Invertebrate accessibility and vulnerability in the analysis of brown trout (Salmo trutta L.) summer habitat suitability

Water discharge regulation can affect food availability, accessibility and vulnerability and thus, the trophic habitat suitability for lotic salmonids. To analyse brown trout habitat suitability, we therefore combined the relative importance of the food availability (overall abundance of benthic and drifting invertebrates), with the potential food vulnerability (accessibility, conspicuousness and ease of handling), the latter depending on both physical habitat characteristics (flow patterns and structural complexity of habitat) and invertebrate traits (size and other biological traits). We considered the trophic patterns of trout at two spatial scales: the reach scale (unregulated reach versus regulated reach) and the macrohabitat scale (e.g. riffles and pools). Discharge regulation reduced trout abundance, biomass, and temperature‐independent growth rates. In the regulated reach, trout had a lower total prey consumption, a higher consumption of terrestrial invertebrates and a higher diet diversity than in the unregulated reach, indicating that trout were food‐limited. However, the potential availability of food supplies per individual trout was similar for the two reaches. Thus, trout prey consumption in the regulated reach should have been predominantly affected by the decrease in both the availability of large invertebrates in the drift and their vulnerability in the total food supplies. There were no macrohabitat‐specific differences in the total prey consumption and in the potential food availability within each reach. However, brown trout diets differed between the macrohabitats of each reach, in relation to differences in potential invertebrate vulnerability. Therefore, the potential vulnerability of invertebrates to predation was more relevant in the ecological evaluation of salmonid habitat suitability than the total food availability. The analysis at the macrohabitat scale provided a better understanding of the switches in brown trout diet and enabled a finer and more realistic analysis of trout feeding patterns. Copyright © 2002 John Wiley & Sons, Ltd.     

Mesohabitat use by brown trout (Salmo trutta) in a small groundwater‐dominated stream

Groundwater‐dominated streams have particular flow regimes that commonly support populations of trout. Meso‐ and micro‐habitat surveys were carried out on a reach of the river Tern that drains a Triassic sandstone aquifer in the English West Midlands, to investigate brown trout (Salmo trutta) habitat use with varying flows. Mesohabitats were mapped over a range of summer and autumn flows and coupled with direct underwater observation (snorkelling) of fish locations together with point measurements of velocity and depth. The number of habitat types recorded was low and dominated by glides, runs, and backwaters. Brown trout showed a strong association with glides and runs with adults being more associated with runs and parr with glides. General habitat use curves showed brown trout to favour depths between 0.30 and 0.40 m and velocities below 0.40 m s^?1. A clear preference was shown for sand and gravel bed materials. However, the differentiation of hydraulic habitats was weak and there was no trend in mesohabitats or change in trout use of mesohabitats with discharge. The study raises limitations of the mesohabitat survey approach when linking fish ecology, flow and physical habitat in small streams with low flow variability and low habitat diversity. In these situations, other factors (especially cover features) appear to strongly influence brown trout distribution. Copyright © 2010 John Wiley & Sons, Ltd.     

Probability-of-use curves applied to brown trout (Salmo trutta fario L.) in rivers of southern France

The aim of this paper is to illuminate any environmental preference of brown trout (Salmo trutta, fario) in streams of the South of France and, in this case, to evaluate the reliability of the Instream Flow Incremental Method, suggested by Bovee (1974) for American streams. At various randomly distributed points on five tributaries of the Tarn and Ariège rivers the occurrence of fishes at different stages of development and selected morphodynamic characteristics (velocity, depth, substratum) were observed simultaneously. The presence of fish was checked by using two special electric fishing techniques: (1) casting the anode ahead of the operator for a sudden immobilization of fish; and (2) connecting previously immersed electrodes on the riverbed. Curves have been plotted for velocity and depth parameters in each river at each season by analysing the frequency of appearance of fish at different stages of development in the different habitats. These curves do not vary greatly from one river to the next and the differences between seasons are not very significant. Single curves have thus been plotted by grouping the data from the different series of measurements. The curves obtained for the velocity parameter are very close to those proposed by Bovee and reveal that the fishes have a preference for low velocities, regardless of their stage of development. As shown by the Bovee curves, the preference obtained for the depth parameter is related to the stage of development: shallow waters are much preferred by fry and juveniles but are not greatly frequented by adults. This study seems to confirm the existence of a real environmental preference among brown trout, indicating that the Instream Flow Incremental Method can be used for French 1st category streams.     

