Substratum Size Research Articles

Predictive models of fish microhabitat selection in multiple sites accounting for abundance overdispersion

AbstractMicrohabitat selection models are frequently used in rivers to evaluate anthropogenic effects on aquatic organisms. Fish models are generally developed from few rivers, with debatable statistical treatments for coping with overdispersed abundance distributions. Analyses of data from multiple rivers are needed to test their transferability and increase their relevance for stakeholders. Using 3,528 microhabitats sampled in nine French rivers during 129 surveys, we developed models for 35 specific size classes of 22 fish species. We used mixed‐effects generalized linear models (accounting for multiple surveys), involving B‐spline transformations (accounting for nonlinear responses) and assuming a negative binomial distribution (accounting for abundance overdispersion). We compared models of increasing complexity: no selection (M1), an “average” selection similar in all surveys (M2), two models with different selection across surveys (M3–M4). Of 132 univariate cases (specific size classes by habitat), 63% indicated selection for depth, 71% for velocity, 45% for substratum size and 13% for substratum heterogeneity. A total of 50 models were retained, involving 26/35 specific size classes. Model fits indicated low explained deviance (R2MF < 0.19) and higher rank correlations (ρ < 0.69) between observed and modelled values. However, Bayesian posterior predictive checks validated these results since excellent fits would generate R2MF lower than 0.59 and ρ lower than 0.78. We found high transferability among rivers and dates, because (a) M2 was the most appropriate in 26/50 cases; (b) the R2MF and ρ values by M2 was, respectively, 72% and 75% of that explained by the complex M4 and (c) independent river cross‐validations showed good transferability. Bivariate models for selected specific size classes improved univariate model fits (ρ from 0.30 to 0.38). Overall, using a nonlinear mixed‐effect approach, our results confirmed the relevance of “average” models based on several rivers for developing helpful e‐flow tools. Finally, our modelling approach opens opportunities for integrating additional effects as the spatial distribution of competitors.

Effects of stream grade control structures on benthic algal communities in a subtropical stream

Stream grade control structures (GCSs) can have strong effects on stream habitat and communities. While most studies of GCSs have examined their impacts on fish and macroinvertebrate assemblages, few have focused on their impacts on benthic algae. Thus, this research studied effects of GCSs by comparing benthic algal communities in a forested stream section with those in a GCS section in a subtropical stream of central Taiwan. We included habitat and water quality surveys and an algal growth experiment with 3 treatments: forested, GCS, and shaded GCS. The GCS section had a greater stream wetted width, percentage of pool area, and algal biomass indicated by chlorophyll a, algal biovolume, and density than the forested section, whereas the forested section had a higher canopy cover, substratum size, and habitat diversity. Filamentous green algae were the dominant group in the GCS section, whereas diatoms were dominant in the forested section. Experimental results showed that GCSs strongly elevated algal biomass and changed community compositions. The shaded treatment significantly reduced >80% of the algal biomass in the GCS section, close to the reduction in the forested treatment. Considering all variables altered by the GCS, canopy cover might be the primary responsible factor. We suggest considerations on design and construction of GCSs.

Distribution and ecology of lampreys Lethenteron spp. in interior Alaskan rivers.

This study describes phenetic characteristics and examines the life history of anadromous Arctic lamprey Lethenteron camtschaticum and freshwater-resident Alaskan brook lamprey Lethenteron alaskense in two tributaries of the middle Yukon River, Alaska. Larval lampreys could not be identified to species using pigmentation density patterns or trunk myomere counts, but adults could be identified to species based on colouration, body size and oral-disc dentition. Although larvae were patchily distributed in both rivers, there was a greater proportion of sample locations where Lethenteron spp. were absent in upper reaches than in middle and lower reaches. Relative abundance, density and median and maximum total length (LT ) of larvae were highly variable among sampling locations. Current velocity, substratum type and coarse woody debris were most strongly correlated with larval Lethenteron spp. density; velocity and substratum size was negatively correlated, whereas woody debris was positively correlated. Water depth, dissolved oxygen levels and specific conductance did not influence habitat selection. Length-frequency distributions, diet composition (organic detritus ≥ 98%), assimilation efficiency (>70%) and gut fullness (<0·2 mg diet ash-free dry mass) were similar for larvae, regardless of river or sampling location. These results increase understanding of Lethenteron spp. biology and ecology in interior Alaska drainages and add to the existing literature on Lethenteron spp. life history.

