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Managers and stakeholders increasingly ask whether predation is a driving force behind the poor status of many species, and whether predator control is likely to be a successful management action to intervene. We review existing literature on Atlantic salmon Salmo salar predation and predator control, as well as general ecological theory on the role of predation in the life cycle of this iconic fish. Many bird, mammal, and fish predators target salmon at different life stages. In healthy salmon populations, predation is likely compensated for by reduced intraspecific competition during the freshwater stage. There is little evidence that predation alone has been an underlying mechanism for driving salmon populations below conservation limits. However, depending on the predator's response to salmon abundance, predation may keep decimated populations from recovering, even when the actual causes of decline have been removed. Under such a scenario, predation control may contribute to recovery, but there are no strong examples that clearly demonstrate the efficacy of managing predators to recover threatened salmon populations, challenging further applications. Key words: predators, predation impact, predator species, Salmo salar, stock recovery, Introduction Few species on the planet inspire as much fascination and sympathy as the Atlantic salmon Salmo salar. Salmon is a symbol of the native ecosystems in the North Atlantic [...]

Evolution of iteroparity is shaped by the trade-off between current and future reproduction. We studied variation in iteroparity among 205 050 individual Atlantic salmon caught in 179 rivers spanning 14[degrees] of latitude. The proportion of repeat spawners (iteroparous individuals) averaged 3.8% and ranged from 0% to 26% across rivers. Females were more often repeat spawners than males and had lower cost of reproduction in terms of lost body mass between spawning events. Proportion of repeat spawners for a given sea age at maturity, and the ratio of alternate to consecutive repeat spawners, increased with increasing population mean sea age at maturity. By combining smolt age, sea age at maturity, and age at additional spawning events, we identified 141 unique life-history types, and repeat spawners contributed 75% of that variation. Our results show that repeat spawners are important for life-history variation and suggest that the association between mean sea age and the frequency of repeat spawning is adaptive rather than a pleiotropic side effect arising from selection on sea age. Key words: age at maturity, life-history evolution, local adaptation, repeat spawning, trade-off, Introduction Allocation of limited resources results in trade-offs shaping life-history evolution. One essential trade-off is the partition of energy between current and future reproduction (Stearns 1992). This trade-off plays out [...]

Many anguillid eel species display facultative catadromy. Some eel spend their entire life cycle in marine coastal areas, but the geographical extent of this, especially at the extremes of their distributional ranges, is unknown. We analysed otolith Sr:Ca and Ba:Ca from yellow-stage European eel (Anguilla anguilla) sampled along the coast of Norway and in several freshwater lakes (58[degrees]N-63[degrees]N), to infer their initial settlement and later life movement patterns with regards to habitat salinity. Most eel (80%) sampled in marine habitats (n = 371) had settled and remained in marine water, but 20% had moved between marine and freshwater habitats and were hence classified as inter-habitat shifters. Among freshwater sampled eel (n = 99), 80% had settled and remained in fresh water, but 20% were classified as inter-habitat shifters. The average growth rates for marine water residents, inter-habitat shifters, and freshwater residents were 35, 27, and 17 mm * [year.sup.-1], respectively. Northern European shallow marine habitats may serve as important yellow eel growth habitats and may be critical to buffer the European eel population against the general decrease in continental recruitment. Key words: otolith microchemistry, anguillids, Anguilla anguilla, salinity, age, growth rate, 1. Introduction The decline of the panmictic European eel (Anguilla anguilla) population is well documented (Drouineau et al. 2018; ICES 2020). The data used to support this apparent decline have [...]

Habitat fragmentation may cut off anadromous salmonids from parts of their potential native habitat and separate previously connected populations. Understanding the consequences of this is vital for fish management and prioritization of restoration activities. Here, we show that there is a significant difference in the body morphology, physiological stress response, and aspects contributing to aerobic capacity between juvenile anadromous brown trout, Salmo trutta, collected at a downstream site and an upstream site, separated by 2 km and several challenging stream sections, in a small unfragmented stream system in western Sweden. Following a standardized stress test, there were significant differences between fish from the upstream and downstream sites (plasma cortisol concentration, plasma osmolality, hematocrit, hemoglobin concentration, and mean corpuscular hemoglobin concentration). Plasma glucose concentration did not significantly differ between fish from the two sites. Fish from the upstream site had larger spleen mass, although there was no evidence of differences in ventricle mass or proportion of compact ventricular myocardium. These physiological differences indicate local variation in stress response and highlight the importance of considering local trait variation in river management. If a section of the river becomes fragmented or degraded, and there are differences in the juveniles in different parts of the river, the consequence for the population might be larger than the proportional loss of habitat. [ABSTRACT FROM AUTHOR]