Dispersal of brown trout (Salmo trutta L.) fry in a low gradient stream - implications for egg stocking practices

Stocking of eggs is a common strategy to support declining or reintroduce extirpated salmonid populations. Data on how juveniles disperse from stocking points is crucial to be able to design efficient stocking programs. Detailed information of dispersal is limited for many salmonids, for example, brown trout. In this study, dispersal distance was measured at the end of the first growing season in a low gradient (0.7%) stream in Sweden where the trout population had been depleted. Eggs from 17 separate sets of parents were stocked as eyed eggs in March. During the following fall fry were sampled throughout the stream. The majority of the fry dispersed downstream and remained within a distance of 200 m from the stocking point with no difference between sizes of fry and the presence of a competing cohort or not. There was no dissimilarity in dispersal distances across offspring originating from different parents indicating absence of genetic influence. Our results suggest that, in streams similar to our study site, stocking points should be separated by approximately 330 m in order to avoid overlap in habitat use of fry from different stocking points and that the presence of competing cohorts, fry size and within population variability in dispersal can be neglected.     

Migratory patterns and return to the catch site of adult brown trout (Salmo trutta L.) in a regulated river

The spawning migration and local homing of adult brown trout was analysed using radio telemetry in a regulated river in central Norway. Twenty‐eight large (37–64 cm) brown trout (Salmo trutta L.) were tracked before, during and after spawning in the River Nea, a watercourse with several obstructions, including an outlet tunnel from a power station and a regulated stretch (26 km) with 45 weirs. Two major patterns of spawning migration were found: (1) about half (n = 16; 57%) of the trout moved very little and remained in the deeper pools of the river from June until November; (2) about half (n = 12; 43%) of the trout migrated relatively long distances (12.5–28 km) up the river prior to the spawning period where they stayed in the outlets of small tributaries, or in rapids on the main river during the spawning period. We assume that these trout belong to a population of lake‐run migratory trout using the River Nea for spawning. There was no significant difference in body length of migratory and stationary brown trout and no significant difference in total distance moved by migratory males (30.5 km, n = 6) and females (20.5 km, n = 6, p > 0.05). Among migratory trout, we found no correlation between body length and migrated distance. Of the 12 migratory trout, nine undertook fast upward migration in periods of high water flow (> 100 m³/s). They passed the outlet tunnel from the power station and negotiated two to 35 weirs before reaching their main reproduction areas. Three trout crossed several weirs when the discharge was low (10–40 m³/s). When there was low discharge, fish remained at the outlet tunnel for up to four weeks and showed a high level of activity. Postspawning downstream migration started between 25 September and 25 October. Most migratory trout (n = 9) wintered in pools on the lower part of the river or in weir basins; only two trout descended to the lake (Selbusjøen) in late autumn. Copyright © 2004 John Wiley & Sons, Ltd.     

An analysis of brown trout (Salmo trutta fario L.) habitat: The role of qualitative data from expert advice in formulating probability-of-use curves

To study the preferred habitat of brown trout, a group of fifteen experts met in the Tarn départment to answer a questionnaire relating to 200 sampled points on the Agout river. The experts were requested to give their opinion on the suitability of various habitats, defined by velocity, depth, substrate, presence of covers, etc. for trout—at each stage of growth, fry, juvenile and adult fish—and the factors which had influenced their choice. The analysis of these questionnaires has allowed probability-of-use curves to be plotted for brown trout. The agreement between the curves derived from this study and those proposed by Bovee is encouraging and gives credit to the validity of the Instream Flow Incremental Methodology. Direct analysis of the data collected has led to conclusions in agreement with the probability-of-use formulation and has identified the best conditions of habitat at different stages of growth of trout for a certain number of parameters. The paper proposes a factor analysis method to secure a direct estimate of the quality of a habitat defined by a group of independent parameters.     