Temperature, invaders and patchy habitat interact to limit the distribution of a vulnerable freshwater fish

AbstractInteracting global‐change drivers such as invasive species and climate warming are likely to have major and potentially unexpected influences on aquatic ecosystems. In river networks, modified water temperature combined with patchy physical conditions will likely cause shifts in the amount and distribution of suitable habitat, with influential invasive species further altering habitat availability. We examined how distributions of a thermally sensitive galaxiid fish native to the alpine rivers of New Zealand, Galaxias paucispondylus, were influenced by these drivers using spatially extensive presence–absence electrofishing surveys of 46 sites spread over four subcatchments. A unimodal response to water temperature and an interaction with substratum size meant G. paucispondylus were limited to streams with average summer water temperatures between 10.6 and 13.8 °C and were absent when average substratum sizes were &lt;36 mm, regardless of temperature. In addition, non‐native trout &gt;150 mm long excluded G. paucispondylus, but were only found in streams with average summer water temperatures &lt;10.6 °C. These influences of trout likely strengthened the unimodal temperature response of G. paucispondylus and led to a very small G. paucispondylus realized niche. When predicted temperature increases were applied to catchment models, G. paucispondylus distributions were patchy and variable across subcatchments. Moreover, local physical characteristics of river networks were particularly important because of the non‐linear and interactive influences of temperature and substratum size on the outcome of species interactions. Therefore, substratum sizes, water temperature and a non‐native predator combined to influence the distribution of this thermally sensitive fish, illustrating how the effects of climate warming will likely be strongly context‐dependent and interactive.

Seeking shelter from the storm: responses of benthic stream invertebrates to natural and experimental floods

Flooding is an important physical disturbance in many streams and rivers. Ecological theory suggests that population persistence will be enhanced if organisms can take shelter from high flow during floods. We investigated whether the black fly larva Simulium tribulatum actively moves to sheltered microhabitats during natural and simulated floods. First, we quantified the relationship between larval abundance and availability of sheltered and exposed microhabitats on a single boulder during and after a natural flood in Taylor Run, Pennsylvania. Changes in abundance were significantly associated with microhabitat type. Abundances in all sheltered microhabitats were greater during than after the flood, whereas abundances in all exposed microhabitats were lower during than after the flood. We used a controlled field experiment to verify this relationship by creating simulated floods over artificial substrata containing exposed and sheltered microhabitats. Larval densities were highest in exposed microhabitats before and after the flood, but were highest in sheltered microhabitats during the flood. Larval densities on constant-flow substrata were always highest in exposed microhabitats. We used hot-film anemometry in a laboratory flume to quantify near-bed velocities in different microhabitats on an artificial substratum during low- and high-velocity conditions, and examined corresponding changes in larval density in those microhabitats. During high-velocity conditions, larval densities declined in microhabitats with near-bed velocities >100 cm/s and increased in sheltered regions with velocities <100 cm/s. Thus, larvae can move quickly to sheltered microhabitats as velocities increase, and return rapidly (order of minutes) to more exposed microhabitats as velocities diminish. This behavior should confer fitness benefits and enhance population persistence, particularly in streams with larger substratum sizes that remain immobile during most floods.

Diversity and distribution of stream bryophytes: does pH matter?

