Ecological Restoration and Large-Scale Ecological Disturbance: The Effects of Drought on the Response by Fish to a Habitat Restoration Experiment

Human‐induced erosion regularly delivers massive quantities of fine sediments into streams and rivers forming large static bodies of sediment known as sand slugs, which smother in‐stream habitat, alter community structure, and decrease biodiversity. Sand slugs are widespread in parts of southeastern Australia as well as in many other parts of the world, and there is now considerable interest in restoring such affected streams. The reintroduction of large timber is widely suggested as a strategy for restoring habitat complexity, but this has rarely been tested in sand slug–affected streams. We examined the response of fish populations to wood addition to two streams in southeastern Australia that have been impacted by sand slugs. Manipulated sites (three per treatment) had either one or four timber structures added, and these sites were compared with (three) unmanipulated (control) sites before and after the manipulation occurred. Despite a supraseasonal drought during the study, we observed short‐term increases in the abundance of Mountain galaxias (Galaxias olidus) at the four‐structure sites, while both the four‐structure and the one‐structure treatments appeared to buffer against drought‐induced declines in two other species, River blackfish (Gadopsis marmoratus) and Southern pygmy perch (Nannoperca australis), relative to controls. However, drought eventually caused the complete loss of surface water from these streams and the loss of fish from both manipulated and unmanipulated sites. Thus, although the study supports the use of timber structures as a means of increasing local fish abundances, these beneficial effects were, in these streams, contingent upon permanency of flow. Because sedimentation has depleted the number of permanent refuge pools in these creeks, recovery rates of the fauna (i.e., resilience) are likely to be slow. We therefore conclude that in streams subjected to frequent disturbance, restoring refugia may be as, if not more, important as restoring what we term residential habitat.     

Dispersal and recruitment of fish in an intermittent stream network

Animal movement is an important process connecting habitats in heterogeneous landscapes, and can play a key role in population persistence. Laboratory swim trials were conducted to determine and compare the dispersal capabilities of two native Australian fish, mountain galaxias (Galaxias olidus, Family Galaxiidae) and southern pygmy perch (Nannoperca australis, Family Nannopercidae) that maintain populations in hydrologically variable and intermittently flowing streams in south‐eastern Australia. These experiments showed that G. olidus had significantly greater swimming endurance under a range of flow velocities. Concurrent field surveys were used to establish whether swimming abilities observed in laboratory studies were consistent with patterns of inferred movement from distribution and abundance patterns observed in the field. Data collected at multiple sites from headwater to lowland reaches along multiple streams revealed substantial temporal changes in the distribution of young‐of‐year (0+) G. olidus, with spawning occurring at upland sites in winter, followed by downstream larval migration and subsequent upstream movement in late spring. Observed spatial and temporal patterns in G. olidus abundances were consistent with a source‐sink population structure, which may be disrupted by prolonged cease‐to‐flow periods during drought years. In contrast, results for N. australis suggested limited dispersal, with restricted local populations that persist at sites with permanent surface water. These field and laboratory findings complement our understanding of the spatial population structure of these two species in intermittent streams, and highlight the importance of understanding the role of dispersal in species conservation and habitat restoration.     

Niche partitioning and the effect of interspecific competition on microhabitat use by two sympatric galaxiid stream fishes

