Prey size,prey abundance,and temperature as correlates of growth in stream populations of cutthroat trout

Growth and maximum size of stream fishes can be highly variable across populations. For salmonid fishes in streams, individuals from populations confined to headwater streams often exhibit small size at maturity in comparison to populations with access to main-stem rivers. Differences in prey size, prey availability, and metabolic constraints based on temperature may explain patterns of maximum size and growth. In this study, cutthroat trout from headwater stream populations that were isolated above a waterfall were compared to individuals from populations in similar sized streams without a movement barrier and from large main-stem rivers. Cutthroat trout from smaller streams with or without a movement barrier were significantly smaller at a given age than fish from main-stem rivers, where individuals were able to achieve a much larger maximum size. Comparisons of invertebrate drift abundance and size in the three types of streams revealed that drift size did not differ between stream categories, but was highest per volume of water in large main-stem rivers. Across all stream types, prey abundance declined from summer to fall. Temperature declined over the course of the season in a similar manner across all stream types, but remained relatively high later in the season in main-stem river habitats. Prey availability and temperature conditions in main-stem rivers may provide more optimal growing conditions for fish as individuals increase in size and become constrained by prey availability and temperature conditions in small streams. Maintaining connectivity between small spawning and rearing tributary streams and main-stem river habitats may be critical in maintaining large-bodied populations of stream salmonids.     

Fish species richness and stream order in Washington State streams

Synopsis We sampled fish at pairs of sites of the same stream order on opposite sides of drainage divides in the Cascade Mountains and in the southwest portion of Washington state. Elevation, gradient, drainage area, and stream order were significantly correlated with number of fish species collected at a site. Elevation accounted for the greatest portion of the variation in number of species and stream order for the least, but in low gradient, low elevation streams, stream order was significantly related to number of species. Species richness was greatest in low elevation, low gradient, high order streams. Species richness of a site reflected species richness of the drainage: in paired comparisons, sites in a drainage with a richer ichthyofauna had more fish species than sites in a drainage with fewer species. Addition of species with increasing stream order occurred in most streams, but replacement was more frequent than in other studies relating fish to stream order. The apparently higher frequency of replacement in this study appeared to be a result of headwater introductions of brook charr, Salvelinus fontinalis, and a tendency for cutthroat trout, Salmo clarki, to occupy headwaters when in freshwater.     

Watershed‐level brook trout genetic structuring: Evaluation and application of riverscape genetics models

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Sibship reconstruction for inferring mating systems,dispersal and effective population size in headwater brook trout (<Emphasis Type="Italic">Salvelinus fontinalis</Emphasis>) populations

Brook trout Salvelinus fontinalis populations have declined in much of the native range in eastern North America and populations are typically relegated to small headwater streams in Connecticut, USA. We used sibship reconstruction to infer mating systems, dispersal and effective population size of resident (non-anadromous) brook trout in two headwater stream channel networks in Connecticut. Brook trout were captured via backpack electrofishing using spatially continuous sampling in the two headwaters (channel network lengths of 4.4 and 7.7 km). Eight microsatellite loci were genotyped in a total of 740 individuals (80–140 mm) subsampled in a stratified random design from all 50 m-reaches in which trout were captured. Sibship reconstruction indicated that males and females were both mostly polygamous although single pair matings were also inferred. Breeder sex ratio was inferred to be nearly 1:1. Few large-sized fullsib families (>3 individuals) were inferred and the majority of individuals were inferred to have no fullsibs among those fish genotyped (family size = 1). The median stream channel distance between pairs of individuals belonging to the same large-sized fullsib families (>3 individuals) was 100 m (range: 0–1,850 m) and 250 m (range: 0–2,350 m) in the two study sites, indicating limited dispersal at least for the size class of individuals analyzed. Using a sibship assignment method, the effective population size for the two streams was estimated at 91 (95%CI: 67–123) and 210 (95%CI: 172–259), corresponding to the ratio of effective-to-census population size of 0.06 and 0.12, respectively. Both-sex polygamy, low variation in reproductive success, and a balanced sex ratio may help maintain genetic diversity of brook trout populations with small breeder sizes persisting in headwater channel networks.     