Migratory behaviour of adult fast-growing brown trout (Salmo trutta,L.) In relation to water flow in a regulated Norwegian river

The migration pattern of spawners of brown trout (Salmo trutta) in relation to water flow was analysed by radio-tracking in the regulated River Gudbrandsdalslågen, Norway. During the upstream spawning migration in the high flow period, trout (2–12 kg) were caught, tagged and released 15 km downstream of a fish ladder at the Hunderfossen waterfall. The released fish displayed a systematic and directional upstream movement to the outlet of the tunnel from the hydroelectricity plant. In autumn and winter there is a minimum water discharge of 20–2 m³/s on p.s. in the river between the dam and the outlet of the tunnel, which has a discharge of 200–300 m³/s. When the water flowing over the Hunderfossen dam decreased to 20 m³/s, the ascent of brown trout up the river stopped and fish periodically entered the power plant tunnel. To determine the flow necessary to attract fish into the spawning reach above the tunnel outlet, two experiments were undertaken using 12 and 17 radio-tagged trout. In the first experiment, 60 m³/s of water released for 24 h resulted in the migration of 50% of the trout up the river. The second experiment, releasing 60 m³/s for 24 h, followed by 30 m³/s for 24 h two days later, resulted in the migration of 60% of the trout. Only one fish ascended the river at a flow of 30 m³/s. It is recommended that a repeated release of water at 60 m³/s is made in periods of minimum water discharge to save the spawning migration. The results demonstrate the advantage of using radio-tracking in experiments dealing with fish migration in relation to water-flow management.     

Causes of mortality of Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) smolts in a restored river and its estuary

During October 2000 to August 2002, the River Skjern Nature Project was implemented by removing dykes and re‐meandering 20.5 km of the lower canalized river. As a consequence the length of the river stretch increased to 23 km. A lake of 250 ha developed in the river valley 5 km upstream from the river mouth because of subsiding soils caused by reclamation and drainage since the 1960s. Using radiotelemetry, the mortality of wild Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) smolts in the River Skjern and its estuary was investigated prior to and after the implementation of the project. Altogether, 77 Atlantic salmon and 66 brown trout smolts were caught, tagged and released in the river upstream of the restoration project during the spring of 2000 and 2002. The in‐river smolt mortality was more than double in 2002 for both Atlantic salmon and brown trout compared with 2000. This was primarily due to bird predation in 2002 which was not observed in 2000. The in‐river bird predation in 2002 was mediated by the new lake, which quickly became an important bird rest area. Estuarine mortality mainly caused by cormorants (Phalacrocorax carbo sinensis) differed significantly between species, but was high for both Atlantic salmon (39%) and brown trout (12%) in both years of investigation. The aggregated smolt mortality in the river and in the estuary (48%) may threaten an indigenous self‐sustaining Atlantic salmon population in the River Skjern. When planning river restoration projects, caution should be used, especially where permanently flooded floodplains (lakes) develop due to subsiding soil. In situations where rivers pass directly through newly developed lakes, migratory species such as Atlantic salmon and brown trout may be severely affected due to increased exposure to predation from predatory fish and birds. Copyright © 2006 John Wiley & Sons, Ltd.     

Effects of transferring glacier-fed water to a clear-water mountain river on the production and food organisms of brown trout (Salmo trutta L.) in Southern Norway

The effects of transferring glacier-fed water to a clear-water river on production and food organisms of brown trout (Salmo trutta L.) were investigated in a mountain river, in southern Norway. Production in unregulated fluvial habitat was 271.5 g 100 m^?2 yr^?1 compared with 103.1 g 100 m^?2 yr^?1 in a glacier-fed reach of the river. This difference was due to low fish density and recruitment rate. The growth rate, at least for younger fish (2+ to 4+ age groups), was significantly higher in the regulated reach of the river. The main difference in food composition was the low abundance of crustaceans Eurycercus lamellatus and Gammarus lacustris in the regulated reach. Trichopterans were the main diet component in both sites.     