1. Numerous interacting abiotic and biotic factors influence niche use and assemblage structure of freshwater fishes, but the strength of each factor changes with spatial scale. Few studies have examined the role of interspecific competition in structuring stream fish assemblages across spatial scales. We used field and laboratory approaches to examine microhabitat partitioning and the effect of interspecific competition on microhabitat use in two sympatric stream fishes (Galaxias‘southern’ and Galaxias gollumoides) at large (among streams and among sites within streams) and small (within artificial stream channels) spatial scales. 2. Diurnal microhabitat partitioning and interspecific competition at large spatial scales were analysed among three sympatry streams (streams with allotopic and syntopic sites; three separate catchments) and four allopatry streams (streams with only allotopic sites; two separate catchments). Electro‐fishing was used to sample habitat use of fishes at 30 random points within each site by quantifying four variables for each individual: water velocity, depth, distance to nearest cover and substratum size. Habitat availability was then quantified for each site by measuring those variables at each of 50 random points. Diet and stable isotope partitioning was analysed from syntopic sites only. Diel cycles of microhabitat use and interspecific competition at small spatial scales were examined by monitoring water velocity use over 48 h in artificial stream channels for three treatments: (i) allopatric G. ‘southern’ (10 G. ‘southern’); (ii) allopatric G. gollumoides (10 G. gollumoides) and (iii) sympatry (five individuals of each species). 3. One hundred and ninety‐four G. ‘southern’ and 239 G. gollumoides were sampled across all seven streams, and habitat availability between the two species was similar among all sites. Galaxias‘southern’ utilised faster water velocities than G. gollumoides in both the field and in channel experiments. Both species utilised faster water velocities in channels at night than during the day. Diet differences were observed and were supported by isotopic differences (two of three sites). No interspecific differences were observed for the other three microhabitat variables in the field, and multivariate habitat selection did not differ between species. Interspecific competition had no effect on microhabitat use of either species against any variable either in the field (large scale) or in channels (small scale). 4. The results suggest that niche partitioning occurs along a subset of microhabitat variables (water velocity use and diet). Interspecific competition does not appear to be a major biotic factor controlling microhabitat use by these sympatric taxa at any spatial scale. The results further suggest that stream fish assemblages are not primarily structured by biotic factors, reinforcing other studies de‐emphasising interspecific competition.     

Spatial variation and effects of habitat on temperate reef fish assemblages in northeastern New Zealand

Reef-associated fishes can respond to changes in habitat structure and the nature of their response can change with different spatial scales of observation. A structured hierarchical mensurative sampling design was used to sample temperate reef fish assemblages in northeastern New Zealand at several spatial scales over 2 years. The three spatial scales examined were tens of meters (transects), hundreds to thousands of meters (sites) and hundreds of kilometers (locations). We tested the hypothesis that fish assemblages differed between kelp forest habitat (relatively dense stands of the kelp, Ecklonia radiata (C. Agardh) J. Agardh, median depth=13.5 m) and barrens habitat (rocky reef dominated by turfing and encrusting red algae and the grazing urchin, Evechinus chloroticus (Valenciennes), median depth=6.7 m). Recently developed multivariate techniques were used to test for and quantify multivariate variation at different spatial scales. There were significant effects of habitat on the spatial distribution of fish assemblages, characterised by greater abundances or frequencies of Parika scaber, Chromis dispilus, Trachurus novaezelandiae, Nemadactylus douglasii, Bodianus unimaculatus, Odax pullus and Pseudolabrus miles in kelp forest habitat, and greater abundances or frequencies of Notolabrus celidotus, Notolabrus fucicola, Girella tricuspidata, Coris sandageri, Chironemus marmoratus, Parma alboscapularis, Scorpis violaceus and Kyphosus sydneyanus in barrens habitat. Some of the more common species, including Upeneichthys lineatus, Scorpis lineolatus and Cheilodactylus spectabilis showed no strong consistent effects of these two differing habitats on their distributions. There was, however, a significant Habitat×Locations interaction: effects of habitat did not occur at all locations. Variability was highest at the scale of individual transects and variability from site to site and from location to location was comparable. Spatial variation was large compared to inter-annual variation, which was minimal, and spatial patterns were consistent in the 2 years examined. Further experiments, including manipulations, are required to understand what mechanisms and processes might be driving these patterns. This study, coupled with results from previous studies, suggests that there may be a dynamic inter-play between effects of habitat on fish and effects of fish on biogenic habitat, such as kelp forests.     

