The migratory behavior of Atlantic salmon (Salmo salar) post-smolts in coastal waters is poorly understood. In this collaborative study, 1914 smolts, from 25 rivers, in four countries were tagged with acoustic transmitters during a single seasonal migration. In total, 1105 post-smolts entered the marine study areas and 438 (39.6%) were detected on a network of 414 marine acoustic receivers and an autonomous underwater vehicle. Migration pathways (defined as the shortest distance between two detections) of up to 575 km and over 100 days at sea were described for all 25 populations. Post-smolts from different rivers, as well as individuals from the same river, used different pathways in coastal waters. Although difficult to generalize to all rivers, at least during the year of this study, no tagged post-smolts from rivers draining into the Irish Sea were detected entering the areas of sea between the Hebrides and mainland Scotland, which is associated with a high density of finfish aquaculture. An important outcome of this study is that a high proportion of post-smolts crossed through multiple legislative jurisdictions and boundaries during their migration. This study provides the basis for spatially explicit assessment of the impact risk of coastal pressures on salmon during their first migration to sea.

Untangling the patterns and proximate drivers of intraspecific genetic and phenotypic structuring informs our understanding of the evolutionary processes shaping diversity. This study investigated morphological and genetic structuring of brown trout (Salmo trutta L.) populations across varying spatial scales in a single, complex, dendritic river catchment and examined the potential natural and anthropogenic environmental features driving this structuring. Morphometric and hierarchical genetic structuring analyses of fish from 22 sampling sites in the River Foyle catchment, Ireland ( 4500km2) identified 19 morphologically distinct groups and 15 genetically distinct populations, separated by river distances ranging from 0.4 km to 188 km. Isolation by Distance was the main factor shaping both genetic and morphological divergence, indicating that strong philopatry is one of the major drivers of the observed population structuring in this system. However, both natural and anthropogenic environmental variables also explained pairwise genetic and morphological differences between sampling sites. Thus, the pairwise differences in the area of woodland in the upstream catchment, water phosphorus concentration, biological oxygen demand, catchment slope, urban area in upstream catchment, altitude, site specific percentage of canopy cover and dissolved oxygen concentration in the river channel were correlated with genetic divergence. The pairwise differences in the concentration of suspended solids, the extent of bankside overhang, the composition of bedrock, boulder and cobble substrates, watercourse width, catchment slope and site altitude were correlated with between-site morphological differences. We hypothesise that local differential selection pressures comprising both natural environmental variation and variation resulting from anthropogenic effects, in combination with strong philopatry and random genetic processes drive the clearly defined genetic and phenotypic patterns described here.

The freshwater phase of the first seaward migration of juvenile Atlantic salmon (Salmo salar) is relatively well understood when compared with our understanding of the marine phase of their migration. In 2021, 1008 wild and 60 ranched Atlantic salmon smolts were tagged with acoustic transmitters in 12 rivers in England, Scotland, Northern Ireland and Ireland. Large marine receiver arrays were deployed in the Irish Sea at two locations: at the transition of the Irish Sea into the North Atlantic between Ireland and Scotland, and between southern Scotland and Northern Ireland, to examine the early phase of the marine migration of Atlantic salmon smolts. After leaving their natal rivers' post-smolt migration through the Irish Sea was rapid with minimum speeds ranging from 14.03 to 38.56 km.day-1 for Atlantic salmon smolts that entered the Irish Sea directly from their natal river, to 9.69-39.94 km.day-1 for Atlantic salmon smolts that entered the Irish Sea directly from their natal estuary. Population minimum migration success through the study area was strongly correlated with the distance of travel, populations further away from the point of entry to the open North Atlantic exhibited lower migration success. Post-smolts from different populations experienced different water temperatures on entering the North Atlantic. This was largely driven by the timing of their migration and may have significant consequences for feeding and ultimately survivorship. The influence of water currents on post-smolt movement was investigated using data from previously constructed numerical hydrodynamic models. Modeled water current data in the northern Irish Sea showed that post-smolts had a strong preference for migrating when the current direction was at around 283° (west-north-west) but did not migrate when exposed to strong currents in other directions. This is the most favorable direction for onward passage from the Irish Sea to the continental shelf edge current, a known accumulation point for migrating post-smolts. These results strongly indicate that post-smolts migrating through the coastal marine environment are: (1) not simply migrating by current following (2) engage in active directional swimming (3) have an intrinsic sense of their migration direction and (4) can use cues other than water current direction to orientate during this part of their migration.

