Brown Trout Density Research Articles

Brown trout density effects on the macroinvertebrate and biofilm communities of headwater streams: a cage experiment

Salmo trutta is a key species of headwater streams that is able to influence the structure of these ecosystems through predation. Populations of this species are being threatened by climate change and hydrological alteration. In order to understand how their disappearance would affect the ecosystem structure of headwater streams, we used cage experiments to analyse different biofilm and macroinvertebrate structural parameters and the biodiversity of their communities under different trout densities. Due to a heavy rain event, our experiment was destroyed on the 10th day, but the role of trout as stream ecosystemic structure drivers was observed, as multiple parameters were modulated by their presence or absence. In the high-density treatments, a significant reduction in the biomass of macroinvertebrate families that fed on biofilms was observed due to direct predation on them, which caused the biofilms to be more autotrophic. A decrease in brown trout population densities could cause cascading trophic effects on the ecosystem structure of headwater streams

Monitoring and Managing Fishes that Are Invisible and Keep Moving Around: Influences of an Invasive Species and Environmental Factors on Capture Probability

AbstractUnderstanding fish population status and trends are fundamental to effective research and management. Challenges in understanding population status include recognizing and accounting for sources of variation in capture probability () that can obscure patterns in count data and bias inferences about the population. In systems where management actions such as invasive species removals are implemented based on population triggers, errors in abundance estimation can propagate to missed management opportunities simply due to variation in , rather than actual population status. We assessed the relative importance of individual heterogeneity (species and fish size), environmental (stream discharge, temperature) and spatial variation, and interspecific interactions (density) on electrofishing for native (Speckled Dace Rhinichthys osculus, Bluehead Sucker Catostomus discobolus, Flannelmouth Sucker Catostomus latipinnis) and invasive (Brown Trout Salmo trutta, Rainbow Trout Oncorhynchus mykiss) fishes. We fit closed‐population depletion models with combinations of covariates using 6 years of data collected during invasive salmonid suppression efforts. We found that the relative importance of spatially and temporally varying environmental influences on differed between native and introduced species. Temperature was important for explaining variability in for two of three native species, and discharge had a significant influence on for both Brown Trout and Rainbow Trout. We also found that declined with increasing Brown Trout density, which would limit precision and potentially bias abundance, for two of the three native species. These results illustrate the potential importance of interspecific interactions in influencing and understanding implications of management actions, such as invasive species suppression. Our results demonstrate the need for cautious interpretation of uncalibrated catch‐per‐unit‐effort data for abundance‐triggered management actions, including those designed to promote conservation of endangered or economically valuable species.

Predatory ability and abundance forecast the ecological impacts of two aquatic invasive species

Characterising interspecific interaction strengths, combined with population abundances of prey and their novel predators, is critical to develop predictive invasion ecology. This is especially true of aquatic invasive species, which can pose a significant threat to the structure and stability of the ecosystems to which they are introduced. Here, we investigated consumer-resource dynamics of two globally-established aquatic invasive species, European green crab (Carcinus maenas) and brown trout (Salmo trutta). We explored the mediating effect of prey density on predatory impact in these invaders relative to functionally analogous native rock crab (Cancer irroratus) and Atlantic salmon (Salmo salar), respectively, feeding on shared prey (Mytilus sp. and Tenebrio molitor, respectively). We subsequently combined feeding rates with each predator’s regional abundance to forecast relative ecological impacts. All predators demonstrated potentially destabilising Type II functional responses towards prey, with native rock crab and invasive brown trout exhibiting greater per capita impacts relative to their trophic analogues. Functional Response Ratios (attack rates divided by handling times) were higher for both invasive species, reflecting greater overall per capita effects compared to natives. Impact projections that incorporated predator abundances with per capita effects predicted severe impacts by European green crabs. However, brown trout, despite possessing higher per capita effects than Atlantic salmon, are projected to have low impact owing to currently low abundances in the sampled watershed. Should brown trout density increase sixfold, we predict it would exert higher impact than Atlantic salmon. Such impact-forecasting metrics and methods are thus vital tools to assist in the determination of current and future adverse impacts associated with aquatic invasive species.