AbstractBryophytes can strongly influence biodiversity and ecosystem function in low-order streams. Mosses and liverworts have substantial biodiversity and biomass in streams, yet few investigators have examined which factors influence bryophyte species distributions, and fewer have examined assemblages across a wide pH gradient. We examined bryophyte assemblages across a pH gradient in 26 southeastern New York (USA) streams. We recorded bryophyte species richness, diversity, and abundance, in conjunction with pH, width, depth, canopy cover, substratum size, temperature, dissolved O2, turbidity, conductivity, current velocity, dissolved organic C, dissolved Ca, Mg, Fe, NH4+, NO3−, soluble reactive P (SRP), and abundance of other autotrophs. pH ranged from ∼4 to 7 and corresponded to the type of underlying bedrock. Nearly all streams had low or undetectable concentrations of SRP. Several species, e.g., Hygrohypnum eugyrium and Codriophorus aduncoides, were apparent pH generalists, whereas others were pH sp...

Predicting the constraint effect of environmental characteristics on macroinvertebrate density and diversity using quantile regression mixed model

Various factors, such as habitat availability, competition for space, predation, temperature, nutrient supplies, presence of waterfalls, flow variability and water quality, control the abundance, distribution and productivity of stream-dwelling organisms. Each of these factors can influence the response of the density of organisms to a specific environmental gradient, inflating variability and making difficult to understand the possible causal relationship. In our study, we used quantile regression mixed models and Akaike’s information criterion as an indicator of goodness to examine two different datasets, one belonging to Italy and one belonging to Finland, and to detect the limiting action of selected environmental variables. In the Italian dataset, we studied the relationships among five macroinvertebrate families and three physical habitat characteristics (water velocity, depth and substratum size); in the Finnish dataset the relationships between taxa richness and 16 environmental characteristics (chemical and physical). We found limiting relationships in both datasets and validated all of them on different datasets. These relationships are quantitative and can be used to predict the range of macroinvertebrate densities or taxa richness as a function of environmental characteristics. They can be a tool for management purposes, providing the basis for habitat-based models and for the development of ecological indices.

Spawning tactics of summer chum salmon Oncorhynchus keta in relation to channel complexity and hyporheic exchange

Vertical hydraulic gradient, channel hydraulic, and substratum data were collected at active chum salmon redds to evaluate their influence on redd site selection in the Kwethluk River, a wandering gravel-bedded tributary of Alaska’s Lower Kuskokwim River. Two distinct summer chum salmon spawning tacitcs: primary- and off-channel groups spawning in different hydraulic and thermal environments. Channel hydraulics, ground-surface water exchanges, and bed sediment data were collected throughout the range of topographic conditions at sites occupied and unoccupied by spawners. Analyses of these data showed that redd site selection was incoherent with channel hydraulics and substratum size, when all channel types were considered. Rather, spawning was associated with the exchange of river and groundwater at four spatial scales. In primary channels, chum salmon spawned only in reaches where the regional hydraulic gradient (regional scale) promoted the penetration of river water, or downwelling, into the bed sediments. Within these reaches, primary-channel spawning was influenced by bed topography (fluvial scale) that promoted strong localized downwelling; a logit model, fitted to constituent hydraulics, discriminated between selected and unselected sites with 89 % accuracy. In contrast, spawners in off-channel habitats (flood and spring channels) selected areas of upwelling groundwater, regardless of channel hydraulics and the regional hydraulic gradient. In parafluvial flood channels, spawners selected localized upwelling associated with short groundwater flow paths (parafluvial scale). In spring channels, within the floodplain’s forested riparia (orthofluvial scale), spawners eschewed areas of localized downwelling and uniformly selected upwelling zones feeding these channels. Thus, redd selectivity differentiated two spawning tactics (groups) in this complex channel network where ground and surface water exchange is an important predictor of spawning habitat.