Fish community composition and diversity at restored estuarine habitats in Tampa Bay,Florida, United States

Estuary restoration in Tampa Bay, Florida, United States, is an ongoing focus of natural resource managers because of pressure from an increasing coastal population, historic habitat loss, and restoration's importance to economic development, recreational activities, and fish habitat. A growing population can also limit future large‐scale restorations due to associations with cost and land availability. This limitation might be overcome by applying the habitat mosaic approach to restoration, which creates distinct habitat types at small spatial scales. This approach was applied to create three types of estuarine habitat, reconnected tidal creek, salt marsh, and tidal pond. The objectives of this study were to (1) initiate monitoring of a restored wetland mosaic and (2) determine how fish diversity and community structure vary among restored habitat types. Replicated sampling using a 3‐mm mesh seine was used to characterize the fish communities. Our results indicate that the habitat mosaic approach creates suitable habitat for a variety of fish species where 37% of fish species were captured in just one habitat type. In particular, the recreationally important Centropomus undecimalis (common snook) was more common in the mangrove‐lined creek and the non‐native Sarotherodon melanotheron (blackchin tilapia) was common in the tidal pond. Greater emphasis should be placed on applied restoration research to identify how habitat types within a larger restoration mosaic contribute to local species diversity and recreationally and commercially important fishes, while limiting non‐natives. This emphasis could reveal how restoration approaches can be modified to include habitat mosaics, maximizing their contribution to productive fish habitat.     

Ecological values of Hamilton urban streams (North Island, New Zealand): constraints and opportunities for restoration

Urban streams globally are characterised by degraded habitat conditions and low aquatic biodiversity, but are increasingly becoming the focus of restoration activities. We investigated habitat quality, ecological function, and fish and macroinvertebrate community composition of gully streams in Hamilton City, New Zealand, and compared these with a selection of periurban sites surrounded by rural land. A similar complement of fish species was found at urban and periurban sites, including two threatened species, with only one introduced fish widespread (Gambusia affinis). Stream macroinvertebrate community metrics indicated low ecological condition at most urban and periurban sites, but highlighted the presence of one high value urban site with a fauna dominated by sensitive taxa. Light-trapping around seepages in city gullies revealed the presence of several caddisfly species normally associated with native forest, suggesting that seepage habitats can provide important refugia for some aquatic insects in urban environments. Qualitative measures of stream habitat were not significantly different between urban and periurban sites, but urban streams had significantly lower hydraulic function and higher biogeochemical function than periurban streams. These functional differences are thought to reflect, respectively, (1) the combined effects of channel modification and stormwater hydrology, and (2) the influence of riparian vegetation providing shade and enhancing habitat in streams. Significant relationships between some macroinvertebrate community metrics and riparian vegetation buffering and bank protection suggest that riparian enhancement may have beneficial ecological outcomes in some urban streams. Other actions that may contribute to urban stream restoration goals include an integrated catchment approach to resolving fish passage issues, active reintroduction of wood to streams to enhance cover and habitat heterogeneity, and seeding of depauperate streams with native migratory fish to help initiate natural recolonisation.     

Non‐native salmonids affect amphibian occupancy at multiple spatial scales

Aim The introduction of non‐native species into aquatic environments has been linked with local extinctions and altered distributions of native species. We investigated the effect of non‐native salmonids on the occupancy of two native amphibians, the long‐toed salamander (Ambystoma macrodactylum) and Columbia spotted frog (Rana luteiventris), across three spatial scales: water bodies, small catchments and large catchments. Location Mountain lakes at ≥ 1500 m elevation were surveyed across the northern Rocky Mountains, USA. Methods We surveyed 2267 water bodies for amphibian occupancy (based on evidence of reproduction) and fish presence between 1986 and 2002 and modelled the probability of amphibian occupancy at each spatial scale in relation to habitat availability and quality and fish presence. Results After accounting for habitat features, we estimated that A. macrodactylum was 2.3 times more likely to breed in fishless water bodies than in water bodies with fish. Ambystoma macrodactylum also was more likely to occupy small catchments where none of the water bodies contained fish than in catchments where at least one water body contained fish. However, the probability of salamander occupancy in small catchments was also influenced by habitat availability (i.e. the number of water bodies within a catchment) and suitability of remaining fishless water bodies. We found no relationship between fish presence and salamander occupancy at the large‐catchment scale, probably because of increased habitat availability. In contrast to A. macrodactylum, we found no relationship between fish presence and R. luteiventris occupancy at any scale. Main conclusions Our results suggest that the negative effects of non‐native salmonids can extend beyond the boundaries of individual water bodies and increase A. macrodactylum extinction risk at landscape scales. We suspect that niche overlap between non‐native fish and A. macrodactylum at higher elevations in the northern Rocky Mountains may lead to extinction in catchments with limited suitable habitat.     