From Gold Mining to Gold Medal Fishery: Evaluating the Fishery Response to Stream Restoration on the Upper Arkansas River, Colorado

AbstractOver a century of metals pollution and channel disturbance associated with historical mining, land use, and water development contributed to degradation of aquatic and riparian habitat within the upper Arkansas River watershed near Leadville, Colorado. Following water quality remediation, habitat restoration was conducted for a 17.7‐km reach characterized as an overwide channel that lacked velocity refuge and overwinter habitat for salmonids. The primary goals of restoration were to improve populations of Brown Trout Salmo trutta and individual fish health, with a target to increase fish metrics by 10% within 5 years after restoration. Fish metrics included Brown Trout density, biomass, quality, and relative weight, where quality was defined as the density of Brown Trout over 356 mm in length. Changes in all fish metrics were evaluated with a before‐after–control‐impact study design that utilized five control sites and five impact sites. Biomass was the only metric that exhibited a significant interaction between site type and period (before and after), with increases of 12% and 21% at control and impact sites, respectively. Increased density (10%) and relative weight (2.4%) were observed across all sites regardless of type. Changes at individual sites were less evident, with only one impact site showing significant increases in biomass (99%) and quality Brown Trout (306%). These results suggest that Brown Trout populations within the upper Arkansas River have continued to improve following large‐scale water quality remediation and stream restoration efforts. Additional monitoring is recommended to evaluate long‐term trends and inform adaptive management.

Size, connectivity and edge effects of stream habitats explain spatio-temporal variation in brown trout (Salmo trutta) density.

Ecological theory postulates that the size and isolation of habitat patches impact the colonization/extinction dynamics that determine community species richness and population persistence. Given the key role of lotic habitats for life-history completion in rheophilic fish, evaluating how the distribution of swift-flowing habitats affects the abundance and dynamics of subpopulations is essential. Using extensive electrofishing data, we show that merging island biogeography with meta-population theory, where lotic habitats are considered as islands in a lentic matrix, can explain spatio-temporal variation in occurrence and density of brown trout (Salmo trutta). Subpopulations in larger and less isolated lotic habitat patches had higher average densities and smaller between-year density fluctuations. Larger lotic habitat patches also had a lower predicted risk of excessive zero-catches, indicative of lower extinction risk. Trout density further increased with distance from the edge of adjacent lentic habitats with predator (Esox lucius) presence, suggesting that edge- and matrix-related mortality contributes to the observed patterns. These results can inform the prioritization of sites for habitat restoration, dam removal and reintroduction by highlighting the role of suitable habitat size and connectivity in population abundance and stability for riverine fish populations.

Combining expert‐based and computational approaches to design protected river networks under climate change

AbstractAimEstimate the current and future distribution of brown trout and identify priority areas for conservation of the species.LocationRhône River basin and Mediterranean streams.MethodsWe first developed a spatially explicit species distribution model to estimate the current and future distribution of brown trout for three time horizons (2030, 2055 and 2080) and two climate change scenarios (RCP 4.5 and RCP 8.5). We then performed a prioritization analysis to identify priority areas for brown trout conservation, accounting for: (a) spatial dependencies along the riverine system, (b) several sources of uncertainty arising from climate‐related forecasts and (c) different protected area scenarios by comparing hypothetical, optimal protected networks to an actual protected network designed by regional fish experts.ResultsFuture projections of brown trout densities exhibited a general trend towards a gradual range contraction, with a significant risk of extirpation across mountainous regions of low to mid‐elevation. Overall, the projected current and future distributions were well‐covered by the existing protected network. In addition, up to 70% of the river reaches included in this expert‐based protection network were also priorities in the optimal priority set (e.g. the best set of areas to maximize biodiversity protection). Finally, a large proportion of these reaches were invariably identified regardless of climate change scenarios and uncertainties or spatial dependencies.Main conclusionsOur analytical approach highlighted priority areas for brown trout conservation which were robust to a set of climate and connectivity assumptions. This core priority network could be further refined by taking into account key fine‐scale processes like thermal refugia. Therefore, we advocate for combining computational and expert‐based approaches in conservation planning of riverine ecosystems to achieve a relevant consensus between regional‐scale management and fine‐grain ecological knowledge.