Polar halogen chemistry has long been known to be active, especially in spring, and has an important influence on the lifetime of some volatile organics, ozone, and mercury. Reactive chlorine and bromine species, produced from snow and aerosols, can have significant impacts on the oxidative capacity of the polar boundary layer. However, halogen production mechanisms from snow remain highly uncertain, making it challenging to include descriptions of halogen snow emissions in models and to understand the impact on atmospheric chemistry. In this work, we investigate the role of Arctic chlorine and bromine emissions from snow on boundary layer oxidation processes using a one-dimensional atmospheric chemistry and transport model (PACT-1D). We explore the impact of halogen snow emissions and boundary layer dynamics on atmospheric chemistry by modelling primary emissions of Cl2 and Br2 from snow, and heterogeneous recycling reactions on snow and aerosols. We present a two-day case study from the 2009 Ocean-Atmosphere-Sea Ice-Snowpack (OASIS) campaign at Utqiagvik, Alaska. The model reproduces both the diurnal cycle and high quantity of Cl2 measured, along with the observed concentrations of Br2, BrO, and HOBr. Due to a combination of chemical emissions, recycling, vertical mixing, and atmospheric chemistry, reactive chlorine is confined to the lowest 15 m of the atmosphere, whilst bromine impacts chemistry up to the boundary layer height. Following the inclusion of halogen emissions and recycling, HOx concentrations (HOx = OH+HO2) increase by as much as a factor of 30 at the surface at mid-day. Consequently, volatile organic compound (VOC) lifetimes are significantly reduced within a shallow layer near the surface, due to chlorine atoms from Cl2 snow emissions and increased HOx attributable to halogen chemistry.

Reactive chlorine and bromine species emitted from snow and aerosols can significantly alter the oxidative capacity of the polar boundary layer. However, halogen production mechanisms from snow remain highly uncertain, making it difficult for most models to include descriptions of halogen snow emissions and to understand the impact on atmospheric chemistry. We investigate the influence of Arctic halogen emissions from snow on boundary layer oxidation processes using a one‐dimensional atmospheric chemistry and transport model (PACT‐1D). To understand the combined impact of snow emissions and boundary layer dynamics on atmospheric chemistry, we model Cl2 and Br2 primary emissions from snow and include heterogeneous recycling of halogens on both snow and aerosols. We focus on a 2‐day case study from the 2009 Ocean‐Atmosphere‐Sea Ice‐Snowpack campaign at Utqiaġvik, Alaska. The model reproduces both the diurnal cycle and high quantity of Cl2 observed, along with the measured concentrations of Br2, BrO, and HOBr. Due to the combined effects of emissions, recycling, vertical mixing, and atmospheric chemistry, reactive chlorine is typically confined to the lowest 15 m of the atmosphere, while bromine can impact chemistry up to and above the surface inversion height. Upon including halogen emissions and recycling, the concentration of HOx (HOx = OH + HO2) at the surface increases by as much as a factor of 30 at mid‐day. The change in HOx due to halogen chemistry, as well as chlorine atoms derived from snow emissions, significantly reduce volatile organic compound lifetimes within a shallow layer near the surface.

Facultative migration occurs when, in response to prevailing conditions, individuals in a population may (or may not) undertake a migration. The brown trout ( Salmo trutta ) is a species that exhibits facultative migration, where some individuals within popula tions may move to mainstem rivers (fluvial–adfluvial migration), lakes (lacustrine–ad fluvial migration), estuaries (partial anadromy) or sea (anadromy) to feed, while others remain resident. This study attempts to separate two alternative hypotheses for the population structuring that underpins the expression of facultative migration in this species: (a) that anadromous and nonanadromous fish comprise two gene pools; (b) that individual genetic variation or individual variation in gene–environment inter actions is responsible for the expression of different life-history tactics within the same gene pool. The study design involved sampling and analyses of anadromous and nonanadromous brown trout from three independent tributary rivers known to produce (sea-run) trout within the same catchment. Results indicate that, in all cases, population genetic divergence was linked to geographical location and not to life-history tactics. Two genetically distinct coexisting population pairs were identified in two separate tributaries. Despite similar environmental conditions in both tributar -ies, the frequency of each life-history tactic (anadromy vs. nonanadromous) within these population pairs differed significantly. The results of this study support the hypothesis that facultative migration in brown trout is likely to be driven by a quanti tative threshold trait, where the threshold value varies both among populations and among individuals within populations.