Persistence at distributional edges: Columbia spotted frog habitat in the arid Great Basin,USA

A common challenge in the conservation of broadly distributed, yet imperiled species is understanding which factors facilitate persistence at distributional edges, locations where populations are often vulnerable to extirpation due to changes in climate, land use, or distributions of other species. For Columbia spotted frogs (Rana luteiventris) in the Great Basin (USA), a genetically distinct population segment of conservation concern, we approached this problem by examining (1) landscape‐scale habitat availability and distribution, (2) water body‐scale habitat associations, and (3) resource management‐identified threats to persistence. We found that areas with perennial aquatic habitat and suitable climate are extremely limited in the southern portion of the species’ range. Within these suitable areas, native and non‐native predators (trout and American bullfrogs [Lithobates catesbeianus]) are widespread and may further limit habitat availability in upper‐ and lower‐elevation areas, respectively. At the water body scale, spotted frog occupancy was associated with deeper sites containing abundant emergent vegetation and nontrout fish species. Streams with American beaver (Castor canadensis) frequently had these structural characteristics and were significantly more likely to be occupied than ponds, lakes, streams without beaver, or streams with inactive beaver ponds, highlighting the importance of active manipulation of stream environments by beaver. Native and non‐native trout reduced the likelihood of spotted frog occupancy, especially where emergent vegetation cover was sparse. Intensive livestock grazing, low aquatic connectivity, and ephemeral hydroperiods were also negatively associated with spotted frog occupancy. We conclude that persistence of this species at the arid end of its range has been largely facilitated by habitat stability (i.e., permanent hydroperiod), connectivity, predator‐free refugia, and a commensalistic interaction with an ecosystem engineer. Beaver‐induced changes to habitat quality, stability, and connectivity may increase spotted frog population resistance and resilience to seasonal drought, grazing, non‐native predators, and climate change, factors which threaten local or regional persistence.     

A laboratory study of the effects of shelter availability and invasive crayfish on the growth of native stream fish

1. The alteration of stream habitats by urbanisation reduces the availability of shelter. Reduced shelter availability may increase both predation risks and metabolic costs, negatively affecting the growth performance of stream fish. Although urbanised streams often allow the establishment of invasive species, the additive or interactive effects of shelter availability and invasive species are rarely explored. The invasive red swamp crayfish (Procambarus clarkii) is a strong shelter competitor and predator in streams. 2. We conducted a stream‐channel experiment to investigate how shelter availability and this invasive crayfish species affect the growth performance (measured as the loss in body mass under starved conditions) of two native benthic species, Japanese crucian carp (Carassius auratus complex) and an endangered cobitid fish (Lefua echigonia). 3. We found that both the absence of shelter and the presence of crayfish increased body mass loss in the crucian carp. However, the growth performance of the crucian carp was improved by the presence of shelter except in the presence of crayfish, in which case the shelter available did not ameliorate the negative effect of the crayfish on growth performance (i.e. an interactive effect). This result suggests that crayfish may affect the growth performance of the crucian carp through chemical cues as well as through shelter occupancy. 4. In contrast, shelter availability is the primary factor affecting the body mass loss of the cobitid fish. However, the growth performance of these fish was unaffected by the presence of crayfish. The change in the growth performance of the cobitid fish in response to the absence of shelter was greater than that of the crucian carp. This finding suggests that the cobitid fish may be more vulnerable than the crucian carp to a reduction in shelter availability. 5. Our study demonstrates that reduced shelter availability and/or crayfish invasion can have significant, indirect negative effects on the growth performance of native fish, but whether those effects are interactive might vary depending on the species. Our findings have clear implications for stream restoration and habitat assessment.     