Habitat restoration for brown trout (Salmo trutta) has limited effects on macroinvertebrate communities in a historically metal-contaminated stream.

Quantifying the success of stream remediation or restoration projects that are designed to improve water quality or habitat, respectively, is often challenging because of insufficient posttreatment monitoring, poorly defined restoration goals, and failure to consider fundamental aspects of ecological theory. We measured the effects of habitat restoration on aquatic and terrestrial prey resources in a system recovering from the long-term effects of mining pollution. The study was conducted in the Upper Arkansas River, a Rocky Mountain stream located in central Colorado, USA. Remediation of California Gulch, a United States Environmental Protection Agency (USEPA) Superfund Site that discharged metals from past mining operations into the stream, was completed in 2000, resulting in significant improvements in water quality, benthic macroinvertebrate communities, and brown trout (Salmo trutta) populations. A large-scale restoration project designed to improve habitat and increase the density and biomass of brown trout was completed in 2014. To assess the effectiveness of these habitat improvements on invertebrate communities in this system, we sampled sites for 9 years before (2010-2014) and after (2015-2018) restoration was completed. In contrast to our expectations, we observed few changes in the abundance of aquatic or terrestrial invertebrates after restoration. The most common response was an overall reduction in abundance resulting from significant instream disturbances during and immediately after restoration, followed by a gradual return to pretreatment conditions. Despite reductions in prey abundance, the number of prey items in the diet of brown trout increased significantly after restoration. We discuss several explanations for these responses, including the effects of residual metals, increased predation by brown trout, and the recalcitrance of novel communities dominated by metal-tolerant species. Our results suggest that the effectiveness of remediation and restoration differed between macroinvertebrates and fish. Benthic macroinvertebrates were more dependent on water quality improvements at the watershed scale, whereas brown trout populations responded to both improvements in water quality and reach-scale improvements in habitat.Integr Environ Assess Manag 2022;18:1047-1055. © 2021 SETAC.

Influence of the Landscape Template on Chemical and Physical Habitat for Brown Trout Within a Boreal Stream Network

We used the distribution of stream-dwelling brown trout (Salmo trutta) in a 67 km2 boreal catchment to explore the importance of environmental organizing factors at a range of spatial scales, including whole-catchment characteristics derived from map data, and stream reach chemical and physical characteristics. Brown trout were not observed at any sites characterized by pH < 5.0 during the spring snowmelt episode, matching published toxicity thresholds. Brown trout distributions were patchy even in less acidic regions of the stream network, positively associated with glaciofluvial substrate and negatively associated with fine sand/silty sediments. A multivariate model including only whole-catchment characteristics explained 43% of the variation in brown trout densities, while models with local site physical habitat characteristics or local stream chemistry explained 33 and 25%, respectively. At the stream reach scale, physical habitat apparently played a primary role in organizing brown trout distributions in this stream network, with acidity placing an additional restriction by excluding brown trout from acidic headwater streams. Much of the strength of the catchment characteristics-fish association could be explained by the correlation of catchment-scale landscape characteristics with local stream chemistry and site physical characteristics. These results, consistent with the concept of multiple hierarchical environmental filters regulating the distribution of this fish species, underline the importance of considering a range of spatial scales and both physical and chemical environments when attempting to manage or restore streams for brown trout.

Fish community and abundance response to improved connectivity and more natural hydromorphology in a post-industrial subcatchment

