Trout Density Research Articles

Fish Distributions and Habitat Associations in Manistee River, Michigan, Tributaries: Implications for Arctic Grayling Restoration

AbstractRestoration and enhancement of North American native freshwater fishes have for several decades been the subject of growing interest among fisheries biologists, natural resource managers, non‐governmental organizations, and the sportfishing public. The Little River Band of Ottawa Indians (LRBOI) and the Michigan Department of Natural Resources (MDNR), along with universities and public interest groups, are re‐examining the potential for re‐introduction of the Arctic Grayling Thymallus arcticus, a species that has been extirpated in Michigan since the 1930s. The Manistee River, Michigan, flows through the LRBOI's reservation and once supported the last known native Arctic Grayling population in the state's Lower Peninsula. The objectives of this study were to (1) identify potential biotic limitations, such as competition and/or predation from other fish species, that may interfere with Arctic Grayling re‐introduction in the Manistee River watershed; and (2) describe how instream habitat features currently relate to populations of potentially interacting species. Field surveys conducted during June–August 2012 in eight Manistee River tributaries identified suitable abiotic habitat for Arctic Grayling in 20 of 22 sampling reaches. However, high densities of Brown Trout Salmo trutta (a nonnative salmonid) may have influenced some of the habitat associations observed for Brook Trout Salvelinus fontinalis and Slimy Sculpin Cottus cognatus, two species that currently and historically co‐occurred in Arctic Grayling habitats. These two species were the most abundant in river reaches with Brown Trout densities less than 0.10 fish/m2. Based on habitat conditions and Brown Trout densities, there appear to be four distinct tributary regions for which management strategies could be developed to enhance the success of Arctic Grayling re‐introduction efforts. Re‐introduction of Arctic Grayling in the Manistee River watershed would support LRBOI and MDNR goals for native species restoration and would provide a unique and historic angling opportunity that has been absent in Michigan for nearly 100 years.

Modelling the factors influencing ontogenetic dietary shifts in stream-dwelling brown trout (Salmo trutta)

Biotic and abiotic variables shape ontogenetic trajectories of animals. This study modelled (i) the body length related timing of the ontogenetic switch from aquatic to surface prey and (ii) the impacts of habitat characteristics, prey availability, and fish densities on the relative contribution of surface prey to the overall diet of native brown trout (Salmo trutta). We used individual-based models of dietary data for 170 fish (length range 48–343 mm). There was a high degree of individual variation in the use of surface prey, but logistic regression suggested that the shift from aquatic to surface prey was established at a body length of 81 mm (range 36–127 mm). Results of linear mixed-effects models highlighted the importance of fish length, benthic invertebrates, brown trout density, and water current velocity to the switch to surface prey by riverine brown trout, with fish length being the most influential variable. Our study provides evidence of the importance of ontogeny (intrinsic features of individuals linked to fish length) and individual differences in feeding behaviour to understand water-column use for feeding by stream-dwelling salmonids.

Relationships between indicators of acid-base chemistry and fish assemblages in streams of the Great Smoky Mountains National Park

The acidity of many streams in the Great Smoky Mountains National Park (GRSM) has increased significantly since pre-industrial (∼1850) times due to the effects of highly acidic atmospheric deposition in poorly buffered watersheds. Extensive stream-monitoring programs since 1993 have shown that fish and macroinvertebrate assemblages have been adversely affected in many streams across the GRSM. Matching chemistry and fishery information collected from 389 surveys performed at 52 stream sites over a 22-year period were assessed using logistic regression analysis to help inform the U.S. Environmental Protection Agency’s assessment of the environmental impacts of emissions of oxides of nitrogen (NOx) and sulfur (SOx). Numerous logistic equations and associated curves were derived that defined the relations between acid neutralizing capacity (ANC) or pH and different levels of community richness, density, and biomass; and density and biomass of brook trout, rainbow trout, and small prey (minnow) populations in streams of the GRSM. The equations and curves describe the status of fish assemblages in the GRSM under contemporary emission levels and deposition loads of nitrogen (N) and sulfur (S) and provide a means to estimate how newly proposed (and various alternative) target deposition loads, which strongly influence stream ANC, might affect key ecological indicators. Several examples using ANC, community richness, and brook trout density are presented to illustrate the steps needed to predict how future changes in stream chemistry (resulting from different target deposition loads of N and S) will affect the probabilities of observing specific levels of selected biological indicators in GRSM streams. The implications of this study to the regulation of NOx and SOx emissions, water quality, and fisheries management in streams of the GRSM are discussed, but also qualified by the fact that specific examples provided need to be further explored before recommendations concerning their use as ecological indicators could be proposed.

