroalgal cover were, on average, approximately two times and five times higher, respectively, in streams containing trout.4. There were no differences in the overall densities of invertebrates in fishless versus paired trout streams; however, invertebrate richness (after rarefaction), evenness, and Simpson and Shannon diversities were 10–20% higher in fishless than in trout streams.5. The densities of invertebrates belonging to the scraper‐algivore and predator functional feeding guilds were higher, and those for the collector‐gatherer guild lower, in fishless than trout streams, but there was considerable variation in the effects of trout on specific taxa within functional feeding groups.6. We found that the densities of 10 of 50 common native invertebrate taxa (found in more than half of the stream pairs) were reduced in trout compared to fishless streams. A similar number of rarer taxa also were absent or less abundant in the presence of trout. Many of the taxa that declined with trout were conspicuous forms (by size and behaviour) whose native habitats are primarily high elevation montane streams above the original range of trout.7. Only a few taxa increased in the presence of trout, possibly benefiting from reductions in their competitors and predators by trout predation.8. These field studies provide catchment‐scale evidence showing the selective influence of introduced trout on stream invertebrate and algal communities. Removal of trout from targeted headwater streams may promote the recovery of native taxa, community structure and trophic organisation.

Density reductions by predatory trout increase adult size and fecundity of surviving caddisfly larvae in a detritus‐based stream food web

Summary1. In some situations, individuals surviving in environments where predation is intense can grow faster because the benefits of release from intraspecific competition outweigh costs associated with anti‐predator responses. Whether these ‘thinning’ effects of predation occur in detritus‐based food webs where resource renewal occurs independently of consumption by consumers was studied. We investigated how effects of predatory brown trout (Salmo trutta) on the larvae of the detritivorous stream caddisfly, Zelandopsyche ingens, influenced the size and fecundity of the caddisfly adults.2. Trout substantially reduced the abundance of Z. ingens larvae, but adult male and female Z. ingens were significantly larger in trout streams compared to fishless streams. Females in trout streams had 33% more eggs than fishless stream females, and egg sizes were not significantly different. In mesocosms, Z. ingens larvae in low density treatments reflecting trout stream abundances grew significantly faster than larvae in high density treatments that were characteristic of fishless stream abundances. Non‐lethal trout presence did not influence case building behaviour, feeding rates or growth or Z. ingens larvae, indicating non‐lethal effects of predators were negligible.3. Increased adult size and fecundity associated with trout stream individuals were probably a result of predator thinning of larval density indirectly releasing surviving Z. ingens from intraspecific competition. Thus, predator thinning did influence interactions between larvae in this detritus‐based food web as larval growth was strongly density‐dependent. However, extrapolating the total number of eggs potentially produced indicates the increased fecundity of females in trout streams would not compensate for losses of larvae to trout predation.

Predator effects on prey population dynamics in open systems

Animal population dynamics in open systems are affected not only by agents of mortality and the influence of species interactions on behavior and life histories, but also by dispersal and recruitment. We used an extensive data set to compare natural loss rates of two mayfly species that co-occur in high-elevation streams varying in predation risk, and experience different abiotic conditions during larval development. Our goals were to generate hypotheses relating predation to variation in prey population dynamics and to evaluate alternative mechanisms to explain such variation. While neither loss rates nor abundance of the species that develops during snowmelt (Baetis bicaudatus) varied systematically with fish, loss rates of the species that develops during baseflow (Baetis B) were higher in streams containing brook trout than streams without fish; and surprisingly, larvae of this species were most abundant in trout streams. This counter-intuitive pattern could not be explained by a trophic cascade, because densities of intermediate predators (stoneflies) did not differ between fish and fishless streams and predation by trout on stoneflies was negligible. A statistical model estimated that higher recruitment and accelerated development enables Baetis B to maintain larger populations in trout streams despite higher mortality from predation. Experimental estimates suggested that predation by trout potentially accounts for natural losses of Baetis B, but not Baetis bicaudatus. Predation by stoneflies on Baetis is negligible in fish streams, but could make an important contribution to observed losses of both species in fishless streams. Non-predatory sources of loss were higher for B. bicaudatus in trout streams, and for Baetis B in fishless streams. We conclude that predation alone cannot explain variation in population dynamics of either species; and the relative importance of predation is species- and environment-specific compared to non-predatory losses, such as other agents of mortality and non-consumptive effects of predators.