Substratum associations and depth distribution of benthic invertebrates in saline Walker Lake, Nevada, USA

Walker Lake, a terminal salt lake in western Nevada, is undergoing rapid changes because of falling lake level and rising salinity, affecting the potential habitat of benthic invertebrates that supply food to native fish and birds. Benthic invertebrate surveys were conducted within different substratum size classes and macrophyte beds in the nearshore littoral shallows ( 10 m) zones of the lake. Samples were dominated by the chironomid midges Cricotopus ornatus and Tanypus grodhausi; the damselfly Enallagma clausum; and an oligochaete worm of the genus Monopylephorus. Midges showed distinct depth preferences, with Cricotopus found primarily in the shallow littoral, and Tanypus found in the lower littoral and profundal regions. Enallagma occurred throughout the littoral region but was reduced in abundance below 10 m. Cricotopus and Enallagma were most abundant on cobble rock substratum and macrophytes. Sand and small gravel substrata supported few invertebrates except oligochaetes, which were most common in shallow littoral areas. The extent of Ruppia beds was determined using hydroacoustic sounding and showed that these beds were most well-developed in a zone from 1.25 to 5 m depth. The estimated area of productive shallow littoral zone habitat at different lake levels showed that coverage was lowest near the current surface elevation. Rising lake levels would result in expansion of suitable habitat area, and while falling levels could also expand nearshore habitat, this would likely occur on areas of poorer substratum quality and under high salinities that may inhibit growth.

Aquatic bryophytes as ecological indicators of the water quality status in the Tiber River basin (Italy)

A survey of 18 watercourses of the Tiber River basin was carried out to define the ecological niche breadth of some aquatic bryophyte species in relation to environmental factors. Aquatic bryophytes were sampled and water environmental parameters were measured at 99 stations distributed along the catchment (from the headwater regions to the downstream reaches). The datasets of the collected species and environmental data were analyzed by using a multivariate statistical analysis (PCA biplot). Ecological responses of the recorded aquatic bryophytes were obtained using a fuzzy set approach, and were compared with data from literature. The results show that the presence of the aquatic bryophytes in watercourses is affected negatively by the reduction of water velocity, clearness, substratum size and the worsening quality of the water physico-chemical status. In fact, aquatic bryophytes show a general preference for stations characterized by medium-large granulometry, and fast-flowing, clear, oxygenated (mean value 9.2 mg/l), cool waters (mean value 15.0 °C), with low loads of nutrients, particularly ammonia (mean value 0.10 mg/l) and phosphates (mean value 0.09 mg/l). However, ecological responses reveal different patterns in the distribution of aquatic bryophyte species mainly in relation to water physico-chemical parameters (e.g. temperature, conductivity, ammonia, phosphates). E.g. Palustriella commutata var. commutata, Cratoneuron filicinum, Fissidens viridulus and Cinclidotus aquaticus show high preference for clear, turbulent and fast-flowing waters, with temperature below 12 °C, conductivity below 300 μS/cm, and concentrations about 0.01 mg/l for phosphates, not exceeding 0.10 mg/l for ammonium ions and 0.90 mg/l for nitrates. Leptodictyum riparium and Riccia fluitans are for their part more linked to turbid and slow waters affected by eutrophication, showing optimum values for about 0.30 mg/l for ammonia concentration, 0.90 mg/l for nitrates and 0.11 and 0.22 mg/l for phosphates respectively. Conversely, Fontinalis antipyretica is not closely related to specific conditions, showing wide ecological ranges for most of the analyzed environmental factors. This paper has evaluated and discussed the possible use of sampled species as bioindicators for biomonitoring of the water quality.