Aquaponics and circular economy in Madeira Island: A Case Study of the Islandap Project

Aquaponics and circular economy in Madeira Island: A Case Study of the Islandap Project

Growth performance, biochemical blood parameters and meat quality of rainbow trout ( Oncorhynchus Mykiss W. ) fed with licorice ( Glycyrrhiza Glabra L. ) supplemented diet

Licorice (Glycyrrhiza glabra) is medicinal plant characterized with sweet flavor which has been used in Bulgarian traditional medicine to improve the condition of the digestive tract and could positively affect digestibility and assimilation of feed nutrients in human and animal organisms. The studies connected with the effect of licorice in fish are limited. The aim of current study was to find the effect of feed supplemented with licorice Glycyrrhiza glabra on growth performance, blood parameters and meet quality in rainbow trout (Oncorhynchus mykiss W.), raised in the recirculation system. The fish were fed with two feeds: control feed (CF) - without the addition of supplement and experimental feed (EF), with supplementation of 300 mg.kg-1 licorice in pellets. The stocking density of rainbow trout in recirculation system was 50 pcs.m-3. The initial average weight of fish in CF variant was 13.30 ±3.07 g and in EF variant was 13.40±3.55 g without differences being statistically significant (p≥0.05). The continuation of the experiment was 60 days. The average final weight, meat quality, and blood biochemical parameters were measured at the end of the experiment. The SGR and FCR were also calculated. Fish from the experimental group fed with a supplement of licorice had with 8.54% higher average final weight compared to the parameter’s value in trouts from the control variant (p<0.05). SGR in rainbow trout’sfrom experimental variants was higher with 6.9% compared to the value of SGR in fish from CF variant. The blood biochemical parameters in experimental trouts were not significantly affected by supplementation of licorice extract. The average values of glucose, ASAT, and ALAT in control variant were higher respectively with 3.96%, 57.2% and 10.3% compared to values in these parameters measured in the blood of fish from the experimental variant, but differences were not statistically proven (p≥0.05). The meat quality parameters were affected from the addition of licorice in feed for trouts and the moisture, fat, and ash quantities were higher respectively with 2.49%, 36.4% and 12.5% compared with the average values of these parameters found for fish from CF group (p<0.05). The extract from licorice added to feed for the feeding of rainbow trout increases the growth of fish and affect the meat quality parameters (p<0.05), but did not affect significantly the blood parameters (p≥0.05).

Hydropower impacts on reservoir fish populations are modified by environmental variation

Global transition towards renewable energy production has increased the demand for new and more flexible hydropower operations. Before management and stakeholders can make informed choices on potential mitigations, it is essential to understand how the hydropower reservoir ecosystems respond to water level regulation (WLR) impacts that are likely modified by the reservoirs' abiotic and biotic characteristics. Yet, most reservoir studies have been case-specific, which hampers large-scale planning, evaluation and mitigation actions across various reservoir ecosystems. Here, we investigated how the effect of the magnitude, frequency and duration of WLR on fish populations varies along environmental gradients. We used biomass, density, size, condition and maturation of brown trout (Salmo trutta L.) in Norwegian hydropower reservoirs as a measure of ecosystem response, and tested for interacting effects of WLR and lake morphometry, climatic conditions and fish community structure. Our results showed that environmental drivers modified the responses of brown trout populations to different WLR patterns. Specifically, brown trout biomass and density increased with WLR magnitude particularly in large and complex-shaped reservoirs, but the positive relationships were only evident in reservoirs with no other fish species. Moreover, increasing WLR frequency was associated with increased brown trout density but decreased condition of individuals within the populations. WLR duration had no significant impacts on brown trout, and the mean weight and maturation length of brown trout showed no significant response to any WLR metrics. Our study demonstrates that local environmental characteristics and the biotic community strongly modify the hydropower-induced WLR impacts on reservoir fishes and ecosystems, and that there are no one-size-fits-all solutions to mitigate environmental impacts. This knowledge is vital for sustainable planning, management and mitigation of hydropower operations that need to meet the increasing worldwide demand for both renewable energy and ecosystem services delivered by freshwaters.

