Need for ecohydrological science in restoration and evidence-based adaptive management

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.

In spring 2022, pink salmon Oncorhynchus gorbuscha smolts were recorded in the UK. Fish were caught in the Rivers Thurso and Oykel in Scotland between 13 and 17 March. To the authors' knowledge, this is the first observation of O. gorbuscha smolts in Europe outside the Scandinavian and Kola peninsulas, including other tributaries of the White and Barents Seas. It also provides evidence of successful spawning in 2021 and completion of the freshwater phase of the life cycle, and indicates the possibility for potential establishment of an O. gorbuscha population in Great Britain.

Migration from fresh water to the marine environment is a crucial, transitional stage in the development of Atlantic salmon (Salmo salar). This study used a combination of acoustic tracking, instrument data, and hydrodynamic modelling to examine behaviour of juvenile salmon (smolts) during their transition from fresh water to the marine environment. The study focuses on a high-energy coastal environment in northern Scotland, which is currently being developed for renewable energy extraction and where there is potential for negative impacts on salmon with energy extraction devices and structures. Thirty-four smolts were captured in the River Wick in Caithness and tagged with acoustic tags transmitting at 69 kHz. The Telemac-Mascaret modelling suite was utilized to construct a three-dimensional model of the study area and surrounding waters to estimate smolt-current interactions during detection times. Timing of migration was linked to low-light conditions, with smolts mainly exiting the river at night and when the moon was below the horizon. The movement of most of the tags conformed with modelled tidal currents and the tracks matched modelled marine tidal patterns. Smolts were detected only on a single tide suggesting that they rapidly cleared the vicinity of the receiver array.

In the UK, large areas of blanket bogs were afforested with non-native conifers between the 1960s and the 1980s. Following recognition of the detrimental effects of such practice on biodiversity and carbon stocks, large-scale restoration trials started in the late 1990s and are further supported by recent changes in policy. The removal of forestry from peatlands is likely to be a widespread land-use change in the coming decades and could affect adjacent freshwater systems. This study aimed to investigate whether forestry removal with drain blocking affected nearby spawning sites used by Atlantic salmon (Salmo salar). We analysed the chemistry of hyporheic (beneath and just above the streambed) and surface water, and measured sediment deposition upstream of, within and downstream of a forestry block in the north of Scotland, during and after restoration management operations. We found no immediate effect of management except on potassium and zinc concentrations, which increased after restoration. The general lack of effect is attributed to catchment properties, including the small proportion of catchment (< 5 %) affected by management, and to dilution effects related to heavy precipitation during the intervention phase. We suggest that longer-term monitoring should be implemented as the sizes of areas undergoing restoration management increases.

Modelling an Atlantic salmon (Salmo salar) population as three phenotypically homogeneous sub-stocks showed different sub-stock dynamics and time-trends, a finding which was robust to three quite extreme variations of marine-mortality scenarios. A process-centred demographic analysis of a phenotypically mixed population (river North Esk, eastern Scotland) separated the life-cycle into two portions: density-dependent (spawner to smolt), represented by a stock-recruitment relationship; density-independent at sea and in both the estuary and river during

Conservation stocking is frequently used by fishery managers to stabilize or increase production of depleted fish stocks. However, in the case of salmonids the benefits are increasingly questioned and generally poorly quantified. We investigated the effects of ova stocking on freshwater emigrant production in a declining Scottish population of "spring" Atlantic Salmon Salmo salar. The stocking program was designed to minimize risks, maximize expected benefits, and bypass population bottlenecks between the ova and fry stages. Long-term data (33 cohorts over 42 years, including 8 years of stocking) on numbers of ova and juvenile emigrant production were used to investigate whether the ova-emigrant stock-recruitment relationship differed between conditions of natural spawning and stocking. We considered Ricker and Beverton-Holt models that included terms for the effects of stocking, intercohort competition, and changes in trap efficiency on emigrant production. The "best model" was considered to be that with the lowest corrected Akaike information criterion (AICc). The strength of evidence for competing models was provided by smooth AICc weights. There was strong support for intercohort competition whereby large cohorts reduced survival rates of subsequent cohorts. There was little support for a stocking effect, and the best model did not include stocking. Changes in juvenile emigrant production could have been detected with 80% power if those changes had increased by an average of around 24% over the eight stocked years, indicating relatively high power to detect stocking effects. Although focused on a particular river system and set of ova stocking protocols, this study suggests that stocking fails to increase Atlantic Salmon production where wild fish populations and suitable habitat remain.

Atlantic salmon (Salmo salar) are iconic and economically important fish, but their migratory behaviour during passage through Scottish coastal seas is not well understood, and there are several key questions which need to be answered in order to predict possible interactions with marine renewable energy arrays. Do migrating salmon (adults and post-smolts) travel through marine renewable energy development areas? Will migrating fish actually encounter arrays of devices as they pass through these areas? What effects might these encounters have on migrating salmon, and what might be the consequences of these interactions for fish populations? In response to these questions, the Environmental Research Institute has established the Pentland Salmon Initiative - a new partnership which aims to engage a growing number of organisations with interests or experience relevant to marine renewables and salmon migration, with a particular focus on the Pentland Firth. This site represents not only a potential bottleneck for migrating salmon but also a key site for the developing marine energy sector. We are actively seeking to build and extend collaborative programmes under four broad themes, which are outlined below.