Influence of habitat restoration on post‐emergence displacement of brown trout (Salmo trutta L.): A case study in a Northern Swedish stream

During the 19th and 20th centuries, Swedish streams were channelized to facilitate the commercial transport of timber on water. Stream‐dwelling fish were affected by the consequent reduction in habitat quality. To mitigate the effects on salmonids, which sustain important recreational fisheries, many streams have been restored through the replacement of boulders into the channel since the early 1980s. However, the effects of restoration on salmonids remain poorly known. This study assesses the effect of habitat restoration on the post‐emergence displacement of brown trout (Salmo trutta L.) fry. Hatchery‐raised fry were released in a third‐order stream canalized for timber floating in northern Sweden, and the effect of restoration on displacement was assessed. The short‐term (24 h) displacement of trout fry was reduced from 10.1 to 2.3% of fry released following restoration. Water velocity accounted for 89.4% of the variation in fry displacement across years and sites. Post‐summer recruitment (the proportion of introduced juvenile trout remaining in the study reaches 60 days after the emergence) increased approximately three fold after restoration. These findings suggest that habitat restoration benefit trout populations through substantial reductions of fry displacement and possible contributions to juvenile recruitment. Copyright © 2009 John Wiley & Sons, Ltd.     

Trophic interactions among algal blooms,macroinvertebrates, and brown trout: Implications for trout recovery in a restored river

Positive correlation between trout abundance and dissolved metal concentrations along the Upper Clark Fork River (UCFR; Montana, USA) have forced restoration practitioners to seek underlying causes of reduced fish density beyond heavy metal contamination. Throughout the river, nutrient enrichment and summer algal blooms may be hindering full recovery of trout populations. In this study, we evaluated the community structure and metal body burdens of benthic invertebrates and characterized existing trophic linkages between brown trout and dominant invertebrate taxa before and during summer algal blooms in a downstream reach of the UCFR where fish densities are low (20–30 trout/km), and where metal contamination is relevant but minimal compared with upstream. In spring, estimated invertebrate abundance was 1,727 ± 217 individuals/m² and dominated by Ephemerellidae and Baetidae families. During summer algal bloom, invertebrate abundance increased 15‐fold (20,580 ± 3,510 individuals/m²) mostly due to greater abundance of Chironomidae, Hydropsychidae, and Simulidae. Copper body burdens (130 ± 42 ppm) were higher than any other heavy metal regardless of season, but detectable concentrations of arsenic, cadmium, and lead were also found. A Bayesian mixing model combining metal burdens and stable isotopes showed that in the spring, trout of average size (355 ± 65 g) relied mostly on epibenthic taxa (Ephemerellidae and Hydropsychidae), contrasting with small (<100 g) and large (>400 g) trout relying heavily on Baetidae, a major component of invertebrate drift. Foraging segregation related to trout size did not occur during summer algal blooms, which may reflect increasing influence of benthic algal proliferation or indicate the indiscriminate use of pool habitats as thermal refugia over summer conditions by trout of different ages.     