Loss of genetic diversity in the North American mayfly Ephemerella invaria associated with deforestation of headwater streams

1. Terrestrial dispersal by aquatic insects increases population connectivity in some stream species by allowing individuals to move outside the structure of the stream network. In addition, individual survival and reproductive success (as well as dispersal) are tightly linked to the quality of the terrestrial habitat. 2. In historically forested catchments, deforestation and altered land use have the potential to interfere with mayfly dispersal or mating behaviours by degrading the quality of the terrestrial matrix among headwater streams. We hypothesised that loss of tree cover in first‐order catchments would be associated with an increase in population substructure and a decrease in genetic diversity of mayfly populations. 3. To test this hypothesis, we investigated spatial patterns of genetic variation in the common mayfly Ephemerella invaria across a gradient of deforestation in the central piedmont region of eastern United States. Intraspecific genetic diversity and population substructure were estimated from data obtained using fluorescent amplified fragment length polymorphism (AFLP) markers. 4. We found that mayfly populations had low population substructure within headwater stream networks and that genetic diversity was strongly negatively correlated with mean deforestation of the first‐order catchments. The large‐scale pattern of population substructure followed a pattern of isolation by distance (IBD) in which genetic differentiation increases with geographical distance, but assignment tests placed a few individuals into populations 300 km away from the collection site. 5. Our results show that loss of genetic diversity in this widespread aquatic insect species is co‐occurring with deforestation of headwater streams. 6. Most arguments supporting protection of headwater streams in the United States have centred on the role of these streams as hydrological and biogeochemical conduits to downstream waters. Our work suggests that headwater stream land use, and specifically tree cover, may have a role in the maintenance of regional genetic diversity in some common aquatic insect species.     

The relative roles of contemporary and ancient processes in shaping genetic variation of a generalist fish in a catchment dominated by agriculture

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Influence of water quality, habitat and species richness on brown trout populations

The influence of water quality, physical habitat and species richness on the occurrence, density and size of brown trout at 216 stream sites in southern Sweden was studied. Discriminant analysis showed that the occurrence of trout at a locality was largely determined by oxygen conditions and medium-sized substrata. At localities where trout occurred, the density of + trout was highest in narrow streams with high oxygen concentrations. For older trout, >0+ in age, stream size and temperature were negatively related to density. Biotic factors also appeared to affect trout density, as trout density was inversely related to abundance of predators and coexisting species. Even intraspecific competition appeared to be important as length of 0+ trout was inversely related to trout density. It is suggested that improvements of water quality may be an effective way to restore sea trout populations in southern Sweden, especially in narrow streams where smolt production has the highest potential.     

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

1. 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 chlorophyll a concentration) 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.     

Geography of Invasion in Mountain Streams: Consequences of Headwater Lake Fish Introductions

The introduction of fish into high-elevation lakes can provide a geographic and demographic boost to their invasion of stream networks, thereby further endangering the native stream fauna. Increasingly, remaining populations of native salmonids are concentrated in fragmented headwater refugia that are protected by physical or biological barriers from introduced fishes that originate in the pervasive source populations established at lower elevations. Although fish introduced near mainstem rivers frequently encounter obstacles to upstream dispersal, such as steep slopes or falls, we found that brook trout (Salvelinus fontinalis) dispersed downstream through channel slopes of 80% and 18-m-high falls. Thus, headwater lake stocking provides source populations that may be capable of invading most downstream habitats, including headwater refugia of native fishes. The extent of additional area invasible from lakes, beyond that invasible from downstream, depends on the geography of the stream network, particularly the density and distribution of headwater lakes and their location relative to barriers inhibiting upstream dispersal. In the thermal and trophic environments downstream of lakes, fish commonly grow faster and thus mature earlier and have higher fecundity-at-age than their counterparts in other high-elevation streams. The resulting higher rates of population growth facilitate invasion. Larger body sizes also potentially aid the fish in overcoming barriers to invasion. Trout introductions to high-elevation headwater lakes thus pose disproportionately large risks to native fishes—even when the place of introduction may appear to be spatially dissociated from populations of the native species. Mapping the potential invasible area can help to establish priorities in stocking and eradication efforts. Received 28 March 2000; accepted 9 February 2001.     