Fish-habitat associations across multiple spatial scales

Important to the study of reef fish ecology is understanding the degree to which fish community structure varies across space, what factors can account for such variation, and whether these factors are scale dependent. This study examined the structure of reef fish communities across four spatial scales (1, 10 100, and 200 m²) visually censused from seven sites within Tague Bay, St. Croix, U.S. Virgin Islands. Relative differences in the number of individuals and species among sites indicated a pattern that was consistent across spatial scales. Spearmans rank correlation revealed significant positive correlation in site rankings, in terms of species richness, between 1 and 10 m², and 100 and 200 m²; and for the abundance of individuals between 100 and 200 m². In order to understand the degree to which quantified habitat variables account for patterns in the abundance of individuals and species, and whether these fish-habitat relationships were consistent regardless of spatial scale, separate canonical correlation analyses were conducted at each scale. Independent of scale, the total number of individuals and species were correlated with specific habitat variables, either negatively (with areas of pavement, sand, no algae, and low structural complexity) or positively (with areas of Amphiroa rigida, Halimeda incrassata, high structural complexity, and diverse algae/seagrass communities). These habitat variables explained 31–81% (at scales of 1–200 m²) of the variation in the number of individuals and species. Similar analyses were also performed on the abundances of the nine most common species, and whether their specific habitat associations were independent of scale. Results indicated that habitat variables explained 19–73% (at scales of 1–200 m²) of the variation in abundances of each species. Unique fish-habitat relationships were observed for each species, and most such relationships were consistent across spatial scales. The structure of reef fish communities of Tague Bay was explained in large part by the composition of coral and algae communities present. Both the spatial variation in community structure and the fish-habitat relationships, at the community and population level, appeared to be largely independent of the spatial scale examined. This suggests that generalizations across Tague Bay are possible. Similar habitat associations reported in the literature are discussed with regard to the possibility for generalizations across regions.     

Re‐building brown trout populations in dredged boreal forest streams: in‐stream restoration combined with stocking of young trout

1. Rivers in boreal forested areas were often dredged to facilitate the transport of timber resulting in channels with simplified bed structure and flow fields and reduced habitat suitability for stream organisms, especially lotic fishes. Currently, many streams are being restored to improve their physical habitat, by replacing boulders and gravel and removing constraining embankments. The most compelling justification behind stream restoration of former floatways has been the enhancement of native fish populations, specifically salmonids. 2. We examined the success of a stream management programme aimed at re‐building diminished brown trout (Salmo trutta) populations by monitoring densities of young‐of‐year and older trout in 18 managed and three reference streams during 2000–2005. Rehabilitation included in‐stream restoration combined with a 5‐year post‐restoration period of stocking young brown trout. Our space‐for‐time substitution design comprised four pre‐management, four under‐management, 10 post‐management and three reference streams. 3. Densities of young‐of‐year brown trout, indicating population establishment, were significantly higher in post‐ compared with pre‐management streams. However, density of young‐of‐year brown trout in post‐management streams was significantly lower compared with near‐pristine reference streams. Furthermore, success of managed brown trout population re‐building varied, indicating stream‐specific responses to management measures. Density of burbot (Lota lota), a native generalist predator, was associated with low recruitment of brown trout. 4. Stream‐specific responses imply that rehabilitation of brown trout populations cannot be precisely predicted thereby limiting application. Our findings support the importance of adaptive stream restoration and management, with focus on identifying factor(s) limiting the establishment of target fish populations.     

The Potential Role of Stream Habitat Restoration in Facilitating Salmonid Invasions: A Habitat‐Hydraulic Modeling Approach

Many successful invasions have taken place in systems where harmful disturbance has changed habitat conditions. However, less attention has been paid to the role of habitat restoration, which modifies habitats and thus also has the potential to facilitate invasions. We examined whether in‐stream habitat restorations have the potential to either facilitate or resist invasion by two widely introduced non‐native stream salmonids, Salvelinus fontinalis Mitchill and Oncorhynchus mykiss Walbaum, in Finland. A physical habitat simulation system was used to calculate whether the habitat area for the target species increased or decreased following the restorations. For comparison, we also reported results for four native stream fish species. The simulations showed that the restored streams provided the highest amount of usable habitat area for the native species, particularly for Salmo salar L. and Gottus gobio L. However, it was interesting to note that the restorations significantly increased habitat quality for the two non‐native species, especially at low flows. Nevertheless, the non‐native species had the lowest amount of usable habitat area overall. The modeling results indicated that not only habitat destruction but also habitat restoration could contribute to the spread of non‐native species. Fisheries and wildlife managers should be aware of the possibility, when restoring habitats in order to preserve native ecosystems, that non‐native species could manage to gain a foothold in restored habitats and use them as population sources for further spread. Knowing the widespread negative effect of non‐native species, this risk should not be underestimated.     