30 years of data reveal dramatic increase in abundance of brown trout following the removal of a small hydrodam

Humans and freshwater ecosystems have a long history of cohabitation. Today, nearly all major rivers of the world have an in-stream structure which changes water flow, substrate composition, vegetation, and fish assemblage composition. The realization of these effects and their subsequent impacts on population sustainability and conservation has led to a collective effort aimed to find ways to mitigate these impacts. Barrier removal has recently received greater interest as a potential solution to restore river connectivity, and reestablish high quality habitats, suitable for feeding, refuge and spawning of fish. In the present study, we present thirty years of data from electrofishing surveys obtained at two sites, both prior to and following the removal of a small-scale hydropower dam in Central Jutland, Denmark. We demonstrate that the dam removal has led to a dramatic increase in trout density, especially in young of the year. Surprisingly, we found that this increase was not just upstream of the barrier, where the ponded zone previously was, but also downstream of the barrier, despite little changes in habitat in that area. These findings suggest that barrier removal may be the soundest conservation option to reinstate fish population productivity.

Simulated Effects of YY‐Male Stocking and Manual Suppression for Eradicating Nonnative Brook Trout Populations

AbstractEradication of nonnative Brook Trout Salvelinus fontinalis populations is difficult to achieve with standard techniques, such as electrofishing removal or piscicides; new approaches are needed. A novel concept is to stock “supermale” hatchery fish with wild conspecifics. Supermales (MYY) have two Y‐chromosomes, resulting in offspring that are all males; over time, successful supermale reproduction could eradicate the wild population. We constructed an age‐structured stochastic model to investigate the effects of manually suppressing wild fish and stocking MYY fingerlings on the long‐term viability of hypothetical nonnative Brook Trout populations. In streams, an annual stocking rate of supermales equivalent to 50% of wild age‐0 Brook Trout density combined with an annual selective suppression rate equivalent to 50% of wild Brook Trout density resulted in a time to extirpation of only 2–4 years if supermale fitness was equivalent to wild male fitness. However, time to extirpation in streams was 5–15 years if supermale fitness was 80% lower than wild male fitness. In alpine lakes, higher supermale stocking rates and nonselective gillnetting were required to eradicate Brook Trout populations. If supermales were assumed to be as fit as wild males, however, any supermale stocking rate greater than 49% in alpine lakes or 60% in streams achieved eradication in 10 years or less, regardless of the suppression rate. Because manual suppression and the stocking of MYY fingerlings can readily be conducted at the levels assumed in our simulations, use of such an integrated pest management (IPM) approach could extirpate undesirable Brook Trout populations within reasonably short periods of time. Given the recent successful development of an MYY Brook Trout broodstock capable of producing large numbers of MYY fingerlings and given the positive results of the present simulations for both streams and alpine lakes, field testing of MYY stocking is warranted within an IPM program that includes manual suppression for eradicating undesirable Brook Trout populations.Received October 14, 2016; accepted June 10, 2017 Published online August 22, 2017