Diel variation in stream fish habitat suitability criteria: implications for instream flow assessment

Abstract Habitat suitability criteria that fail to incorporate temporal variability in habitat preferences of stream fish may mis‐represent critical habitat requirements and lead to setting of inappropriate flow targets when used in instream flow assessments. Developing suitability criteria from daytime observations alone relies on the assumption that habitat preferences are constant over the diel cycle. Few studies have tested these assumptions, particularly for small‐bodied, cryptic, benthic species. During summer in two gravel‐bed rivers, bluegill bullies (Gobiomorphus hubbsi), torrentfish (Cheimarrichthys fosteri) and upland bullies (Gobiomorphus breviceps) exhibited strong preferences with respect to water depth, velocity and substratum size. All three species underwent a diel shift in microhabitat preference for at least two of these variables. Microhabitat preferences were generally weaker when fish were active at night; bluegill bullies, upland bullies and especially torrentfish were observed over a broader range of depths, velocities and substratum sizes at night than during the day. Observations of fish in a stream simulator confirmed that bluegill bullies and torrentfish showed a preference for runs at dusk and return to riffles before dawn, but habitat preferences of upland bullies remained static across the diel cycle. Diel microhabitat shifts affected the assessment of flow requirements. Instream habitat analysis of the Waipara River using separate day and night suitability criteria predicted differing amounts of habitat available at a given flow, and the relationships between fish abundance, fish density and flow. The presence of diel microhabitat shifts in stream fishes suggests that instream habitat analyses will produce more effective and defensible flow recommendations when patterns of nocturnal microhabitat preferences are known and critical habitat bottlenecks can be identified. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Bryozoan assemblages on hard substrata: species abundance distribution and competition for space

Bryozoans are colonial invertebrates that often dominate epibenthic assemblages on the lower surfaces of hard substrata. Competition among neighbouring organisms is usually a critical process regulating biodiversity, and hard substrata have proved to be a suitable model habitat to analyse spatial interactions. We explored the relationships among abundance, species richness, diversity, competitive ability, coverage, available surface, depth and substratum size in an assemblage of bryozoans encrusting pebbles and cobbles in a bank off the eastern mouth of the Strait of Magellan. We also tested whether overgrowth competition can be regarded as hierarchical, and whether the species abundance distribution shows a mode of rare species and a decreasing frequency of increasingly abundant species. Abundance, species richness, diversity and overgrowth competition were highest on the largest substrata. Smaller pebbles tended to be encrusted by the commonest bryozoans, while the rarest species were mainly found on relatively larger clasts. A high proportion of the lower surfaces of most substrata was available for growth. Diversity values of relatively shallow stations were lower than expected under Caswell’s neutral model. Interspecific competition was hierarchical, but the abundance of colonies was not related to the competitive ability of each species. The species abundance distribution was bimodal, with a main mode of rare species and a second one partly composed of relatively abundant bryozoans with poor competitive abilities. This study shows that even in an encrusting assemblage where competition is hierarchical, the weakest competitors persist and the dominant species are far from being capable of monopolizing space.

Ecology and biogeography of Himalayan diatoms: distribution along gradients of altitude, stream habitat and water chemistry

Himalayan streams, an important natural resource, are subject to numerous threats such as pollution and hydrological changes with deteriorating water quality in some areas. In order to develop diatom-based monitoring tools, we investigated diatom species composition at 206 sites in relation to variation in hydrochemistry, habitat characteristics (land use, shading, substrate), altitude and geographical position across a ca. 900 km east-west distance and a 4300 m altitudinal range in Nepal and north-west India. A total of 261 species were found, of which naviculoid (55), monoraphid (47), Gomphonema and fragilarioid (30), cymbelloid (26) taxa and Nitzschia (22) were most abundant. Many species were cosmopolitan, but some had restricted distributions. Of these, most were confined to the eastern Himalaya. More than one third of the species could not be identified. Richness and diversity increased in streams with higher Si concentrations and smaller substratum size, but were not correlated with altitude as a single factor. Canonical correspondence analysis and non-metric multidimensional scaling ordination analyses showed that different regions had distinct diatom assemblages. Important environmental gradients included basestatus, reflecting differences in geology, concentrations of Si (0.5-11.5 mg/L) and Na (0.2-9.0 mg/L), reflecting impacts from agriculture, and habitat, such as substratum size, land use and stream size. The importance of these factors varied in different climatic zones. Across the east-west range and altitudinal gradient hydrochemistry and habitat characteristics explained more of the variation in species composition than altitude and geographical position, but these factors were inter-correlated and their effects difficult to disentangle. Some species had restricted distributions along the altitudinal gradient, and particularly in the eastern Himalaya a change in species composition occurred most clearly at 2000 m a.s.l. Consequently, areas above and below this altitude should be considered separately when developing diatom-based monitoring methods.