This study investigated the cumulative impact of weirs on the downstream migration of wild Atlantic salmon (Salmo salar) smolts in the River Foyle, Northern Ireland. In spring of 2013 fish were released in two tributaries of similar length; one tributary (impacted) had seven low-head weirs along the migration pathway and the other was devoid of such structures (un-impacted). Salmon smolts fitted with acoustic transmitters were monitored via a passive acoustic telemetry array during downstream migration. In 2014 the study was repeated only in the impacted tributary. Overall freshwater survival rates were high (>94%). There was no significant difference in mortality, movement pattern, delay or travel speeds between rivers or between years at any phase of migration. Escapement of salmon smolts through Lough Foyle (a marine sea lough) to the open ocean was low, approximately 18% in each year. Escapement did not differ between impacted and un-impacted rivers. This study showed no postpassage effects of weirs on mortality, migration speed or escapement of downstream migrating smolts. This suggests that the elevated mortality at low-head obstacles described in other studies is not inevitable in all river systems. Migration through rivers with natural riffle-pool migration may result in similar effects as those from low-head weirs. Causes of apparent high mortality in the early part of marine migration in this study, are unknown; however similar studies have highlighted the impact of fish predators on smolts.

Atlantic salmon populations appear to fluctuate stochastically through time. It is suspected that both density-dependent and density-independent factors cause these fluctuations but the relative importance of each, and the life stages at which they operate, is not well known. In this study, a long-term data set on Atlantic salmon migrants returning to the Foyle catchment, Ireland, was used to determine the role of density-dependent and life stage-specific environmental factors regulating population size. A Ricker density-dependent model showed that spawning adult population size significantly predicted variation in the resultant filial generation; however, a large amount of variation (ca. 68%) remained unexplained. It was shown that environmental factors were significant in explaining some of the remaining variance and that these influences were linked to specific life stages. Three life stages-spawning and incubation, fry emergence and marine survival-were shown to have significant environmental effects that resulted in changes in the returning cohort strength. It is concluded that these life stage-specific environmental effects are likely to contribute to the stochastic variation in population size resulting from the application of traditional stock-recruitment models. The identification and quantification of these effects should allow improved model accuracy.

Electrofishing techniques are widely used for the estimation of the size of stream-dwelling fish populations for both fishery management and scientific study. In contrast to multiple pass, population depletion methods, single-pass catch-per-unit-effort (CPUE) methods are less time-consuming and labour-intensive. A possible issue with the commonly used fixed total time CPUE protocol is that it does not differentiate between the time spent actively fishing and the time incurred while not actively fishing (e.g. removing fish from nets and navigating the site). This issue is likely related to fish density. This study compared two methods of CPUE electrofishing and tested the hypothesis that time spent handling fish and navigating a site can be a source of error in the commonly used fixed total time electrofishing method. Seventy-one sites were sampled across three subcatchments in the Foyle catchment in Northern Ireland. We found a difference in the catch per unit time between the two methods and that this difference increased with fish density. The fixed-time CPUE method also failed to detect a species presence in low-density sites.

The behaviour of returning Salmo salar (Linnaeus, 1758) approaching, and attempting to pass low-head weirs remains relatively unknown. A radio telemetry array was created at a low-head weir to enable the behaviour of S. salar ( n = 120) to be observed as they approached and attempted to pass the barrier. The majority of fish successfully passed the barrier on their first or second attempt, some individuals required 11 attempts prior to successful passage occurring. Mean delay at the barrier per fish was 47.8 h (±SD 132.0 h), range 15 min to 31 days. Passage success on a fish’s initial attempt was significantly predicted by the amount of searching a fish undertook, fork length, and fat content. Fish were more likely to have a successful first passage attempt if it was smaller with a low fat content and exerted a greater effort in searching for a passage channel. Small-scale barriers cause delays and increased energy expenditure in migrating fish. Barriers may be creating an anthropogenic selection pressure for traits which enable passage success. The impact of a delay at a barrier and increased energy expenditure on reproduction and gonad development remains unknown but is likely to be negatively impacted by instream anthropogenic structures.

Globally, there are a large and growing number of researchers using biotelemetry as a tool to study aquatic animals. In Europe, this community lacks a formal network structure. The aim of this study is to review the use of acoustic telemetry in Europe and document the contribution of cross-boundary studies and inter-research group collaborations. Based on this, we explore the potential benefits and challenges of a network approach to identify future priorities and best practices for aquatic biotelemetry research in Europe.