Population dynamics of brown trout in a Minnesota (USA) stream: A 25‐year study

AbstractBrown trout (Salmo trutta) were surveyed by mark recapture in a 200‐m section of Gilmore Creek, Minnesota, annually during fall 1989–2013 to assess long‐term trends in abundance. Young‐of‐year (YOY) fish comprised >68% of the population annually, but age 3 and older fish were present in 23 of 25 years. Trout abundance varied irregularly, peaking every 4 to 6 years. Fall densities of YOY brown trout were positively correlated with median annual stream discharge but inversely correlated with 10% exceedance discharge in May, at a nearby gaged stream. Changes in brown trout abundances were synchronized with those of trout in 2 nearby streams. Annual mortality rates (mean = 74%) and sizes of YOY trout were correlated with YOY densities, with high densities (>1.0 fish/m2) producing small size during fall and high cohort mortality. High YOY densities resulted in low proportional size structure‐quality (PSSQ, <20%) 1 and 2 years later. If similar brown trout population dynamics occur in other streams within the region, interpretation of short‐term studies of brown trout (e.g., regulation evaluations, creel surveys, population response to habitat improvement, seasonal movements, and growth rates) may be confounded.

The effects of population density and lake characteristics on growth and size structure of brook trout Salvelinus fontinalis (Mitchill, 1815) in boreal forest lakes in Canada

This paper examines the effect of lake characteristics on population density and how this variation affects growth, mortality and population size structure of brook trout, Salvelinus fontinalis. The study was conducted on 17 recreationally fished, reproductively isolated boreal forest lakes in Newfoundland, Canada from 1993 to 2000. A standardized sampling program, the Fyke Littoral Index Netting program (FLIN) was used to collected data that describes brook trout population parameters and life history attributes. Regression analyses showed significant relationships between fish density and biomass and characteristics of the lakes. Variation in fish density and biomass was explained by lake surface area and littoral habitat area. Significant relationships were found when growth, mortality and size structure were regressed against density. The proportional stock distribution and theoretical maximum size of brook trout were negatively related to density, and natural mortality was positively related to density. The largest maximum length and highest proportional stock densities occurred at brook trout densities of less than 30 fish/ha. In general, the higher the proportion of littoral habitat area the higher the densities of brook trout, which correspondingly had important effects on growth, natural mortality and size structure of the brook trout populations. This information is critical to the development of management strategies aimed at altering size distribution to produce specific fisheries management outcomes.

Spatial and Temporal Consumption Dynamics of Trout in Catch‐and‐Release Areas in Arkansas Tailwaters

AbstractRestrictive angling regulations in tailwater trout fisheries may be unsuccessful if food availability limits energy for fish to grow. We examined spatial and temporal variation in energy intake and growth in populations of Brown Trout Salmo trutta and Rainbow Trout Oncorhynchus mykiss within three catch‐and‐release (C‐R) areas in Arkansas tailwaters to evaluate food availability compared with consumption. Based on bioenergetic simulations, Rainbow Trout fed at submaintenance levels in both size‐classes (≤400 mm TL, >400 mm TL) throughout most seasons. A particular bottleneck in food availability occurred in the winter for Rainbow Trout when the daily ration was substantially below the minimum required for maintenance, despite reduced metabolic costs associated with lower water temperatures. Rainbow Trout growth rates followed a similar pattern to consumption with negative growth rates during the winter periods. All three size‐classes (<250 mm TL, 250–400 mm TL, >400 mm TL) of Brown Trout experienced high growth rates and limited temporal bottlenecks in food availability. We observed higher mean densities for Rainbow Trout (47–342 fish/ha) than for Brown Trout (3–84 fish/ha) in all C‐R areas. Lower densities of Brown Trout coupled with an ontogenetic shift towards piscivory may have allowed for higher growth rates and sufficient consumption rates to meet energetic demands. Brown Trout at current densities were more effective in maintaining adequate growth rates and larger sizes in C‐R areas than were Rainbow Trout. Bioenergetic simulations suggest that reducing stocking levels of Rainbow Trout in the tailwaters may be necessary in order to achieve increased catch rates of larger trout in the C‐R areas.Received August 31, 2016; accepted January 9, 2017 Published online March 21, 2017

Highly dilute water chemistry during late snowmelt period affects recruitment of brown trout (Salmo trutta) in River Sira, southwestern Norway