Potential factors controlling the population viability of newly introduced endangered marble trout populations

While several population viability analyses (PVAs) have been performed on anadromous salmonids, less attention has been given to stream-living salmonids. In this work, we explore the role of PVA as a tool in the recovery of threatened stream-living salmonid species. The analysis has been performed with reference to marble trout Salmo marmoratus, a salmonid with a limited geographic distribution and at risk of extinction due to hybridization with the non-native introduced brown trout. Demographic parameters, such as survival, fecundity and density-dependent patterns were estimated from an eight year on-going monitoring program of two translocated marble trout populations in pristine, previously fishless streams (Zakojska and Gorska) in the Soca and Idrijca river basins (Slovenia). To explore the importance of disturbance events such as floods on marble trout population dynamics, we performed a PVA under three scenarios: (1) occurrence of both severe and moderate floods; (2) occurrence of only moderate floods; (3) no flood events. Our analysis shows that population viability is threatened only by severe flood events, otherwise the two populations prove to be fairly stable with population abundance fluctuating around stream carrying capacity. A sensitivity analysis performed on model parameters highlighted that density-dependence in first-year survival and the magnitude of reduction in population size after a severe flood are the two most crucial parameters affecting population abundance and quasi-extinction probability, respectively. While only extreme floods can drive the population to extinction, the increase in juvenile survival when population abundance collapses after a major flood may allow the populations to quickly recover from few reproductive individuals back to stream carrying capacity.

Hybridization mechanisms between the endangered marble trout ( Salmo marmoratus) and the brown trout ( Salmo trutta) as revealed by in-stream experiments

Populations of the marble trout ( Salmo marmoratus) are critically declining due to introgression by brown trout ( S. trutta) strains. Hybrids between the two forms are fertile and presently predominant in most rivers of the species’ range. The involved hybridization mechanisms have been studied through two large scale in-stream experiments (Driselpoh and Stopnikarca) as 50% of each species have been stocked at the age of one year in fishless streams, each fish being individually marked. Diagnostic molecular markers were applied to test a partial reproductive isolation between the two species. Emphasis was put on survival and growth patterns of stocked fish (parental generation) as well as on fish hatched within the stream (F 1 generation). No evidence of a partial reproductive barrier between the two species was observed. Survival of the parental generation depended on the year in both streams, as well as on the species in Stopnikarca, but was identical for both species in Driselpoh. In both streams survival of instream hatched individuals measured from 0+ to 2+ was lower for brown trout. In Driselpoh, F 1 0+, 1+ and 2+ hybrids were larger than pure individuals. Larger hybrids were only observed in Stopnikarca when analyses focused on individuals in inter-specific competition suggesting that heterosis and stress effects may explain the observed size differences. Our results point out important ecological differences between marble and brown trout and have shown that hybridization can easily take place. The findings indicate that high F 1 hybrid presence, survival and heterosis effects may impede marble trout rehabilitation in the area.

Endangered crayfish in northern Japan: Distribution, abundance and microhabitat specificity in relation to stream and riparian environment

Japan’s only native crayfish species Cambaroides japonicus has been declining dramatically in the past few decades. For the purpose of conservation planning, twenty-two coastal streams were surveyed to investigate summer distributions of crayfish in relation to stream and riparian environment. Classification and regression trees were used to predict the occurrence and abundance of crayfish. The classification tree model with stream variables as predictors showed that crayfish would occur in swift or high gradient streams (correct classification rate = 91%). Within those streams, however, crayfish only inhabited depositional microhabitats, in which the areas are limited in availability. Crayfish were not found in gentle, low gradient streams containing abundant depositional microhabitats. This paradoxical distribution pattern was attributed to availability of boulder substrates in swift or high gradient streams. The regression tree model indicated that crayfish abundance was determined primarily by the percentage of boulder substrates and the presence of fish (observed vs. predicted r = 0.64). The classification tree model using only riparian variables indicated that the total woody plant (mainly broadleaf species) density followed by the percentage of early successional species such as alder and willow determined the splits of the tree model (correct classification rate = 95%). A leaf processing experiment on 10 riparian plant species suggested that crayfish preferred high nitrogen (or low C/N) leaves. These results suggest that swift or high gradient fishless streams associated with abundant cover in dense broadleaf forest serve conservation areas for this endangered crayfish, and that consideration of riparian composition may facilitate conservation efforts.