Barrier removal and fish pass construction are increasingly used as tools to restore river connectivity and improve habitat quality, but the effectiveness of subcatchment-scale connectivity restoration on recovery of fish communities is poorly understood. We used a before-after-downstream-upstream methodology to determine the effects of subcatchment-scale connectivity restoration on fishes in a fragmented tributary of the River Wear, Northeast England, between 2013 and 2019. Following restoration (three barriers removed, five barriers fitted with fish passes, two barriers unaltered), riffle habitat increased, fine sediment decreased, and most fish species benefitted. Total fish abundance, comprising seven native species, increased 3 years after the restoration and remained elevated to the end of the study. Mean brown trout (Salmo trutta) density increased from 20.9 ± 6.3 to 33.8 ± 16.8 per 100m2 from 2013 to 2019, with Young-of-Year trout increasing from 10.6 ± 4.6 to 19.8 ± 11.8 per 100m2. Connectivity restoration reduced the mean age of trout, suggesting a change to an increased migratory component of the population. Density of bullhead (Cottus perifretum), a species with poor dispersal ability, increased from 4.6 ± 2.7 to 32.6 ± 17.9 per 100m2 over 2013 to 2019. Stone loach (Barbatula barbatula), also a less mobile species but tolerant to fine sediment, decreased in abundance where barriers were removed. Atlantic salmon (Salmo salar) were absent over the study timescale, despite being common in the Wear, and despite suitable habitat and water quality in the restored subcatchment, suggesting a hysteresis effect. Our findings indicate that, where good water quality exists, restoring river connectivity and hydromorphology at a subcatchment scale is beneficial for most native resident and migratory fishes. However, the ecological benefits of connectivity restoration, especially in rivers with many barriers, may take several years to develop. We encourage well-controlled long-term studies reporting the outcomes of large-scale connectivity restoration.

Long-term effects and cost-benefit analysis of eight spawning gravel augmentations for Atlantic salmon and Brown trout in Norway

River regulation alters flow and sediment regime, habitat availability, and ultimately the ecological functioning of rivers. Various restoration and mitigation measures have been developed to improve ecological function, and among them is the addition of gravel to enhance the reproduction of gravel bed spawning fishes. However, information on long-term efficiency, costs, and maintenance needs of gravel additions are scarce. Here, we study the functioning of gravel additions at eight sites in three rivers in western Norway for up to 18 years. Gravel was added between 2002 and 2010 to enhance spawning of Atlantic salmon and anadromous Brown trout. We monitored changes in the size of the gravel-covered areas, sediment composition, interstitial oxygen, egg survival, and juvenile fish densities. Additionally, we report monetary costs, identify potential maintenance needs, and calculate a cost-benefit ratio. Juvenile densities of Atlantic salmon and Brown trout increased significantly after the gravel augmentations. After 10–18 years, the median egg survival was still high (> 90%) and sediment conditions were still suitable for salmonid fish reproduction. The areas were, however, shrinking across time (median area reduction 26%), mostly caused by scouring of gravel in the steep, supply-limited, and partly regulated rivers. The average construction costs of spawning gravel augmentations were 11.2 NOK (1.12 €) m−2 year−1. Compared to similar measures elsewhere, the measures have had a long life span (up to 18 years) at relatively low costs. Gravel augmentation was concluded to be a successful management measure that contributed to significantly increased Atlantic salmon and Brown trout reproduction.

Rapid response of fish and aquatic habitat to removal of a tidal barrier

Abstract River barrier removal is used increasingly as a conservation tool to restore lotic habitat and river connectivity, but evidence of its efficacy is incomplete. This study used a before–after methodology to determine the effects of removing a tidal‐limit barrier on the fishes, macroinvertebrates, and habitats of an English coastal stream. Following barrier removal, habitat diversity increased immediately upstream and remained similar downstream. Mobilized silt altered the substrate composition immediately downstream, but this was temporary as silt was flushed out the following winter. Changes to macroinvertebrate communities occurred upstream and downstream of the former barrier but these were transient. A dramatic and sustained increase in fish density occurred immediately upstream of the barrier after its removal, but effects downstream were minor. The fish community upstream changed, largely due to rapid recruitment and dispersal of endangered European eel (Anguilla anguilla). Eel density in the formerly impounded zone increased from 0.5 per 100 m2 before barrier removal to 32.5 per 100 m2 5 months after removal. By 17 months after barrier removal there was no difference in eel density across the six sections sampled. Although resident stream fishes such as bullhead (Cottus gobio species complex, protected under the European Habitats Directive) were abundant in middle and upper‐stream sections, brown trout (Salmo trutta, a listed species for biodiversity conservation in England and Wales) density remained low during the study and recruitment was poor. This suggests that although colonization access for anadromous trout was available, habitat upstream may have been unsuitable for reproduction, indicating that wider catchment management is required to complement the restoration of connectivity. These findings suggest that tidal barrier removal is an effective method of restoring lotic habitats and connectivity, and can be beneficial for resident and migratory fishes including those of conservation importance (e.g. European eel) in coastal streams.