During the past two-three decades the water chemistry in southern Norway has recovered considerably from acidification. The biological response, however, has been more variable. Thus, it has become increasingly apparent that decades of acidification may have masked other restrictors to fish populations in this area. The current study compiles data on young brown trout ( Salmo trutta ) density and water chemistry from 16 sites in River Sira in southwestern Norway during 2003–2014. The water chemistry during late snowmelt period was highly dilute, having a median conductivity and Ca of 10.5 μS/cm and 0.42 mg/l, respectively (n = 208). The corresponding minimum values were 5.0 μS/cm and 0.15 mg/l. With a pH median value of 5.89, the water was only slightly acidic. No effect of pH on the density of young brown trout was found, suggesting that acidification is no longer a limiting factor. However, both conductivity in year n-1 and Ca in year n correlated significantly with the density of 1+ brown trout. Al in year n , and surprisingly Ca in year n-1 , correlated negatively on their densities. We conclude that very dilute water chemistry during late snowmelt is a limiting factor for the recruitment of brown trout in the study river. This effect has probably existed in the past as well, but has been less apparent due to decades of severe acidification.

Long-term monitoring reveals the success of salmonid habitat restoration

The growing concern on declining salmonid populations has resulted in numerous restoration projects with variable responses worldwide. In this spatially replicated multiyear study, we assessed the long-term (12 years postrestoration) effects of in-stream habitat restoration (i.e., addition of boulders or large woody debris (LWD) together with boulders) on densities of three age-classes of juvenile brown trout (Salmo trutta) in six forest streams in northern Finland. LWD combined with boulders was more beneficial, particularly for the larger trout (age-2 and older), than were boulder structures alone, indicating that the more diverse habitat created by LWD may have provided a safeguard against drought for the larger fish. Density of age-0+ trout showed a significant long-term increase in boulder-restored sections, providing evidence that log structures may need to be complemented by stony enhancement structures to guarantee the availability of suitable stream habitat for all trout age-classes. As trout densities are known to exhibit inherently wide interannual variability that tracks climatically induced hydrological variation, long-term postrestoration monitoring that encompasses extreme hydrological events is critical for evaluating the success of restoration projects.

Challenging convention: the winter ecology of brown trout (Salmo trutta) in a productive and stable environment

Summary Understanding of the winter ecology of stream salmonids is biased by research conducted in northern temperate and boreal regions dominated by hard rock geology. Such systems are driven by highly dynamic surface‐flow regimes and tend to be physically diverse, nutrient poor and influenced by ice. This study investigated how the behaviour of brown trout, Salmo trutta, inhabiting a stable groundwater‐fed, productive and comparatively warm southern English chalk stream differs from that described for other systems, and how this is translated to performance, measured as growth. Physical characteristics were mapped, and high‐resolution temperature data collected using a spatial array of data loggers installed throughout the study reach during the winter. A combination of passive integrated transponder and radio telemetry was used to monitor distribution, density, and movement of trout. Micro‐archival data storage tags inserted in some individuals provided information on temperature regimes experienced. Growth performance was calculated for recaptured fish. Trout density was positively related to depth and there was no evidence that temperature influenced microhabitat selection. Three patterns of movement were observed. Over three‐quarters of tracked fish exhibited high site fidelity and tended to remain in a single focal position throughout the study. Fourteen per cent of trout exploited more than one distinct location, while the remainder were detected at multiple locations and showed no preference for any one. Trout exhibited regular daily activity patterns and highly periodic local movements at dusk and dawn and tended to experience positive growth performance during periods that included winter. This study challenges the conventional view of salmonid winter ecology, which is biased towards populations that inhabit hard rock surface‐flow dominated rivers that experience the influence of ice. Despite inhabiting a distinctly different winter habitat template than more commonly studied populations, trout occupying a hydrologically stable and productive chalk stream exhibited behaviours similar to those described for elsewhere, yet performed considerably better.