Effects of stream restoration and wastewater treatment plant effluent on fish communities in urban streams

1. Fish community characteristics, resource availability and resource use were assessed in three headwater urban streams in Piedmont North Carolina, U.S.A. Three site types were examined on each stream; two urban (restored and unrestored) and a forested site downstream of urbanisation, which was impacted by effluent from a wastewater treatment plant (WWTP). Stream basal resources, aquatic macroinvertebrates, terrestrial macroinvertebrates and fish were collected at each site. 2. The WWTPs affected isotope signatures in the biota. Basal resource, aquatic macroinvertebrate and fish δ¹⁵N showed significant enrichments in the downstream sites, although δ¹³C signatures were not greatly influenced by the WWTP. Fish were clearly deriving a significant part of their nutrition from sewage effluent‐derived sources. There was a trend towards lower richness and abundance of fish at sewage‐influenced sites compared with urban restored sites, although the difference was not significant. 3. Restored stream sites had significantly higher fish richness and a trend towards greater abundance compared with unrestored sites. Although significant differences did not exist between urban restored and unrestored areas for aquatic and terrestrial macroinvertebrate abundances and biotic indices of stream health, there appeared to be a trend towards improvements in restored sites for these parameters. Additional surveys of these sites on a regular basis, along with maintenance of restored features are vital to understanding and maximising restoration effectiveness. 4. A pattern of enriched δ¹³C in fish in restored and unrestored streams in conjunction with enriched δ¹³C of terrestrial invertebrates at these sites suggests that these terrestrial subsidies are important to the fish, a conclusion also supported by isotope cross plots. Furthermore, enriched δ¹³C observed for terrestrial invertebrates is consistent with some utilisation of the invasive C4 plants that occur in the urban riparian areas.     

Linking microhabitat availability and local density of rainbow trout in low-gradient Japanese streams

Quantification of reach-based microhabitat availability for rainbow trout (Oncorhynchus mykiss), considering their microhabitat requirements in two low-gradient streams, northern Japan, was attempted to test for habitat space limitation of local trout density. Underwater observations revealed that fish selected microhabitats of moderate current velocity, relatively greater depth and shorter distance to overhead cover in both streams, although habitat features used and available differed slightly between the streams. Habitat space for fish potentially available in the channel environment was evaluated using principal component analysis (PCA) of both available and used microhabitat. A close relationship was evident in both streams between reach-based microhabitat availability and fish density, which was assessed by a three-pass removal method. Direct estimates of fish microhabitat availability using PCA can contribute to accurate predictions of local fish density and provide insight into the mechanisms responsible for fish–habitat relationships in streams.     

Location of demographic sources affects the distributions of a vulnerable native fish in invaded river networks

1. Invasive predators negatively affect native prey to varying degrees across landscapes, and spatial configuration of invader‐free refugia may affect prey distributions across invaded river networks. In New Zealand, introduced trout (Salmo trutta and Oncorhynchus mykiss) create source‐sink dynamics in native Galaxias vulgaris populations, and their co‐occurrence with trout may be enhanced by immigration from trout‐free reaches. 2. We investigated how network configuration of trout‐free demographic sources affected the distribution of G. vulgaris across trout‐invaded riverscapes. Using quantitative biomass surveys and spatially extensive presence–absence surveys, the interaction between habitat variability and location relative to sources in limiting distributions of G. vulgaris in trout‐invaded reaches was assessed. 3. Galaxias vulgaris biomass at invaded sink sites decreased with increasing network distance to the nearest trout barrier. The maximum distance to barriers at which G. vulgaris occurred in the riverscape was limited, so that galaxiids were excluded from small and stable streams far from sources. 4. Large predatory trout (i.e. >150 mm fork length) occurred in high densities at stable sites all year round and were seasonally excluded from sites disturbed by flooding. Large streams probably provide increased refugia for galaxiids to avoid predation from trout, but narrowness and habitat stability may act synergistically to extirpate G. vulgaris from sites that are too far from sources to receive regular immigrants. 5. The interaction between immigration and habitat configuration in mediating effects of trout on G. vulgaris distributions indicates the spatial dependency of predator–prey interactions in river networks. 6. These results indicate creating new invader‐free source habitat should enhance co‐occurrence in nearby invader‐occupied reaches. Moreover, adding source habitat in sink streams far from existing sources, and ensuring barriers prevent future invasion, will also allow native fish dispersal between sources and sinks and will maximise the conservation gains from management across invaded riverscapes.     