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

Summary1. 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.

Winter behaviour of juvenile Atlantic salmon Salmo salar L. in experimental stream channels: effect of substratum size and full ice cover on spatial distribution and activity pattern

Activity and choice of areas offering different cover (substratum or surface ice) for juvenile Atlantic salmon Salmo salar were studied in experimental stream channels during winter. Channels were completely ice covered between December and March. During this period, the ice thickness increased from 50 to 300 mm after which 50% of the ice was experimentally removed and followed by c. 2·5‐fold increase in discharge to simulate the effects of spring flood. Large substrata provided preferred habitats but areas with small substratum sizes were also used when full surface ice provided above‐stream cover and the stream discharge was relatively low. The fish remained nocturnal throughout the study but the level of day activity significantly increased as the surface ice became thicker. Maximum movement distance during a 24 h period and homing‐at‐dawn behaviour remained at a constant level throughout the main winter, but significantly changed during the simulated spring flood (mean ±s.e. maximum extent of movements within 24 h increased from 1·1 ± 0·1 to 3·0 ± 0·5 m; homing behaviour decreased from the highest level of 89·3 to 34·6% during spring flood). Overwinter survival was high (92·9%). Relative mass increase during the study ranged from –8·3 to 28·5%, and 84% of the juvenile Atlantic salmon gained mass. The highest rates of mass increase were associated with frequent movements between areas of different substratum size. The results indicate that during winter: (1) Atlantic salmon parr preferred large substratum cover compared with surface ice cover at the fish densities studied here, (2) juvenile Atlantic salmon were predominantly nocturnal but diurnal activity increased as surface ice became thicker and (3) increase in water discharge during spring altered the behaviour of juvenile Atlantic salmon and may have caused additional habitat shifts.

Hyporheic rehabilitation in rivers: restoring vertical connectivity

Summary1. The hyporheic zone below the channel and banks of many rivers where surface water and ground water exchanges plays a crucial functional role in the biogeochemical transformation of water, mediated by active microbial biofilms. This zone also harbours assemblages of invertebrates that graze biofilms, contribute to secondary production, and can alter the porosity of the hyporheic zone through their movement or burrowing activities.2. Many human activities cause interstitial sedimentation or disrupt surface–groundwater hydrological linkages, impacting upon ecological processes in the hyporheic zone. However, strategies for river rehabilitation seldom explicitly consider the hyporheic zone or seek to restore lost vertical linkages with groundwater. Instead, restoration goals target surface, riparian or floodplain features even though current river ecosystem theory emphasises the three dimensions of hydrological connectivity. To guide effective, holistic river restoration, scientists and managers therefore need information on the mechanisms by which energy and material are transferred in the hyporheic zone and which ecosystem services are thus provided.3. Other gaps in our understanding of hyporheic zone rehabilitation include recruitment processes of the hyporheos and the relative importance of groups of hyporheic invertebrates in rivers differing in substratum size, disturbance frequency and groundwater linkages. Carefully designed experiments that assess responses to hyporheic rehabilitation strategies will provide valuable data at varying scales (e.g. distribution of hyporheic habitat types at the reach scale) for management as well as providing insights into the mechanisms controlling hyporheic invertebrate assemblages and ecological processes. Fully successful river rehabilitation must include restoration of vertical linkages between the river and its shallow groundwater aquifers.

Spatial and temporal segregation of spawning habitat by catostomids in the Savannah River, Georgia and South Carolina, U.S.A.