Measurements of individual body length at different life stages are critical in understanding fish ecology and evolution. Such data can be obtained via back-calculation from measurements of fish scales or by using the size of the scale as a direct proxy for body length. Using data from Atlantic salmon, we test key assumptions associated with each approach and their implications for investigating how variation in growth earlier in life is associated with age at maturity. The scaling of scale size approximated isometry and was similar among individuals who matured at different ages—validating a key assumption of back-calculation. However, we observed that individuals genetically predisposed to delay maturation have smaller scales for their body size—challenging a key assumption of the "direct scale-size" approach. Depending on the method of body length estimation, the relationship between body length growth earlier in life and age at maturity was observed to differ. Thus, when using scale material to study the relationship between growth and maturation in salmon, we recommend back-calculation and an assumed allometric scaling coefficient. [ABSTRACT FROM AUTHOR]

Jack Brand and colleagues introduce the salmonid fishes comprising over 200 species. [ABSTRACT FROM AUTHOR]

Human activities have the potential to accelerate population-level decline by contributing to climate warming and decreasing the capacity of species to survive warming temperatures. Here we build a predictive model to test interactions between river warming and catch and release mortality in recreational fisheries for Atlantic salmon (Salmo salar) by compiling and analyzing published data. We then test whether warming has occurred in rivers where angling occurs and whether angling opportunities have been restricted through increased river closures due to high water temperatures. We find that catch and release mortalities are low ( Les activites humaines ont le potentiel d'accelerer les declins de populations en participant au rechauffement climatique et en reduisant la capacite des especes de survivre a des temperatures en hausse. Nous elaborons un modele predictif pour examiner les interactions entre le rechauffement des rivieres et la mortalite associee a la peche avec remise a l'eau dans la peche sportive au saumon atlantique (Salmo salar), en compilant et en analysant des donnees publiees. Nous verifions ensuite si un rechauffement s'est produit dans les rivieres ou une peche a lieu et si les occasions de peche ont ete restreintes par l'augmentation du nombre de fermetures de riviere en raison de temperatures de l'eau elevees. Nous constatons que les taux de mortalite associes a la peche avec remise a l'eau sont faibles (, Introduction Average global air temperature has increased 0.74 [degrees]C since 1906, with 17 of the 18 warmest years on record occurring since 2001 (IPCC 2018) and with projections of further [...]

Evolution of phenotypic plasticity requires genotype–environment interaction. The discovery of two large‐effect loci in the vgll3 and six6 genomic regions associated with the number of years the Atlantic salmon spend feeding at sea before maturation (sea age), provides a unique opportunity to study evolutionary potential of phenotypic plasticity. Using data on 1246 Atlantic salmon caught in the River Surna in Norway, we show that variation in mean sea age among years (smolt cohorts 2013–2018) is influenced by genotype frequencies as well as interaction effects between genotype and year. Genotype–year interactions suggest that genotypes may differ in their response to environmental variation across years, implying genetic variation in phenotypic plasticity. Our results also imply that plasticity in sea age will evolve as an indirect response to selection on mean sea age due to a shared genetic basis. Furthermore, we demonstrate differences between years in the additive and dominance functional genetic effects of vgll3 and six6 on sea age, suggesting that evolutionary responses will vary across environments. Considering the importance of age at maturity for survival and reproduction, genotype–environment interactions likely play an important role in local adaptation and population demography in Atlantic salmon. [ABSTRACT FROM AUTHOR]

Abstract Considerable technical developments over the past half century have enabled widespread application of electronic tags to the study of animals in the wild, including in freshwater environments. We review the constraints associated with freshwater telemetry and biologging and the technical developments relevant to their use. Technical constraints for tracking animals are often influenced by the characteristics of the animals being studied and the environment they inhabit. Collectively, they influence which and how technologies can be used and their relative effectiveness. Although radio telemetry has historically been the most commonly used technology in freshwater, passive integrated transponder (PIT) technology, acoustic telemetry and biologgers are becoming more popular. Most telemetry studies have focused on fish, although an increasing number have focused on other taxa, such as turtles, crustaceans and molluscs. Key technical developments for freshwater systems include: miniaturization of tags for tracking small-size life stages and species, fixed stations and coded tags for tracking large samples of animals over long distances and large temporal scales, inexpensive PIT systems that enable mass tagging to yield population- and community-level relevant sample sizes, incorporation of sensors into electronic tags, validation of tag attachment procedures with a focus on maintaining animal welfare, incorporation of different techniques (for example, genetics, stable isotopes) and peripheral technologies (for example, geographic information systems, hydroacoustics), development of novel analytical techniques, and extensive international collaboration. Innovations are still needed in tag miniaturization, data analysis and visualization, and in tracking animals over larger spatial scales (for example, pelagic areas of lakes) and in challenging environments (for example, large dynamic floodplain systems, under ice). There seems to be a particular need for adapting various global positioning system and satellite tagging approaches to freshwater. Electronic tagging provides a mechanism to collect detailed information from imperilled animals and species that have no direct economic value. Current and future advances will continue to improve our knowledge of the natural history of aquatic animals and ecological processes in freshwater ecosystems while facilitating evidence-based resource management and conservation.