Current and projected impacts of the parasite Tetracapsuloides bryosalmonae (causative to proliferative kidney disease) on Central European salmonid populations under predicted climate change

Abstract Proliferative kidney disease (PKD) caused by the myxozoan parasite Tetracapsuloides bryosalmonae is an emerging salmonid disease implicated in recent declines in salmonid populations. Laboratory experiments have shown that the clinical symptoms of PKD exacerbate with increasing temperature. However, empirical evidence for a relationship between climate change and PKD driven declines in wild salmonid stocks is scarce. The current study uses both empirical data and ecological niche modelling to extrapolate future changes in temperature and precipitation on the spread of PKD in relation to changes in suitability of rivers for its primary bryozoan and secondary salmonid hosts. A 20‐year dataset on brown trout density using standardised method shows a decline of more than 50% in the population of the river Wutach, south‐western Germany at 536–755 m elevation. The decline coincides with higher prevalence of PKD. This decline was temperature related and driven by reduced local survival of parr to yearlings. Kidney hyperplasia was highest at low elevations and correlated negatively with trout density. Niche models based on state‐wide data show strong overlap in the areas suitable for T. bryosalmonae and its primary bryozoan hosts, and a strong link with temperature. Projections based on moderate to high emission‐case climate change scenarios predict a 50%–82% decrease in the area suitable for salmonids within this century, linked to PKD. The empirical data identify temperature‐mediated PKD as the underlying cause of the population decline. This relationship was corroborated by the niche modelling results. The highest losses are expected in projected salmonid suitable areas where salmonid habitats currently overlap with areas suitable for T. bryosalmonae. With ongoing climate change, both current and future populations are at severe risk to T. bryosalmonae and its associated disease PKD. Conservation efforts of salmonid stocks are dependent on climate mitigation and measures to help salmonids adapt to the disease.

What are the best upland river characteristics for glass eel restocking practice?

The fitness of restocked European eel (Anguilla anguilla), an endangered fish species, was studied in relation to the environmental variables of habitats in six upland rivers that are typologically different in terms of their hydromorphological and physicochemical characteristics, food resources and fish communities. These rivers received a total of 76,370 imported glass eels in 2017. During a three-year period, we monitored eels with respect to total length, annual growth rate, condition factor and density using capture-mark-recapture experiments to understand the effects of the characteristics of receiving rivers on restocking success levels. Our results showed the survival of the restocked eels in the six rivers and revealed significant differences between them in terms of total length, condition factor and density. Better performance in eel yield variables was observed in a eutrophic alkaline river with greater roughness of riverbed substrates, dominant pool- and riffle-type flow facies and lower brown trout density. The variables conductivity and total hardness had higher explanatory power and were strongly associated with increased eel density. This study suggests that a well-selected habitat/river in a restocking programme can be beneficial for the species and recommends restocking practice as a management tool to achieve eel conservation goals.

Factors Associated with Detection and Distribution of Native Brook Trout and Introduced Brown Trout in the Driftless Area of Iowa