Genetic structure of a native cyprinid in a reservoir‐altered stream network

1. Reservoirs modify riverine ecosystems worldwide, and often with deleterious impacts on native biota. The immediate effects of reservoirs on native fish species below dams and in impounded reaches have received considerable attention, but it is unclear how reservoirs may affect fish species at larger spatial and temporal scales. Documented declines of stream fish populations in direct tributaries of reservoirs suggest reservoir pools may reduce gene flow among historically connected populations. 2. Because of increased predator densities in reservoirs and the extent of habitat alteration in impounded reaches, I predicted reservoir habitats would reduce gene flow among small‐bodied fish populations separated by reservoir habitat. I used microsatellite markers to assess the spatial genetic structure of populations of the red shiner (Cyprinella lutrensis), in a reservoir‐fragmented stream network (Lake Texoma, U.S.A.). I also tested the prediction that populations in two direct tributaries that have experienced population declines would have low genetic diversity. Individuals were collected from six sites upstream of the reservoir, three sites in the reservoir and three sites in direct tributaries of the reservoir during 2008 and 2009. 3. Results indicate that most populations were isolated by distance with little divergence among populations. In one direct tributary population, however, there was substantial genetic divergence, and genetic diversity was significantly lower than in other populations. Gene flow also seemed to be lower in reservoir habitats than in intact stream habitats, suggesting reservoir habitats may be reducing gene flow among the reservoir‐separated populations. These results indicate that reservoirs may reduce gene flow among reservoir‐fragmented stream fish populations, altering the evolutionary trajectories of fragmented populations.     

Influence of irrigation infrastructures and water quality on fish assemblages in Lake Tana tributaries,north-west Ethiopia

Lake Tana is one of East Africa’s largest freshwater bodies, yet many of its fishes are migratory and utilize in-flowing tributaries as critical spawning habitat. However, factors such as expanding water resources developments and sand mining along these rivers and streams may disrupt this ecosystem function. We monitored juvenile and adult fish abundance and water quality across five lake tributaries from August 2014 to April 2015 to examine how irrigation schemes and water quality affect assemblage and population structure. Adult assemblages were dominated by Labeobarbus cyprinids and varied between tributaries, albeit without separation by irrigation development or sand mining. Overall, adult abundances of the dominant migratory Labeobarbus species were four-fold higher below the Shini River irrigation weir than upstream. Contrastingly, juvenile abundances were often significantly higher above these structures. Juvenile abundances decreased on average by 46% along the first 1000 m of two irrigation canals, suggesting poor habitat suitability or high mortalities from water withdrawals. Water quality varied more between rivers than sampling times, but without any separation of tributaries by irrigation or sand mining. Conductivity and turbidity-related parameters had the highest correlation with adult assemblage structure and individual species abundances. These findings indicate that Lake Tana tributaries must be managed on a case by case basis, with more focus given to mechanisms allowing fish to bypass irrigation developments and the direct assessment of fish populations between sand mining and other sites.