There have been few mechanistic studies linking local ecological characteristics of streams to morphological pressures. This paper reports the findings of an investigation of the impacts of canalisation on salmonid habitat. Habitat use by Atlantic salmon and brown trout, fry and parr was assessed using Generalised Additive Models (GAMs) based on seasonal electrofishing survey data and output from 2-dimensional hydraulic models. By overlaying the habitat models onto the physical characteristics of the canalised reach, it was possible to determine where and when environmental bottlenecks occurred, and for which species and life stages. The canalised reach was characterised by relatively uniform sedimentary and hydraulic characteristics compared to other reaches. Although it was generally well suited to fry, the lack of coarser substrate and low velocity areas during winter made the canalised reach unfavourable for salmon and trout parr respectively. Given the insights provided by this study, we suggest the approach of combining physical channel characterisation with locally derived seasonal habitat models is well suited to future research focussed on assessing the ecological impact of morphological pressures.

Spawning patterns in female brown trout Salmo trutta were examined by documenting the construction of nests in a small stream and later excavating them to recover progeny. The maternal provenance of nests was determined by genetic typing of embryos using microsatellite markers. Seventy-two nests, for which position and date of construction were known, were made by 59 individuals. Position and date of construction were known for a further 35 nests, comprising 11 Atlantic salmon Salmo salar nests and 24 nests which contained few or no progeny. Salmo trutta showed a behavioural preference for spawning near (≤1 m) prior nests; nests made by different individuals tended to accumulate in a spatial sequence that progressed upstream. The directionality of the association between prior and new nests suggests that later spawners use the residual depressions created by previous spawners as the first element of their own nests.

Interstitial velocity within spawning bed substrates is frequently used to predict salmonid embryo survival and often forms the basis of habitat assessment or sediment-based predictive models. In this study, the relationships between interstitial velocity, hyporheic dissolved oxygen (DO), and survival of Atlantic salmon Salmo salar embryos were examined in an experimental field study that incorporated natural complexity in the hyporheic environment. Hyporheic DO and interstitial velocity were not clearly related over the observed range of velocities. There was strong evidence of a relationship between survival and DO but only weak evidence of a relationship with interstitial velocity. Conceptual models of hyporheic environmental variation and salmonid embryo survival should include the appropriate range of environmental processes. Given the findings of this study, we suggest that interstitial velocity should not be used to assess or predict spawning habitat quality; direct measurements of DO offer a more defensible approach.

Agricultural practices have the potential to influence hyporheic exchange through increased fine sediment loads and through reduced hydraulic conductivity, bed roughness and morphological diversity. These impacts can reduce connectivity between stream and hyporheic waters to the detriment of salmonid embryos. Salmonids bury their eggs in streambed gravels, typically to depths of up to 300 mm and for incubation periods of up to 6 months. Embryo survival is dependant on a complex range of factors which critically includes the delivery of oxygen from surface waters. This paper investigates hyporheic zone exchange processes, oxygen concentration and embryo survival in a canalised agricultural stream at a range of nested spatiotemporal scales. Results are contrasted with those from the Girnock Burn, a relatively undisturbed catchment. Conservative tracer experiments were used to assess reach average hyporheic exchange using the USGS OTIS model. Artificial redds were used to assess reach scale variability in hyporheic processes. Incubation stacks were used to assess embryo mortality and hyporheic water quality with depth at the scale of individual redds. Optodes (optical dissolved oxygen sensors) were used at a sub-set of sites to assess fine scale temporal variability in hyporheic oxygen and temperature. Stream and hyporheic hydrochemistry were used to infer source water contributions and provenance. Data showed that reach average hyporheic exchange in the Newmills Burn was ca. five times less than in the Gimock Burn. Dissolved oxygen (DO) concentrations related strongly to spatiotemporal patterns of groundwater (GW) discharge. Embryo survival was significantly correlated with DO. It is suggested that low DO in the Newmills Burn relates to groundwater upwelling and limited hyporheic exchange caused by a combination of low morphological diversity, hydraulic conductivity and bed roughness. It is suggested that future studies of embryo survival should look beyond the single issue of fine sediment effects to include a broader understanding of hyporheic zone processes.

This paper reports an investigation of stock–recruitment relations for Atlantic salmon ( Salmo salar ). We regard these relations as stochastic functions characterized by an expected stock–recruitment relation and deviations from this expectation driven by observational error and uncharacterised environmental variability. We estimate model parameters by standard Bayesian methods. Analysis of the input–output characteristics of segments of the salmon life cycle in the Girnock Burn (Northeast Scotland) reveals two independent regulatory processes, one between ova and fry and the other between fry and smolts. Comparison of stock–recruitment relations for Atlantic salmon in Scotland, Ireland, and Canada, reinforced by an extended series of simulation studies, shows that even when comparatively long time series of high quality data are available, it is frequently difficult to exclude the possibility of low stock depensation — an effect whose implication of enhanced extinction risk implies that precautionary management policy would pay special attention to the posssibility of its occurence. A particular feature of our simulation results is their demonstration that inappropriate combination of distinct subpopulations both increases process noise and distorts the expected stock–recruitment relation, thereby greatly reducing the accuracy of any system identification process.