Spawning aggregations of five species of catostomids were observed on the two mid‐channel gravel bars of the Savannah River, Georgia and South Carolina, in 2004 and 2005 to assess the degree of spatial and temporal overlap in the use of this habitat and determine the habitat preferences leading to segregation. Spawning catostomids showed a considerable amount of temporal overlap in their use of these mid‐channel gravel bars. The observed temporal overlap was consistent between 2004 and 2005 and corresponded to temperatures at which species were present. The distribution of catostomids was not uniform at the upstream gravel bar. Carpsuckers Carpiodes sp., spotted sucker Minytrema melanops and robust redhorse Moxostoma robustum both demonstrated some spatial overlap with notchlip redhorse Moxostoma collapsum; however, their overall distributions were different from one another. Northern hogsucker Hypentelium nigricans was present across the gravel bars, apparently as an egg predator. Spawning catostomids segregated based on flow, depth, slope and substratum size. Whether due to limited habitat availability or changes in the timing of reproduction due to altered cues, temporal and spatial overlap occurs between spawning catostomids despite the apparent partitioning of available spawning habitat. It is unclear, however, if this overlap results in excessive mortality in the early life‐history stages of these species. Results suggest spatial overlap among catostomid species was minimized due to species spawning in areas within a narrow range of conditions. Intraspecific interactions such as nest site superimposition or disturbance may be a concern.

Refuge‐use strategies of stream fishes in response to extreme low flows

Low flows were simulated in an artificial stream to determine the refuge‐use strategies of two benthic New Zealand fish species, Canterbury galaxias Galaxias vulgaris and upland bullies Gobiomorphus breviceps, and to investigate how refuge availability and the rate of flow recession affect the type and incidence of refuge use. When riffles dewatered, upland bullies tended to move to runs, whereas Canterbury galaxias showed a stronger propensity to burrow into the substratum. Both species showed a strong and consistent tendency to move upstream when emigrating from riffles. Burrowing was more frequent on coarse substrata and during rapid flow recessions. The incidence of surface stranding increased with the rate of flow recession, but only when interstitial refugia were unavailable, and was higher on gravel than on cobble substrata. The effect of rate of flow recession and substratum size on the probability of stranding depended upon the type of refuge‐use strategy adopted by the fishes. Rate of flow recession affected upland bullies more than Canterbury galaxias, whereas substratum size affected Canterbury galaxias more than upland bullies. These results suggest that the impact of disturbance is contingent upon species‐specific refuge‐use strategies, which result from interplay between refuge availability, the nature of the disturbance and species’ behaviours and morphologies. When component species adopt contrasting refuge‐use strategies, disturbance events may not consistently favour one species over another but rather inflict species‐specific mortality that varies both temporally and spatially.

The Relationships Among Disturbance, Substratum Size and Periphyton Community Structure

The Relationships Among Disturbance, Substratum Size and Periphyton Community Structure

Habitat selection by juvenile Atlantic salmon: the interaction between physical habitat and abundance

The effect of physical river habitat variables on the distribution of juvenile Atlantic salmon Salmo salar L. in the Rivière de la Trinité, Québec, Canada, was examined using generalized additive modelling. A survey of Atlantic salmon fry and parr densities and habitat variables (flow velocity, water column depth and substratum size) was conducted in the summer months from 1984 to 1992. Clear patterns of habitat use existed: specific ranges of habitat variables were selected, with parr preferring greater velocities, depths and substratum sizes than fry. There was a large variation, however, in juvenile densities for given velocities, depths or substratum sizes, with this variation being greatest in optimal habitats. On examination of an individual year, interaction between the variables was found to explain some of the variation. On a year‐to‐year basis the juvenile Atlantic salmon population was found to exhibit an ‘Ideal Free Distribution’, which resulted in greatest variation in optimal habitats with year‐to‐year changes in population abundance.