More than a corridor: use of a main stem stream as supplemental foraging habitat by a brook trout metapopulation

Coldwater fishes in streams, such as brook trout (Salvelinus fontinalis), typically are headwater specialists that occasionally expand distributions downstream to larger water bodies. It is unclear, however, whether larger streams function simply as dispersal corridors connecting headwater subpopulations, or as critical foraging habitat needed to sustain large mobile brook trout. Stable isotopes (δ¹³C and δ¹⁵N) and a hierarchical Bayesian mixing model analysis was used to identify brook trout that foraged in main stem versus headwater streams of the Shavers Fork watershed, West Virginia. Headwater subpopulations were composed of headwater and to a lesser extent main stem foraging individuals. However, there was a strong relationship between brook trout size and main stem prey contributions. The average brook trout foraging on headwater prey were limited to 126 mm standard length. This size was identified by mixing models as a point where productivity support switched from headwater to main stem dependency. These results, similar to other studies conducted in this watershed, support the hypothesis that productive main stem habitat maintain large brook trout and potentially facilitates dispersal among headwater subpopulations. Consequently, loss of supplementary main stem foraging habitats may explain loss of large, mobile fish and subsequent isolation of headwater subpopulations in other central Appalachian watersheds.     

Genetic effects of habitat fragmentation and population isolation on Etheostoma raneyi (Percidae)

The use of genetic methods to quantify the effects of anthropogenic habitat fragmentation on population structure has become increasingly common. However, in today’s highly fragmented habitats, researchers have sometimes concluded that populations are currently genetically isolated due to habitat fragmentation without testing the possibility that populations were genetically isolated before European settlement. Etheostoma raneyi is a benthic headwater fish restricted to river drainages in northern Mississippi, USA, that has a suite of adaptive traits that correlate with poor dispersal ability. Aquatic habitat within this area has been extensively modified, primarily by flood-control projects, and populations in headwater streams have possibly become genetically isolated from one another. We used microsatellite markers to quantify genetic structure as well as contemporary and historical gene flow across the range of the species. Results indicated that genetically distinct populations exist in each headwater stream analyzed, current gene flow rates are lower than historical rates, most genetic variation is partitioned among populations, and populations in the Yocona River drainage show lower levels of genetic diversity than populations in the Tallahatchie River drainage and other Etheostoma species. All populations have negative F_IS scores, of which roughly half are significant relative to Hardy–Weinberg expectations, perhaps due to small population sizes. We conclude that anthropogenic habitat alteration and fragmentation has had a profoundly negative impact on the species by isolating E. raneyi within headwater stream reaches. Further research is needed to inform conservation strategies, but populations in the Yocona River drainage are in dire need of management action. Carefully planned human-mediated dispersal and habitat restoration should be explored as management options across the range of the species.     