Effect of flow‐related substrate alteration on physical habitat: a case study of the endemic river loach Sinogastromyzon puliensis (Cypriniformes,Homalopteridae) downstream of Chi‐Chi Diversion Weir,Chou‐Shui Creek,Taiwan

This paper addresses the variation of substrate status with flow in the framework of habitat simulation. The Physical Habitat Simulation System (PHABSIM) was developed as a tool for water management and has become one of the most popular methods worldwide. Despite its many advantages, the variation of channel properties with flow is not addressed in PHABSIM. We modified PHABSIM by incorporating a sub‐program that can perform substrate analyses under five different schemes for evaluating the substrate suitability. These schemes include the conventional methods to specify the substrate attribute (scheme 1) and to use the mean grain size of the original bed material for determining the substrate suitability (scheme 2), or employ the threshold conditions for sediment entrainment to evaluate the nominal grain size of the stable substrate (schemes 3–5). As a case study, Sinogastromyzon puliensis in Chou‐Shui Creek (Taiwan) is selected as the target species for the habitat simulation. For schemes 1 and 2, the substrate attribute specified for each cell of a cross‐section does not change with flow. The nominal grain size of the stable armour material (scheme 5) is generally greater than the nominal grain sizes of the above‐critical bed material (scheme 3) and the static armour layer (scheme 4). The simulation results indicate that the suitability values specified in scheme 1 and the resulting habitat area are on the optimistic side, whereas scheme 2 tends to undervalue the suitability of the hydraulically worked substrate. The overall variation trends of the weighted usable area (or percentage usable area) curves for different substrate schemes are similar, especially in the in‐phase regions of the water velocity and depth. The results of this study appear to imply that the flow‐related habitat analyses are not usually sensitive to substrate. Copyright © 2002 John Wiley & Sons, Ltd.     

Effects of a small hydropower station upon brown trout Salmo trutta L. in the River Hoz Seca (Tagus basin,Spain) one year after regulation

A small hydroelectric power station was built in 1993 on the River Hoz Seca (Tagus basin, central Spain). Pre‐ and post‐regulation studies provided the opportunity to test the early effects of this disturbance on the brown trout Salmo trutta L. population. Before and after comparisons of population density and biomass, age composition, growth and production were made upstream and downstream of the diversion dam. The effects of disturbance on benthic macroinvertebrates were also analysed but no changes in abundance were detected. The downstream estimated population densities and biomass of trout showed a decrease of about 50 and 43%, respectively, following regulation. Examination of length‐for‐age tables revealed no obvious change in growth but a significant difference in age structure. The main consequence of the imposed fluctuating flow regime was a serious reduction in trout production caused by a loss of suitable habitat and a loss of juveniles. Copyright © 1999 John Wiley & Sons, Ltd.     

Exploitation rate,survival and movements of brown trout (Salmo trutta L.) stocked at takeable size in the regulated rivers Lågen and Otta,southern Norway

Takeable size (age 4⁺, length 27–67cm), hatchery reared brown trout (Salmo trutta L.) were released into the rivers Lågen and Otta, the Glomma River System, southern Norway, in May-July 1981 and 1983–87. The brown trout released in May gave significantly lower recapture rates compared to those released in June/July, i.e. 22.8 per cent compared to 41.6–58.0 per cent, respectively. Mean exploitation rate and survival rates ranged from 0.23 to 0.58 and 0.2 to 0.4, respectively, Within 15 days after release 50 per cent were recaptured, and 90 per cent were caught within 67 days. The frequency of recapture increased significantly with fish length in the length interval 30–45 cm, but decreased for bigger fish. The stocked brown trout were stationary and 95 per cent of the recaptures were caught less than 1 km from the site of release. Frequency of recapture ranged from 19.2 to 54.7 per cent in 6 experiments with takeable sized brown trout in different tributaries of the Glomma River System, and the variations are probably mainly due to differences in fishing pressure.     

Effects of stream regulation on atlantic salmon,Salmo salar L., and brown trout,Salmo trutta l., in the upper severn catchment,U.K.

Data for three rivers in the upper Severn catchment are examined to assess the influence of regulation strategies on the Atlantic salmon and brown trout populations. In the regulated Afon Clywedog, juvenile recruitment of both species declined steadily following increased utilization of the impoundment for regulation. The decline was considered the response to the variable and rapidly changing discharge regime adopted in the river. Spawning success and juvenile survival of salmonids in the Vyrnwy was relatively stable suggesting the flow regime in this river was less devastating.