Recovery of Native Brook Trout Populations Following the Eradication of Nonnative Rainbow Trout in Southern Appalachian Mountains Streams

AbstractNonnative Rainbow Trout Oncorhynchus mykiss have displaced native Brook Trout Salvelinus fontinalis in many southern Appalachian Mountains streams. We monitored the population recovery of Brook Trout following Rainbow Trout eradication at 10 sites in seven allopatric Rainbow Trout streams located in Great Smoky Mountains National Park, USA. Rainbow Trout were successfully eradicated by electrofishing or Fintrol (also known as antimycin‐A), and Brook Trout were reintroduced at low densities (39–156 fish/km) from streams located within the park. Within 2 years after reintroduction, the density and biomass of adult Brook Trout recovered to levels comparable to the prerestoration density and biomass of Rainbow Trout. Spawning in the first autumn after reintroduction was assumed by the presence of young‐of‐the‐year fish in seven out of nine sites surveyed during the following summer. Brook Trout density and biomass 3–5 years after restoration did not significantly differ from those in natural allopatric populations within the park in young‐of‐the‐year fish but were significantly lower in adults. Individual body size of adult and young‐of‐the‐year fish were density dependent after restoration, indicating that Brook Trout populations had recovered to a point that habitat saturation triggered intraspecific competition. We conclude that Rainbow Trout removal has been a viable management technique to restore Brook Trout populations in the park.Received March 4, 2016; accepted July 17, 2016 Published online October 20, 2016

In-stream water quality, invertebrate and fish community health across a gradient of dairy farming prevalence in a New Zealand river catchment

Dairying is an intensive form of agriculture influencing stream ecosystems worldwide via increased levels of nutrients, deposited fine sediment and other contaminants. However, it is not fully understood how dairy farming affects food supply for stream fish. We investigated relationships between dairy farming prevalence in the catchments of nine tributaries of a New Zealand river (0% to 79% of the catchment area) and fish and invertebrate communities. Streams were sampled four times at monthly intervals for brown trout density, fitness/growth-related trout response variables, native fish density, invertebrate community metrics as well as physical and chemical water quality variables. Densities of both brown trout and native fish declined as dairying increased, with no trout found in streams where dairy farms covered more than 50% of the catchment area. Increasing dairy farming prevalence was also associated with higher in-stream levels of dissolved nutrients and deposited fine sediment. These findings suggest that increasing the extent of dairy farming in New Zealand based on practices at the time of sampling results in less abundant and diverse fish communities.

Hydrological drivers of brown trout population dynamics in France

AbstractAlteration of natural river flow regimes is a major threat to freshwater biodiversity. Restoration of natural flow regimes requires an understanding of linked hydrological and ecological processes. In this study, we investigated how annual seasonal flow characteristics and physical habitat attributes could interact to explain yearly changes in brown trout population densities (for 0+ fish, corresponding to the young of the year, and for the >0+ age class) in 112 sites widespread in France. Using an information theoretic approach and general linear modeling, we identified the physical habitat attributes and seasonal hydrological variables which explained 0+ proportion, total, >0+ and 0+ brown trout density dynamics. A decrease in total and 0+ brown trout densities were mainly linked to high water levels in both high and low flows during the emergence period and to a lesser extent, to physical habitat attributes reflecting the river size. 0+ proportions were only linked to the level of low flows, and to a lesser extent to high flows, during the emergence period. Our study demonstrated that seasonality of flow is a main driver of brown trout population dynamics, especially magnitude of high and low flows for the 0+ age class, and should be considered in the definition of environmental flows.

Recovery of young brown trout (Salmo trutta) in acidified streams: What are the critical values for acid-neutralizing capacity?