Macroinvertebrate diversity in headwater streams: a review

1. Headwater streams are ubiquitous in the landscape and are important sources of water, sediments and biota for downstream reaches. They are critical sites for organic matter processing and nutrient cycling, and may be vital for maintaining the 'health' of whole river networks.
2. Macroinvertebrates are an important component of biodiversity in stream ecosystems and studies of macroinvertebrate diversity in headwater streams have mostly viewed stream systems as linear reaches rather than as networks, although the latter may be more appropriate to the study of diversity patterns in headwater systems.
3. Studies of macroinvertebrate diversity in headwater streams from around the world illustrated that taxonomic richness is highly variable among continents and regions, and studies addressing longitudinal changes in taxonomic richness of macroinvertebrates generally found highest richness in mid-order streams.
4. When stream systems are viewed as networks at the landscape-scale, α-diversity may be low in individual headwater streams but high β-diversity among headwater streams within catchments and among catchments may generate high γ-diversity.
5. Differing ability and opportunity for dispersal of macroinvertebrates, great physical habitat heterogeneity in headwater streams, and a wide range in local environmental conditions may all contribute to high β-diversity among headwater streams both within and among catchments.
6. Moving beyond linear conceptual models of stream ecosystems to consider the role that spatial structure of river networks might play in determining diversity patterns at the landscape scale is a promising avenue for future research.     

Multiple electrofishing as a mitigate tool for removing nonnative Atlantic brown trout (<Emphasis Type="Italic">Salmo trutta</Emphasis> L.) threatening a native Mediterranean brown trout population

In the native range of the brown trout (Salmo trutta L.) in Europe, the hybridization of native populations by nonnative domesticated strains introduced by stocking is one of the most serious threats to the long-term conservation of diversity within this species. With the objective of conserving and restoring the native gene pool, fishery managers are beginning to implement various management strategies at the local scale. Nevertheless, few case studies have been published that investigate the effectiveness of the various different conservation strategies for native brown trout populations. In the Chevenne Creek, a small French mountain stream, we tested the strategy of removing nonnative individuals by multiple electrofishing carried out by fishery managers in order to evaluate its feasibility and effectiveness for eliminating a nonnative population threatening a native population. Electrofishing produced major reductions in the nonnative population between 2006 and 2009, with 82–100% of nonnative individuals being removed over a period of 4 years. Nevertheless, despite multiple-electrofishing campaigns, this nonnative population was not entirely eradicated, and some natural recruitment persisted. The young of the year and subadults were less effectively removed than the adults. The results suggest that repeated electrofishing campaigns can be used by managers to reduce the nonnative brown trout population with the objective of conserving the native gene pool, but the removal operation must be continued for at least 4 consecutive years. This strategy, which is feasible in small streams, has to be followed by complementary operations to allow the restoration of a new, native, self-sustainable brown trout population.     

Responses of stream fish populations to farming intensity and water abstraction in an agricultural catchment

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Diatom communities along stream longitudinal gradients

1. Summer diatom communities on artificial substrates were sampled weekly for a month in three first- to sixth-order tributaries of the Kentucky River to determine how community structure varied with stream size. 2. Diatom cell abundances were generally higher in the headwaters. Species diversity increased in a downstream direction in two of the streams, and in an upstream direction in the third. However, diversity in general seemed more closely related to current regimes than to stream size per se, with highest species diversity at intermediate current velocities. 3. Variation in diatom accumulation rates was greater in downstream communities than in the headwater assemblages of two streams, suggesting that downstream communities may experience greater fluctuations in abundance, at least under low-flow conditions. 4. Patterns of species distributions suggested a relationship between morphological growth forms (guilds) and stream size, as well as the influence of current. Achnanthes spp., Eunotia spp., erect, and stalked taxa were more commonly associated with headwater assemblages. Filamentous and centric diatoms occurred with greater frequency downstream.     

Static habitat partitioning and dynamic selection by sympatric young Atlantic salmon and brown trout in south-west England streams