A fixed receiver array was used to examine the movement patterns and space use of the European eel Anguilla anguilla in an oligotrophic Irish lake between July and September. We assessed home range size, temporal change in spatial behaviour and activity patterns of broad-headed ( n = 11) and narrow-headed ( n = 8) morphotypes. Broad-headed individuals displayed a larger home range (mean KUD 95 (km 2 ):0.296 ± 0.04 S.E.) in comparison to narrow-headed individuals (mean KUD 95 (km 2 ):0.143 ± 0.02 S.E.). Eel activity was strongly dependent on light conditions. Narrow-headed individuals’ movement peaks occurred at dawn and dusk in comparison to broad-headed individuals which exhibited a more stable movement pattern throughout night and into dawn, suggesting that narrow-headed eels are more crepuscular in nature whereas broad-headed individuals are more nocturnal. Lunar phase period also influenced eel movement within the lake. These results provide valuable insights into the spatio-temporal distribution of yellow eels in a lake system, demonstrating that individuality in foraging behaviour has direct influence on spatial patterns.

Surface‐based temperature inversions (SBIs) occur frequently over Antarctica and play an important role in climate and weather. Antarctic SBIs are examined during the austral spring of 2010 using measurements from dropsondes, ERA‐Interim Atmospheric Reanalysis Model, and the recently released version 6 of the Infrared Atmospheric Sounding Interferometer (IASI) level 2 product. A SBI detection algorithm is applied to temperature profiles from these data sets. The results will be used to determine if satellite and reanalysis products can accurately characterize SBIs, and if so, then they may be used to study SBIs outside of the spring 2010 study period. From the dropsonde data, SBIs occur in 20% of profiles over sea ice and 54% of profiles over land. IASI and ERA‐Interim surface air temperatures are found to be significantly warmer than dropsonde observations at high plateau regions, while IASI surface air temperatures are colder over sea ice. IASI and ERA‐Interim have a cold bias at nearly all levels above the surface when compared to the dropsonde. SBIs are characterized by their frequency, depth, and intensity. It is found that SBIs are more prevalent, deeper, and more intense over the continent than over sea ice, especially at higher surface elevations. Using IASI and ERA‐Interim data the detection algorithm has a high probability of detection of SBIs but is found to severely overestimate the depth and underestimate the intensity for both data sets. These overestimation and underestimation are primarily due to the existence of extremely shallow inversion layers that neither satellite nor reanalysis products can resolve.

As part of the Uinta Basin Winter Ozone Study, January-February 2013, we conducted 937 tethered balloon-borne ozone vertical and temperature profiles from three sites in the Uinta Basin, Utah (UB). Emissions from oil and gas operations combined with snow cover were favorable for producing high ozone-mixing ratios in the surface layer during stagnant and cold-pool episodes. The highly resolved profiles documented the development of approximately week-long ozone production episodes building from regional backgrounds of similar to 40ppbv to >165ppbv within a shallow cold pool up to 200m in depth. Beginning in midmorning, ozone-mixing ratios increased uniformly through the cold pool layer at rates of 5-12ppbv/h. During ozone events, there was a strong diurnal cycle with each succeeding day accumulating 4-8ppbv greater than the previous day. The top of the elevated ozone production layer was nearly uniform in altitude across the UB independent of topography. Above the ozone production layer, mixing ratios decreased with height to similar to 400mabove ground level where they approached regional background levels. Rapid clean-out of ozone-rich air occurred within a day when frontal systems brought in fresh air. Solar heating and basin topography led to a diurnal flow pattern in which daytime upslope winds distributed ozone precursors and ozone in the Basin. NOx-rich plumes from a coal-fired power plant in the eastern sector of the Basin did not appear to mix down into the cold pool during this field study.

Mortality rates of wild Atlantic salmon Salmo salar smolts implanted with acoustic transmitters were assessed to determine if mortality was size dependent. The routinely accepted, but widely debated, ‘2% transmitter mass: body mass’ rule in biotelemetry was tested by extending the transmitter burden up to 12·7% of body mass in small [mean fork length (LF) 138·3 mm, range 115–168 mm] downstream migrating S. salar smolts. Over the short timescale of emigration (range 11·9–44·5 days) through the lower river and estuary, mortality was not related to S. salar size, nor was a relationship found between mortality probability and transmitter mass: body mass or transmitter length: LF ratios. This study provides further evidence that smolt migration studies can deviate from the ‘2% rule’ of thumb, to more appropriate study‐specific measures, which enables the use of fishes representative of the body size in natural populations without undue effects.

