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Recent interest in landscape re‐wilding and ecological restoration has resulted in a proliferation of large‐scale projects in many countries that have the potential to cause significant ecohydrological change. In Scotland an increasing number of watershed "restoration" schemes are motivated by declining Atlantic salmon populations and the threat of climate change. These usually involve riparian planting to shade salmon streams and re‐engineering of river channels to "enhance" salmon habitat. However, the need for, and objectives of, these schemes are often highly uncertain and there is no compelling scientific evidence to suggest that they are likely to be successful in halting salmon declines. Remarkably, these schemes ‐ which affect can affect rivers with the highest conservation designations in protected landscapes ‐ have been subject to limited environmental assessment. In some cases, engineering activities pose significant potential risk to juvenile salmon, and existing high quality salmon habitat may be degraded in the re‐engineering of streams. This commentary highlights the urgent need for more evidence‐based approaches in the management of complex ecohydrological systems. [ABSTRACT FROM AUTHOR]

Long‐term data are crucial for understanding ecological responses to climate and land use change; they are also vital evidence for informing management. As a migratory fish, Atlantic salmon are sentinels of both global and local environmental change. This paper reviews the main insights from six decades of research in an upland Scottish stream (Girnock Burn) inhabited by a spring Atlantic salmon population dominated by multi‐sea‐winter fish. Research began in the 1960s providing a census of returning adults, juvenile emigrants and in‐stream production of Atlantic salmon. Early research pioneered new monitoring techniques providing new insights into salmon ecology and population dynamics. These studies underlined the need for interdisciplinary approaches for understanding salmon interactions with physical, chemical and biological components of in‐stream habitats at different life‐stages. This highlighted variations in catchment‐scale hydroclimate, hydrology, geomorphology and hydrochemistry as essential to understanding freshwater habitats in the wider landscape context. Evolution of research has resulted in a remarkable catalogue of novel findings underlining the value of long‐term data that increases with time as modelling tools advance to leverage more insights from "big data". Data are available on fish numbers, sizes and ages across multiple life stages, extending over many decades and covering a wide range of stock levels. Combined with an unusually detailed characterization of the environment, these data have enabled a unique process‐based understanding of the controls and bottlenecks on salmon population dynamics across the entire lifecycle and the consequences of declining marine survival and ova deposition. Such powerful datasets, methodological enhancements and the resulting process understanding have informed and supported the development of fish population assessment tools which have been applied to aid management of threatened salmon stocks at large‐catchment, regional and national scales. Many pioneering monitoring and modelling approaches developed have been applied internationally. This history shows the importance of integrating discovery science with monitoring for informing policy development and assessing efficacy of management options. It also demonstrates the need to continue to resource long‐term sites, which act as a focus for inter‐disciplinary research and innovation, and where the overall value of the research greatly exceeds the costs of individual component parts. [ABSTRACT FROM AUTHOR]

The article discusses determination of size of Atlantic salmon an important characteristic in determining the number and size of eggs spawned by salmonids. It mentions that reductions in female salmon size played a relatively minor role in determining long-term trends in egg deposition compared with abundance, the role of size remains important. It presents information on importance of changing abundance and size in determining long-term trends in egg deposition.