Direct underwater observation of micro‐habitat use by 1838 young Atlantic salmon Salmo salar [mean L_T 7·9 ± 3.1(s.d.) cm, range 3·19] and 1227 brown trout Salmo trutta (L_T 10·9 ± 5·0 cm, range 3·56) showed both species were selective in habitat use, with differences between species and fish size. Atlantic salmon and brown trout selected relatively narrow ranges for the two micro‐habitat variables snout water velocity and height above bottom, but with differences between size‐classes. The smaller fishes <7 cm held positions in slower water closer to the bottom. On a larger scale, the Atlantic salmon more often used shallower stream areas, compared with brown trout. The larger parr preferred the deeper stream areas. Atlantic salmon used higher and slightly more variable mean water velocities than brown trout. Substrata used by the two species were similar. Finer substrata, although variable, were selected at the snout position, and differences were pronounced between size‐classes. On a meso‐habitat scale, brown trout were more frequently observed in slow pool‐glide habitats, while young Atlantic salmon favoured the faster high‐gradient meso‐habitats. Small juveniles <7 cm of both species were observed most frequently in riffle‐chute habitats. Atlantic salmon and brown trout segregated with respect to use of habitat, but considerable niche overlap between species indicated competitive interactions. In particular, for small fishes <7 cm of the two species, there was almost complete niche overlap for use of water depth, while they segregated with respect to water velocity. Habitat suitability indices developed for both species for mean water velocity and water depth, tended to have their optimum at lower values compared with previous studies in larger streams, with Atlantic salmon parr in the small streams occupying the same habitat as favoured by brown trout in larger streams. The data indicate both species may be flexible in their habitat selection depending on habitat availability. Species‐specific habitat overlap between streams may be complete. However, between‐species habitat partitioning remains similar.     

Productivity and significance of headwater streams: population structure and biomass of the black-bellied salamander (Desmognathus quadramaculatus)

1. Headwater streams are a significant feature of the southern Appalachian landscape, comprising more than 70% of the total stream length in the region. Salamanders are the dominant vertebrate within headwater‐riparian forest ecosystems, but their ecological role is not clearly understood. 2. We studied a population of black‐bellied salamanders (Desmognathus quadramaculatus) at a headwater stream in the southern Appalachian Mountains using radio‐telemetry and mark‐recapture methods. The length and area of headwater streams in the region were estimated using GIS. 3. Home ranges of radio‐tracked salamanders were relatively small (mean = 1.06 m²). Adult salamanders in our telemetry study inhabited edge microhabitats significantly more often than either stream or riparian microhabitats, and the same trend was observed in the mark‐recapture study. 4. We estimated the population density at this site to be 11 294 salamanders ha^?1, amounting to 99.30 kg ha^?1 of biomass, an estimate that is six times greater than reported in previous studies. The majority of this biomass was found within the stream, but 22% was found in the surrounding riparian habitat more than 1 m from the stream. Using headwater stream length and area estimates, we extrapolated biomass estimates for black‐bellied salamanders inhabiting stream and riparian microhabitats across the study region. 5. We report one of the largest estimates of secondary consumer biomass for a headwater ecosystem, attesting to the overall productivity of headwater streams. Headwaters are known to be important for ecological and ecosystem processes and our biomass estimates suggest that salamanders are a critical component to these systems.     

Changes in habitat structure, benthic invertebrate diversity, trout populations and ecosystem processes in restored forest streams: a boreal perspective

1. Most Finnish streams were channelised during the 19th and 20th century to facilitate timber floating. By the late 1970s, extensive programmes were initiated to restore these degraded streams. The responses of fish populations to restoration have been little studied, however, and monitoring of other stream biota has been negligible. In this paper, we review results from a set of studies on the effects of stream restoration on habitat structure, brown trout populations, benthic macroinvertebrates and leaf retention. 2. In general, restoration greatly increased stream bed heterogeneity. The cover of mosses in channelised streams was close to that of unmodified reference sites, but after restoration moss cover declined to one‐tenth of the pre‐restoration value. 3. In one stream, densities of age‐0 trout were slightly lower after restoration, but the difference to an unmodified reference stream was non‐significant, indicating no effect of restoration. In another stream, trout density increased after restoration, indicating a weakly positive response. The overall weak response of trout to habitat manipulations probably relates to the fact that restoration did not increase the amount of pools, a key winter habitat for salmonids. 4. Benthic invertebrate community composition was more variable in streams restored 4–6 years before sampling than in unmodified reference streams or streams restored 8 years before sampling. Channelised streams supported a distinctive set of indicator species, most of which were filter‐feeders or scrapers, while most of the indicators in streams restored 8 years before sampling were shredders. 5. Leaf retentiveness in reference streams was high, with 60–70% of experimentally released leaves being retained within 50 m. Channelised streams were poorly retentive (c. 10% of leaves retained), and the increase in retention following restoration was modest (+14% on average). Aquatic mosses were a key retentive feature in both channelised and natural streams, but their cover was drastically reduced through restoration. 6. Mitigation of the detrimental impacts of forestry (e.g. removal of mature riparian forests) is a major challenge to the management of boreal streams. This goal cannot be achieved by focusing efforts only on restoration of physical structures in stream channels, but also requires conservation and ecologically sound management of riparian forests.     