P>Individually tagged farmed Atlantic salmon, Salmo salar L., were released from fish farms in simulated escapes in Scotland (n = 678) and Norway (n = 597) to compare migratory behaviour and dispersal. Large fish (510-870 mm fork length) were released to simulate the escape of aquaculture growers. Fish were released in spring and all recaptures of tagged fish were reported during summer and autumn of the year of release. Recapture rates were respectively 7.0 and 0.6% of the salmon released in Norway and Scotland, indicating a higher mortality or a lower exploitation or reporting rate for Scottish fish. Recaptures of Norwegian fish were all from Norway and mostly within 150 km of the release site; 64% were taken by anglers in fresh water. By contrast, the three salmon recaptured from the release in Scotland were reported from Norway (Hardangerfjord and Lofoten) and western Sweden (River Gota); two detached tags were found on beaches in Scotland north of the release site. These findings establish the capacity for long distance dispersal among escapees from aquaculture facilities and suggest a net easterly bias in long distance dispersal of salmon escaping from Scottish fish farms.

Sea age, size, and condition of adult Atlantic salmon (Salmo salar) are prime determinants of individual, and hence population, productivity. To elucidate potential mechanisms, 151 000 records of salmon returning to six Scottish coastal sites over 44 years were analysed for length, weight, and condition, by site, sex, sea age, and river age. After correcting for capture effort biases, all sites showed seasonal increases in length and weight for both 1 sea winter (1SW) and 2SW fish. However, whereas condition increased slightly with season for 2SW, it decreased notably for 1SW. Sites showed common decadal trends in length, weight, and condition. Within years, length and weight residuals from trends were coherent across sites, but residuals from condition trends were not. Rates of seasonal condition change also showed decadal trends, dramatically different between sea ages, but common across sites within sea-age groups. Longer salmon were disproportionately heavy in all seasons. 1SW condition was markedly lower in 2006. Detrended correlations with oceanic environmental variables were generally not significant, and always weak. A published correlation between the condition of 1SW salmon caught at a single site and sea surface temperatures in the Northeast Atlantic could not be substantiated for any of the six fisheries over the wider time-scales examined.

There is increasing realisation of the importance of groundwater–surface water (GW–SW) interactions in understanding freshwater ecology. A study that assessed the influence of local GW–SW interactions on shallow (<250 mm) hyporheic water quality at two contrasting salmon spawning locations in Scotland, UK is reported. At a groundwater-dominated site, continuous logging sensors revealed that hyporheic dissolved oxygen (DO) concentrations changed rapidly in response to changing hydrological conditions. Low volume (25 ml) spot samples revealed fine-scale spatial variability (<0.05 m) consistent with a vertically shifting boundary layer between source waters. At a surface-water-dominated location, hyporheic water was typically characterised by high DO and electrical conductivity values, characteristic of surface water. Small reductions in DO at this site are hypothesised to be associated with short residence hyporheic discharge. A comparison between in-situ (logging DO sensor data) and ex-situ (small volume sampling) methods revealed good agreement, potentially allowing deployment of the two methods in stratified sampling programmes. This study demonstrates that hyporheic water quality varies over fine spatial and temporal scales and that future studies need to design sampling strategies that consider the scales appropriate to both the ecology and the hyporheic processes of interest.

Over a 3.5 year period, levels of dissolved oxygen (DO) saturation were continuously monitored in surface waters and at depths of 150 and 300 mm in the hyporheic zone of a riffle in a montane stream where Atlantic salmon spawn. Throughout this period, DO in surface waters remained close to 100% saturation, but exhibited daily variations related to CO(2) cycling driven by diurnal patterns of respiration and photosynthesis. However, in the hyporheic zone, variations were much more dynamic over storm event, seasonal and inter-annual timescales. At 300 mm, DO saturation was generally close to 100% during summer low flows, though levels occasionally fell during warm periods which appeared to be related to diffusion gradients caused by benthic respiration. Such DO decreases at low flows were much more common and marked at 150 mm. During wetter conditions, DO saturation at 300 mm fell to zero for prolonged periods; this is consistent with increased fluxes of groundwater discharging through the hyporheic zone. During the wettest periods this also affects DO saturation at 150 mm. However, during hydrological events, hyporheic water quality is 're-set' as head reversals cause streamwater ingress which results in transient periods of re-oxygenation, which end during the hydrograph recession. This is consistent with stream-ward hydraulic gradients being re-established in riparian ground water as the stream stage falls. The connectivity between groundwater and streamwater through the hyporheic zone is driven by climatic conditions and is reflected in marked inter-annual variability in water quality characteristics. In some cases, this variability may have implications for the ecology of the hyporheic environment-including the survival of salmon eggs-particularly if oxygen levels are affected. Copyright (C) 2009 John Wiley & Sons, Ltd.