     

Reach and catchment‐scale characteristics are relatively uninfluential in explaining the occurrence of stream fish species

The objectives of this study were (1) to determine whether the presence or absence of prairie fishes can be modelled using habitat and biotic characteristics measured at the reach and catchment scales and (2) to identify which scale (i.e. reach, catchment or a combination of variables measured at both scales) best explains the presence or absence of fishes. Reach and catchment information from 120 sites sampled from 1999 to 2004 were incorporated into tree classifiers for 20 prairie fish species, and multiple criteria were used to evaluate models. Fewer than six models were considered significant when modelling individual fish occurrences at the reach, catchment or combined scale, and only one species was successfully modelled at all three scales. The scarcity of significant models is probably related to the rigorous criteria by which these models were evaluated as well as the prevalence of tolerant, generalist fishes in these stochastic and intermittent streams. No significant differences in the amount of reduced deviance, mean misclassification error rates (MER), and mean improvement in MER metrics was detected among the three scales. Results from this study underscore the importance of continued habitat assessment at smaller scales to further understand prairie‐fish occurrences as well as further evaluations of modelling methods to examine habitat relationships for tolerant, ubiquitous species. Incorporation of such suggestions in the future may help provide more accurate models that will allow for better management and conservation of prairie‐fish species.     

Biogeography,ecoregions, and geomorphology affect fish species composition in streams of eastern Oklahoma,USA

Stream fish assemblages are structured by biogeographical, physical and biological factors acting on different spatial scales. We determined how physical factors, geomorphology and stream habitat, influenced fish species composition (presence–absence) in eastern Oklahoma, USA relative to the ecoregion and biogeographic effects previously reported. We sampled fish assemblages and surveyed geomorphology and habitat at 107 stream sites in the Boston Mountains, Ouachita Mountains, and Ozark Highlands ecoregions in eastern Oklahoma. Partial canonical correspondence analyses (pCCAs) and variance partitioning showed that patterns of endemism related to drainage basins and ecoregions explained important variation in fish species composition in all streams, but stream size and local channel morphology explained more variation overall. Stream size effects were most important in explaining variability in fish species composition in both northeastern and southeastern Oklahoma streams. Local channel morphology and substrate characteristics were secondarily important. Variables typically considered important as fish habitat (aquatic vegetation, etc.) had little effect on fish species composition.     

Scale-dependence of fish habitat associations in a North American river system

1. Determining the appropriate measurement scale to assess habitat variables is critical for ecologists assessing biological or ecological conditions. Depth, velocity, substrate, woody debris and other fish cover variables occur on both reach and microhabitat scales, and fish habitat associations with these variables may be scale-dependent. The aim of this work was to better understand the importance of scale for fish–habitat associations with these variables in a framework consistent with environmental filtering and to test the hypothesis that habitat variable importance is scale-dependent.
2. I used prepositioned areal electrofishing in wadeable streams of the Delaware River basin to evaluate the associations of fish with the same variables summarised on different reach and microhabitat scales. The importance of scale for fish–habitat associations was assessed using two approaches that approximate an environmental filtering framework: variance partitioning with (1) ordination and (2) generalised linear mixed models.
3. Variables on both the reach and microhabitat scales explained a significant fraction of the total variation in fish community composition (p < 0.05). Variation decomposition of reach- and microhabitat-scale effects revealed 20.2% and 2.0% of all variation were due uniquely to reach and microhabitat scales, respectively. Measures of coarseness, embeddedness, amount of riffle and areal coverage of five fish cover variables were significant explanatory variables of community composition at the reach scale only (p < 0.05). Velocity and mesohabitat (amount or presence of riffle) were the only two habitat features that were significant explanatory variables of fish community composition at both the reach and microhabitat scales (p < 0.05). Individual models of species occurrence revealed similar patterns as seen with analyses of community composition.
4. For many fishes, habitat features quantified at the reach scale were more explanatory than at the microhabitat scale. Longnose dace (Rhinichthys cataractae) were more dependent upon microhabitat variables than reach-scale variables, relative to other fishes. Mean velocity at the reach scale was the most important explanatory variable for explaining fish community composition and indicated support for the concept of environmental filtering at the reach and microhabitat scales.
5. Few studies of fish occurrence have incorporated a study design and analytical framework that approximates the hierarchical nature of habitat. This study identifies important scales and predictors, demonstrates the importance of a multiscale approach, and provides support for the environmental filtering concept at the reach and microhabitat scales. These findings will allow ecologists to better account for scale-dependent habitat associations and justify the use of fish habitat associations on reach and microhabitat scales for assessing biotic integrity, restoration and conservation of fishes.