The recovery of young allopatric brown trout (Salmo trutta) grouped into YoY (age 0+) and older parr (age ≥1+) fish, was studied in acid-sensitive streams in a Norwegian watershed during a 24-year-period (1987–2010). Their abundance was assessed by electrofishing. Most sites typically had 5.0–5.5 in pH, 0.4–0.7 mg L−1 Ca, 10–20 μg L−1 inorganic toxic aluminum (Ali) and acid-neutralizing capacity adjusted for organic acids (ANCOAA) of - 15 to +25 μeq L−1. Densities of both YoY and older parr increased significantly during the study period. Water quality also improved in recent years with respect to pH (5.8–6.0), Ali (5–15 μg L−1) and ANCOAA (10–20 μeq L−1). However, some negative trends in both fish density and water chemistry were found during both the first (1987–1993) and last years (2004–2008) of the study. Initially, YoY densities remained at about 16–20 specimens 100 m−2 (1987–1990), declined to 10–15 specimens 100 m−2 in the early/mid 1990s, and rosed to 30–50 specimens 100 m−2 in recent years (1997–2010). Their densities correlated significantly with ANCOAA, and at least three stages in the recovery process were recognised: (i) Low density with 10–20 specimens 100 m−2 at −18 to −5 μeq L−1, (ii) medium and unstable density with 20–30 specimens 100 m−2 at −5 to 10 μeq L−1, and (iii) increasing density to 40–50 specimens 100 m−2 at 10–25 μeq L−1. The decline in brown trout density in the early-mid 1990s coincided with high sea salt depositions, which caused increased acidification. Component 1 in a PCA explained 51% of the variation in fish densities, including conductivity, Mg, Ca, Na, alkalinity and TOC. Component 2 explained an additional 31% of the variation, including pH, Ali and ANCOAA. Multiple regression analysis coefficients showed that the two components explained 41% of the variance in total fish density. Young brown trout suffered a high mortality during the initial phase of the study in spite of relative low levels of Ali. This is probabaly because the study streams have very diluted water. The densities of young brown trout have levelled off in recent years, indicating a development towards reaching carrying capacity and hence full recovery. However, still some annual fluctuations in density are recorded, which may be related to an unstable water chemistry.

Main potential drivers of trout population dynamics in bypassed stream sections

AbstractThe key role of hydrological variability in structuring brown trout populations is well established. However, the influence of additional drivers is more difficult to identify. The implementation of long‐term monitoring and the development of reliable tools can help to reveal fine local drivers structuring fish populations in contrasted flow regimes. This study used data series for nine reaches monitored for nine to nineteen years in four French salmonid streams. Study reaches were within five bypassed sections influenced by instream flow. A deterministic trout population dynamics model was applied on each reach, with calibration and validation procedures. Results revealed that biological drivers structured all reaches similarly. In addition, seven other drivers were identified. Among these additional drivers, hydrology mainly explained temporal fluctuations in trout density, regardless of reach. Three drivers independent of hydrology were also revealed: poor water quality, limited spawning area and the effect of power plant operations (overtopping, flushing or plant shutdown). All drivers influenced the whole bypassed section and were never limited to the scale of the reach (sampling area). Further analyses of each driver are now needed, to regionalise and quantify their respective impact precisely. Thus, assessment of trout population status would be simplified, enabling implementation of efficient management rules.

Optimal flow for brown trout: Habitat – prey optimization

The correct definition of ecosystem needs is essential in order to guide policy and management strategies to optimize the increasing use of freshwater by human activities. Commonly, the assessment of the optimal or minimum flow rates needed to preserve ecosystem functionality has been done by habitat-based models that define a relationship between in-stream flow and habitat availability for various species of fish.We propose a new approach for the identification of optimal flows using the limiting factor approach and the evaluation of basic ecological relationships, considering the appropriate spatial scale for different organisms. We developed density-environment relationships for three different life stages of brown trout that show the limiting effects of hydromorphological variables at habitat scale. In our analyses, we found that the factors limiting the densities of trout were water velocity, substrate characteristics and refugia availability. For all the life stages, the selected models considered simultaneously two variables and implied that higher velocities provided a less suitable habitat, regardless of other physical characteristics and with different patterns. We used these relationships within habitat based models in order to select a range of flows that preserve most of the physical habitat for all the life stages. We also estimated the effect of varying discharge flows on macroinvertebrate biomass and used the obtained results to identify an optimal flow maximizing habitat and prey availability.