The importance of conceptualizing the dynamics of storage-driven saturation area connectivity in runoff generation has been central to the development of TOPMODEL and similar low parameterized rainfall-runoff models. In this contribution, we show how we developed a 40-year hydrometric data base to simulate storage-discharge relationships in the Girnock catchment in the Scottish Highlands using a simple conceptual model. The catchment is a unique fisheries reference site where Atlantic salmon populations have been monitored since 1966. The modelling allowed us to track storage dynamics in hillslopes, the riparian zone and groundwater, and explicitly link non-linear changes of streamflows to landscape storage and connectivity dynamics. This provides a fundamental basis for understanding how the landscape and riverscape are hydrologically connected and how this regulates in-stream hydraulic conditions that directly influence salmonids. We use the model to simulate storage and discharge dynamics over the 40-year period of fisheries records. The modelled storage-driven connectivity provides an ecohydological context for understanding the dynamics in stream flow generation which determine habitat hydraulics for different life stages of salmon population. This new, long-term modelling now sets this variability in the riverscape in a more fundamental context of the inter-relationships between storage in the landscape and stream flow generation. This provides a simple, robust framework for future ecohydrological modelling at this site, which is an alternative to more increasingly popular but highly parameterized and uncertain commercial ecohydrological models. It also provides a wider, novel context that is a prerequisite for any model-based scenario assessment of likely impacts resulting from climate or land use change. Copyright © 2016 The Authors Hydrological Processes Published by John Wiley & Sons Ltd. Copyright © 2016 The Authors Hydrological Processes Published by John Wiley & Sons Ltd. [ABSTRACT FROM AUTHOR]

Flow regulation is a major constraint on fish populations in many river systems. Current scientific understanding of the direct and indirect influence of hydrology on salmonids between spawning and emergence is reviewed and used to identify key knowledge gaps. The review emphasises difficulties in linking ecological response to physical process understanding and, conversely, the difficulties in applying detailed process understanding to large-scale management. It is suggested that the lack of process understanding in the development of current environmental standards reflects differences in perspective, language and scale consideration between researchers and river managers. This presents considerable difficulties for the development of new and more defensible environmental standards. Although upscaling of process understanding should remain a focus for researchers, it is also necessary to acknowledge the 'uniqueness of place' and consequently that local data collection may frequently be required. [ABSTRACT FROM AUTHOR]

This study investigated the effects of diurnal temperature variability (>7° C) on the growth of 1+ year Atlantic salmon Salmo salar. Experimental manipulation of water temperature was used to simulate: (1) constant and (2) naturally varying thermal regimes with similar daily mean values. Data from two replicates of four treatments (two thermal and two feeding regimes) were collected over 6 months corresponding to the main spring to summer growth period. Fish growth was assessed at fortnightly intervals. Small but significant differences in mean fork length ( L) and mass were observed between temperature treatments, with smaller, lighter fish under the variable temperature regime. The effects of temperature regime on growth were independent of food ration. At termination of the experiment, the median L and mass of fish exposed to the variable temperature regime were estimated, respectively, to be 2· 6 and 8· 0% less than those under the constant regime. Given the relatively small differences in growth attributable to variable temperature regime in these experiments, it is suggested that mean daily temperatures are adequate to inform juvenile growth models for field-based studies. [ABSTRACT FROM AUTHOR]

This case study sought to examine how temporal variability in hydrological and hydraulic conditions might affect the feeding opportunities of juvenile Atlantic salmon in two hydrologically contrasting years of 2002 and 2003, which were characterised by high and low flows respectively. Firstly, measures of hydraulic influence were calculated to define what might be ecologically meaningful disturbance periods during high flows. Secondly, for identifying such periods, the parameter Critical Displacement Velocity (CDV) was derived from a river discharge time series as a first approximation of the amount of time in two hydrologically extreme years when fish foraging strategies for specific age classes of juvenile Atlantic salmon ( Salmo salar) might be disrupted by flows. The CDV estimates the threshold velocity above which juvenile salmon are unable to hold station and it is dependent upon fish size and stream temperature. In the wet year 2002, the CDV was exceeded on 18% and 21% of days for 0+ and 1+ fish respectively. In 2003 these respective numbers fell to 6% and 15%. The data suggest that hydrological conditions during certain times of the year have the potential to affect foraging behaviour. This in turn might have implications for recruitment and growth rates for juvenile salmon in upland streams. [ABSTRACT FROM AUTHOR]