This study presents evidence of the decline in European eel abundance in a transitional water body in the Northern Ireland from 1967–2013. The available historical data is from an important period, representative of the baseline abundance of eels in a transitional water body before recruitment collapse. The results from this study indicate the current eel population in the Foyle estuary is 3.38% of historical levels.

The unimpeded downstream movement patterns and migration success of small female and male Anguilla anguilla through a catchment in north-west Europe were studied using an acoustic hydrophone array along the River Finn and into the Foyle Estuary in Ireland. Twenty silver-stage A. anguilla (total length, LT , range: 332-520 mm) were trapped 152 km upstream from a coastal marine sea-lough outlet and internally tagged with acoustic transmitters of which 19 initiated downstream migration. Migration speed was highly influenced by river flow within the freshwater (FW) compartment. Anguilla anguilla activity patterns were correlated with environmental influences; light, tidal direction and lunar phase all influenced the initiation of migration of tagged individuals. Migration speed varied significantly between upstream and lower river compartments. Individuals migrated at a slower speed in transitional water and sea-lough compartments compared with the FW compartment. While 88·5% survival was recorded during migration through the upper 121 km of the river and estuary, only 26% of A. anguilla which initiated downstream migration were detected at the outermost end of the acoustic array. Telemetry equipment functioned efficiently, including in the sea-lough, so this suggests high levels of mortality during sea-lough migration, or less likely, long-term sea-lough residence by silver A. anguilla emigrants. This has important implications for eel management plans.

During the 2010 Concordiasi field experiment, 635 dropsondes were released from the lower stratosphere providing in situ atmospheric profiles from the release height (similar to 60hPa) to the surface over Antarctica. They provide a unique data set of high vertical resolution temperature profiles over the entire Antarctic continent and surrounding ocean. This study uses temperature profiles and derived surface-based inversion (SBI) properties from the sonde data set to evaluate Atmospheric Infrared Sounder (AIRS) versions 5 (v5) and 6 (v6) temperature profiles. A total of 1486 matched pairs of profiles are available for analysis. The AIRS averaging kernel, representing the AIRS measurement sensitivity, is applied to the dropsonde profiles. The AIRS data are compared to kernel-averaged dropsonde profiles and found, on average, to have a small cold bias (similar to 0.5 degrees C) (for v6) in the troposphere. AIRS v6 is improved over v5 with both profile-averaged bias and root-mean-square errors reduced by over 25%. Compared to the kernel-averaged dropsonde profiles, AIRS v6 accurately detects the existence of SBIs in 79% of the profiles and agrees on the inversion depth 79% of the time. AIRS correctly identifies SBIs in 59% of cases when compared to the full-resolution sonde. AIRS systematically underestimates the SBI intensity. This is due to warmer reported AIRS surface air temperatures (T-a) than T-a measured with the dropsonde. Replacement of AIRS T-a with that measured by the dropsonde improves the agreement in both SBI detection and intensity. If AIRS T-a could be improved, AIRS has the potential to be a stand-alone SBI detection tool over Antarctica.

Management of many salmon populations is based on maintaining individual stocks above a conservation limit defined as the maximum sustainable yield point from a stock recruitment (SR) relationship. A crucial input parameter of any SR analysis is an accurate measure of fecundity. Atlantic salmon (Salmo salar L) fecundity data from nine rivers in Ireland were collated and analysed, providing the first comprehensive multi-river assessment of egg numbers for Irish stocks. The number of eggs in wild 1 SW fish (1 sea winter, also known as grilse) varied with fish size and also according to the source river. When expressed as eggs per kg of fish, river was a less important explanatory variable. The number of eggs in wild fish averaged 3441 for 1 SW fish and 6059 for MSW (multi-sea winter) fish, respectively. Egg diameter also varied with fish size and source river. A negative binomial model of egg number per fish, with weight as the only explanatory variable, accounted for 60% of the explained deviance in this dataset. When applied to a large national dataset of fish weights from angling records, this model allows river-specific fecundity to be estimated for salmon populations with varying life history characteristics. The model predicts that the fecundity of Irish salmon is at the lower end of the range reported for Atlantic salmon across Europe. (C) 2014 Elsevier B.V. All rights reserved.