Downstream Warming and Headwater Acidity May Diminish Coldwater Habitat in Southern Appalachian Mountain Streams

Stream-dwelling species in the U.S. southern Appalachian Mountains region are particularly vulnerable to climate change and acidification. The objectives of this study were to quantify the spatial extent of contemporary suitable habitat for acid- and thermally sensitive aquatic species and to forecast future habitat loss resulting from expected temperature increases on national forest lands in the southern Appalachian Mountain region. The goal of this study was to help watershed managers identify and assess stream reaches that are potentially vulnerable to warming, acidification, or both. To our knowledge, these results represent the first regional assessment of aquatic habitat suitability with respect to the combined effects of stream water temperature and acid-base status in the United States. Statistical models were developed to predict July mean daily maximum water temperatures and air-water temperature relations to determine potential changes in future stream water temperatures. The length of stream considered suitable habitat for acid- and thermally sensitive species, based on temperature and acid neutralizing capacity thresholds of 20°C and 50 μeq/L, was variable throughout the national forests considered. Stream length displaying temperature above 20°C was generally more than five times greater than the length predicted to have acid neutralizing capacity below 50 μeq/L. It was uncommon for these two stressors to occur within the same stream segment. Results suggested that species’ distributional shifts to colder, higher elevation habitats under a warming climate can be constrained by acidification of headwater streams. The approach used in this study can be applied to evaluate climate change impacts to stream water resources in other regions.     

Are native cyprinids or introduced salmonids stronger regulators of benthic invertebrates in South African headwater streams?

The introduction of nonnative salmonids in the Southern Hemisphere generally leads to a reduction in invertebrate abundance and changes in assemblage composition. In the Cape Floristic Region of South Africa, introduced rainbow trout Oncorhynchus mykiss is the dominant predator in many headwater streams, where they have replaced small‐bodied native fishes such as Breede River redfin Pseudobarbus burchelli. To examine the consequences of this species replacement on food web structure, we used a month‐long field experiment to compare the top‐down effects of Breede River redfin and rainbow trout on benthic invertebrate assemblages (abundance and composition) and basal resources (periphyton and particulate organic matter) in 1 × 1.5 m of plastic cages. Benthic invertebrate abundance was more strongly depleted in the cages with redfin than in the cages with trout, and redfin and trout had distinct effects on invertebrate assemblage composition. On the other hand, neither redfin nor trout had a significant influence over standing stocks of periphyton or organic matter, implying that their differential effects on benthic invertebrates did not cascade down to the base of the stream food web in our experiment. Gut content analysis showed that aquatic invertebrates contributed more to the diet of redfin, while terrestrial invertebrates contributed more to the diet of trout, which may be responsible for the relatively weak effect of trout on aquatic invertebrates. This pattern contrasts with nonnative salmonid impacts elsewhere in the Southern Hemisphere. That trout can strongly alter the structure of benthic invertebrate assemblages, in addition to severely depleting native fish abundance, in Cape Floristic Region headwater streams should be weighed into management decisions, and our findings highlight the need for a detailed understanding of species‐specific top‐down effects where native predators are replaced by invasive predators.