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The hydraulic and sedimentary characteristics of the spawning habitat of Atlantic salmon (Salmo salar) in tributary and mainstem locations in a river system in north-east Scotland are described. Salmon used spawning sites with a relatively wide range in sediment characteristics, although measures of central tendency were all in the gravel (2–64 mm) size-class. The dominant factor differentiating the sediment characteristics of study sites was the level of fine sediment, which accounted for significant differences between tributary and mainstem samples. The ranges of depth and velocity in areas used for spawning by salmonids were found to be similar in all tributary study sites. However, due to the interdependence of depth and velocity, major differences were observed between tributary and mainstem study sites in that spawning in larger streams tended to be associated with deeper, faster flowing water. Spawning locations were shown to have similar Froude number, despite different sized streams and species of salmonid. Due to its dimensionless nature and significance in characterising flow hydraulics, the Froude number is proposed as a potentially useful variable for describing the habitat of aquatic organisms. [Copyright &y& Elsevier]

This paper relates spatial patterns of spawning by Atlantic salmon (Salmo salar L.) to channel geomorphic and hydrologic characteristics in two upland Scottish streams. The full length of each stream was surveyed and classified using a process-based typology. The precise locations of 845 spawning events were mapped over 3 successive years. These locations were classified by reach type, and a location-specific discharge was calculated for each spawning event. Fish used ‘response’ reach types (pool-riffle and transitional pool-riffle/plane bed) preferentially, avoiding ‘transport’ reach types (plane bed and step pool). The spatial distribution of reach types contrasted markedly between the streams, and as a consequence, the distribution of spawning activity differed. In each stream, fish utilised relatively high flows for spawning with almost all spawning occurring at discharges greater than location-specific median annual flows, while optimum discharges (defined as those where values of use most exceeded values of availability) were as high as two to three times greater than reach median flows. In each stream, spawning occurred over a greater flow range in response than transport reach types, primarily as a result of higher maximum utilised flows in the former. Because the channel classification procedure is process based (and therefore transferable to other geographic areas), the present study suggests that reach type may be used to predict the spatial distribution of Atlantic salmon spawning activity within a catchment. Since the data indicate that discharge use by spawning fish is reach-type specific, the results also suggest that channel morphology should be considered when identifying environmental flow requirements. [Copyright &y& Elsevier]

Many upland rivers in the Northern Hemisphere contain important habitat for Atlantic salmon ( Salmo salar L.). Owing to their sensitivity to environmental change, salmon are often used as bio-indicators. In Scotland, rivers containing potentially suitable habitat for salmon fry are often also regulated for hydropower. Regulated flow regimes can differ substantially spatially and temporally. Thus, where river management may be needed to maintain, restore, and protect their ecological functioning, this needs to be based on evidence of such spatio-temporal effects. This study investigated the effects of different types of river regulation on the hydraulic characteristics of downstream river reaches and the inferred consequences for salmon fry using hydraulic habitat quality models. The study focussed on the River Lyon (390 km 2 ), a tributary of the Tay (4587 km 2 ), Scotland, UK. Hydraulic habitat variability was assessed for three reach-scale sites with contrasting flow regimes characterised by (a) releases from hydropower generation, (b) compensation flow and (c) partly re-naturalised flow conditions. For each site, high resolution Digital Terrain Models (DTMs) were developed from bathymetric surveys and 2D hydraulic models were used to assess hydraulic characteristics. Discharge time series were used to simulate hydraulic conditions for regulated and simulated natural flows. Depth and velocity data were extracted from the hydraulic models and used to infer habitat quality using a habitat model developed for Atlantic salmon fry in similar-sized Scottish rivers. Results showed the effects of regulation can vary substantially within reaches and between seasons. Comparison to natural flow regimes suggested that flow alteration has a variable influence on habitat quality depending on the type of regulation and time of year. This work has improved understanding of the effects of regulation on biophysical processes and may also be useful for managing trade-offs between management, restoration, and societal benefits. [ABSTRACT FROM AUTHOR]