AbstractFish populations at the fringe of their geographic range often inhabit marginal habitats and are vulnerable to ecological disturbances, such as species invasion, land cover conversion, and climate change. The Driftless Area of northeastern Iowa, USA represents the southwestern edge of the native range of Brook Trout Salvelinus fontinalis, where endemic populations have been greatly reduced due to habitat degradation and introduced Brown Trout Salmo trutta. Therefore, documenting Brook Trout occurrence patterns at the southern tip of their range may prove useful for the conservation of peripheral populations. We fitted single‐species occupancy models at multiple spatial scales to estimate the relative effects of biotic (e.g., Brown Trout density) and habitat (instream, riparian, and watershed) variables on Brook and Brown Trout occupancy to predict their current distribution in the Iowa Driftless Area. Species occurrence and physical habitat data were collected at 138 stream segments of the Upper Iowa, Yellow, and Little Maquoketa River watersheds (8‐digit hydrologic unit codes) during May–September in 2018 and 2019. Brook Trout occupancy was low (19 of 138 sites) and affected by local (instream and riparian) habitat covariates, particularly August–September stream temperatures, but no adverse effects of Brown Trout density were detected. Brown Trout occupied 54% of streams (74 of 138 sites), and occupancy was influenced by a combination of local‐scale (mean stream velocity, percentage of run macrohabitat, and August–September stream temperatures) and catchment‐scale (percent forested land cover and upstream catchment area) habitat variables. Our results provide evidence that instream thermal conditions are a critical determinant of Brook Trout distribution, while Brown Trout exhibit plasticity in habitat suitability and increased colonization in areas where they are introduced. Given the dependence of Brook Trout on cold stream temperatures, the projected loss of thermally suitable habitats due to climate change may facilitate their replacement by naturalized Brown Trout at the southwestern extent of their range.

Geomorphological features drive spatiotemporal dynamics of young‐of‐the‐year brown trout populations in a large New Zealand river catchment

Abstract Determining the factors driving population dynamics of stream salmonids across broad landscapes is important for understanding stream ecosystem functioning and for the management and conservation of the populations of such fish. A few studies have described the determinants of the early life history of salmonids across catchments, but none of them have examined how this spatial variation develops temporally. We hypothesised that: (1) spatiotemporal dynamics of young‐of‐the‐year (YoY) trout populations vary within the catchment, with loss rates (emigration plus mortality) being higher in lower elevation, downstream reaches than in higher elevation, upstream ones; and (2) loss rate would be density dependent, leading to reduced intra‐ and inter‐cohort competition and supporting establishment of resident populations. We tested the first hypothesis by examining relationships between geomorphic characteristics of spawning streams and temporal dynamics of YoY brown trout density, loss rate and biological traits through an austral summer across the catchment of New Zealand's fourth‐longest river, the Taieri River. To test the second hypothesis, we examined whether initial YoY density in spring and other biotic determinants affected loss rate and cohort structure dynamics, resulting in density‐dependent self‐thinning of YoY fish to avoid resource limitation. Spring YoY density and subsequent loss rate was higher in low elevation sites, corresponding to stream accessibility to migratory spawners. These significant correlations of YoY density and related biotic variables with elevation disappeared later in the season, indicating that regardless of their origin, brown trout YoY tend to form stream resident populations with multicohort structure and densities low enough to limit intraspecific competition. Our findings provide evidence that geomorphological features of the landscape affect temporal dynamics of YoY trout populations driven by density‐dependent self‐regulatory mechanisms that can control the abundance of growing fish.

Above parr: Lowland river habitat characteristics associated with higher juvenile Atlantic salmon (Salmo salar) and brown trout (S. trutta) densities

AbstractUnderstanding juvenile salmonid habitat requirements is critical for their effective management, but little is known about these requirements in lowland rivers, which include important but unique salmonid habitats. We compared the relative influence of in‐stream Ranunculus cover, water depth, prey abundance, distance upstream and two previously unexplored factors (water velocity heterogeneity and site colonisation potential) on summer densities of juvenile Atlantic salmon and brown trout. We applied electrofishing, habitat surveys and macroinvertebrate kick sampling, and calculated the site colonisation potential from salmon redd surveys across 18–22 sites in a lowland river in 2015–2017. Due to a recruitment crash in 2016, models including and excluding this unusual year were explored. Excluding 2016 data, juvenile salmon densities showed a positive association with Ranunculus cover and numbers of nearby upstream redds, and a negative association with distance upstream from the tidal limit. Trout densities were positively associated with velocity heterogeneity, indicating a potential indirect influence of Ranunculus mediated by water velocity. When including 2016, year had the largest effect on densities of both species, highlighting the impact of the recruitment failure. These findings uncover interspecific differences in the habitat requirements of juvenile salmonids in lowland rivers. Velocity heterogeneity and site colonisation potential had high explanatory power, highlighting that they should be considered in future studies of habitat use. These findings demonstrate that temporal replication and recruitment dynamics are important considerations when exploring species–habitat associations. We discuss potential management implications and argue that Ranunculus cover could be an important management tool in conservation of lowland salmonids.

Population Assessment and Habitat Use of Brown Trout Following Severe Overwinter Predation from Common Mergansers in a Western New York Stream

Heavy predation by Mergus merganser (Common Merganser) during severe winters of 2013–2014 and 2014–2015 resulted in substantial reductions of wild Salmo trutta (Brown Trout) in open-water, groundwater-fed reaches of Oatka Creek in western New York State. New York State Department of Environmental Conservation considered the need for habitat manipulation to reduce the severity of future overwinter-predation by mergansers in Oatka Creek Park (OCP) but lacked data to make an informed decision. Thus, this study sought to estimate population abundance, density, and year-class distribution of Brown Trout; quantify the availability of cover and habitat; and identify habitat features used by Brown Trout in OCP. We recorded data for 100 Brown Trout (101–512 mm TL) during spring 2016, autumn 2016, winter 2017, and spring 2017. Despite the absence of mergansers in ensuing warmer winters, trout population indices decreased as the study continued, likely affected by high streamflow during sampling events. Year-class distributions typical of a small stream, however, suggest that the population is recovering. Velocity refuges and structural cover were the primary factors determining habitat selection. Large woody debris was the most favored cover type by all Brown Trout; however, boulders were also important, especially during low streamflow. Large trout (>300 mm) showed a strong preference for deepwater habitats with slow currents and high densities of woody debris and boulders, while small trout (<200 mm) preferred shallower complex habitats with slow currents, course substrates, and high cover densities. Quality trout habitat and instream cover is abundant throughout OCP, but the abundance of highly complex overwinter habitats capable of providing protection from mergansers may be limited. Adding complex structural cover to areas favored by smaller trout (<300 mm) could increase habitat complexity and likely reduce the severity of future overwinter predation.

Effects of altered river network connectivity on the distribution of Salmo trutta: Insights from a metapopulation model

Abstract Network connectivity is a key feature of rivers that affects patterns and processes in lotic ecosystems. Few studies have considered how changes in river reach connectivity might affect ecosystem attributes at a whole river network scale. The use of population dynamics models of keystone species at a river network scale allows exploration of how the effects of altered natural connectivity patterns might propagate through a river network. In this study, we present a metapopulation model to estimate the spatial distribution of the population density of brown trout (Salmo trutta), an ecologically and socioeconomically important top predator, in a river network in northern Spain. The model accounts for the presence of barriers that limit longitudinal connectivity in upstream and downstream directions. The model estimates the spatial distribution of densities of three age‐classes (young‐of‐the‐year, juveniles, and adults) in all river reaches that make up the network based on topology, connectivity and population dynamics (e.g. age‐class specific mortality, spawning, age‐class dispersal, and spawning migration patterns). Seventy‐five percent of the modelled population densities fell within the 95% confidence intervals of the empirical data (84.6% for young‐of‐the‐year, 69.2% for juveniles, and 69.2% for adults) collected in 13 reaches. The simulated removal of all longitudinal barriers to migration in the river network (re‐naturalisation of the whole catchment) produced a modelled increase in brown trout density in the most downstream reaches of the river network and lowered fish densities in the upstream portion of the network when bias in juvenile and adult movement direction was assumed. Furthermore, the simulated removal of a single obstacle affected fish density even in distant tributaries. The proposed model is an appropriate tool for the evaluation of spatial patterns of brown trout density at a river network scale and for the assessment of the impact of altered connectivity. This might help simulate the results of management strategies regarding river connectivity and show where population decreases or increases could be expected, although empirical knowledge of overall trout movement in the studied river networks is required for drawing realistic scenarios.

A new fish based multi-metric assessment index for cold-water streams of the southern Caspian Sea Basin in Iran

Numerous riverine ecosystems in Iran are currently suffering due to various human activities, which are directly affecting the physico-chemical conditions of running waters and influencing the aquatic biota. In this study, we chose the cold-water streams of the Southern Caspian Sea Basin in Iran to analyse human pressures and develop a multi-metric fish index for the cold-water streams dominated by brown trout, Salmo trutta in this Basin. To do this, the fish and environmental data were sampled in 67 medium sized streams in the Caspian Mountains. In total, nine fish metrics were calculated. Moreover, 27 criteria describing major anthropogenic pressures for the entire sampling area were used to generate a regional pressure index that accounted for potential effects of multiple human pressures. For the index development, we first defined reference sites and then quantified differences of fish metrics between the reference and impacted sites. The multiple linear regressions were used for describing metric responses to natural environmental differences in the absence of any human pressures. Finally, by testing the ability of each metric for discrimination of impacted and reference sites as well as their sensitivity to a gradient of regional pressure index, the metrics density of brown trout and biomass of sub-adult brown trout explained human pressures best. Our multi-metric fish index performed well in discriminating between reference and impacted sites, giving a significant negative linear response to a gradient of regional pressure index. In general, the development of such an index offers an opportunity to enhance national bio-monitoring programmes in Iran.

Roles of environmental factors in the recruitment of the European grayling Thymallus thymallus, near its southern range limit in England

Event Abstract Back to Event Roles of environmental factors in the recruitment of the European grayling Thymallus thymallus, near its southern range limit in England Tea Basic1, 2, 3*, Robert Britton2, Richard Cove4, Anton Ibbotson3 and Stephen Gregory3 1 Centre for Environment, Fisheries and Aquaculture Science (CEFAS), United Kingdom 2 Bournemouth University, United Kingdom 3 Game & Wildlife Conservation Trust, United Kingdom 4 Natural Resources Wales, United Kingdom Density-dependent and independent factors are driving the recruitment of salmonids, with conspecific density, discharge and temperature especially important. Major drivers of the Atlantic salmon Salmo salar and brown trout Salmo trutta recruitment are well known, however there is a paucity of knowledge on the potamodromous salmonid European grayling Thymallus thymallus, whose post-emergence time coincide with period of increasing temperature and low discharge. Consequently, this study assessed the importance of different density-dependent and independent parameters for grayling survival during incubation and emergence period in a southern chalk stream. Linear and non-linear stock recruitment models were fitted to derive residuals from the “best” performing model and use in mixed effect models as a response variable. River temperature and discharge derived variables, conspecific and heterospecific densities were used as explanatory variables in a set of mixed effect models, with river site as a random effect on the intercept. The final mixed effect model revealed the significant effect of both the incubation and post-incubation temperature anomaly, number of days with low flow post-incubation and 0+ brown trout densities on 0+ grayling densities. The relationship with incubation temperature anomaly and 0+ brown trout abundance was positive, while increasing the number of days with low flow following incubation period negatively affected their survival. In addition, post-incubation temperature anomaly had a concave quadratic relationship with 0+ grayling densities, with threshold set at 13.5 °C. These outcomes are discussed in relation to future population persistence and management in a changing climate. Keywords: salmonids, recruitment, temperatur, discharge, Chalk streams Conference: XVI European Congress of Ichthyology, Lausanne, Switzerland, 2 Sep - 6 Sep, 2019. Presentation Type: Oral Topic: GRAYLING (GENUS THYMALLUS): EVOLUTION, SYSTEMATICS, MANAGEMENT AND CONSERVATION Citation: Basic T, Britton R, Cove R, Ibbotson A and Gregory S (2019). Roles of environmental factors in the recruitment of the European grayling Thymallus thymallus, near its southern range limit in England. Front. Mar. Sci. Conference Abstract: XVI European Congress of Ichthyology. doi: 10.3389/conf.fmars.2019.07.00071 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 24 Jun 2019; Published Online: 14 Aug 2019. * Correspondence: Dr. Tea Basic, Centre for Environment, Fisheries and Aquaculture Science (CEFAS), Lowestoft, United Kingdom, Tea.basic@cefas.co.uk Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Tea Basic Robert Britton Richard Cove Anton Ibbotson Stephen Gregory Google Tea Basic Robert Britton Richard Cove Anton Ibbotson Stephen Gregory Google Scholar Tea Basic Robert Britton Richard Cove Anton Ibbotson Stephen Gregory PubMed Tea Basic Robert Britton Richard Cove Anton Ibbotson Stephen Gregory Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.