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Genetic identity analysis and PIT (passive integrated transponder) tagging were used to examine the freshwater return rates and phenotypic characteristics of n = 1791 downstream migrating juvenile Salmo trutta in the Burrishoole catchment (northwest Ireland) across the period September 2017 to December 2020. In this system, juveniles out‐migrate (move from freshwater into brackish or marine habitats) in every month of the year, with distinct seasonal peaks in spring (March through June; mostly silvered smolts) and autumn (September through December; mostly younger, unsilvered fry or parr). Both types exhibited a sex‐bias towards females, which was stronger in spring (78% females) than in autumn outmigrants (67%). Sixty‐nine returning fish were matched back to previous juvenile outmigrants, and similar return rates were found for spring outmigrants (5.0%), autumn outmigrants (3.3%) and fish that out‐migrated outside of spring or autumn (2.8%). Spring and autumn outmigrants returned at similar dates (typically mid to late July), but autumn fish were away for longer periods (median = 612 days; spring outmigrants = 104 days). Autumn outmigrants were 25% smaller than spring outmigrants at outmigration and 6% smaller on their return, and within both groups smaller/younger outmigrants spent longer away than larger/older outmigrants. Autumn outmigrants were more likely to return unsilvered as "slob" trout (84%) than spring outmigrants (31%), suggesting they make greater use of brackish habitats that might be safer, but less productive, than fully marine habitats. Nonetheless, both types also produced silvered "sea trout" (≥1+ sea‐age), implying neither is locked into a single life‐history strategy. The findings emphasise that autumn outmigrants and the transitional habitats that support their persistence should not be overlooked in salmonid management and conservation. [ABSTRACT FROM AUTHOR]

Abstract: Identifying differences in quantitative life history traits between cultured and native or non-native wild populations is important in assessing the impact of accidental and deliberate introductions of hatchery-reared fish into the wild. As the ability to exploit the marine environment is the defining life history characteristic of anadromous salmonids, knowledge of variation in smoltification characteristics among populations is crucial in determining how these introductions affect fitness in recipient populations. Data are presented here describing the timing and extent of the autumn migration; the propensity for male parr maturation; the timing of the spring migration; and the size of autumn and spring migrants from Atlantic salmon (Salmo salar) populations from various genetic backgrounds. These experiments were carried out under common garden conditions over a decade in the Srahrevagh River in the west of Ireland. Population specific genetically determined differences in quantitative life history traits associated with smoltification were apparent. These differences may reflect smolt quality and therefore impact on marine survival and ultimately lifetime fitness. Both hatchery domestication and geography (different selective environments) were found to be important factors determining smolt phenotypes, although it was difficult to measure the relative contribution of each. These results indicate that farm, native hatchery, non-native wild salmon (even from a neighbouring catchment) and their hybrids with native wild fish, are likely to produce less well adapted and thus poorer quality smolts than native wild populations and, where wild populations are extant, such stocks should not be used for enhancement purposes. [Copyright &y& Elsevier]

Animals often exhibit extensive flexibility in movement behaviours on a range of temporal and spatial scales in response to cues that reliably predict fitness outcomes. The annual timing of movements between distinct habitats can be crucial, particularly in seasonal environments with narrow ecological windows of opportunity. In polygamous species, sexual selection may further shape sex-specific phenology and movement behaviours. Here, we characterized seasonal, daily and diel movement patterns in adult brown trout, Salmo trutta , between a lake feeding habitat and two spawning streams in northwestern Ireland, using passive integrated transponder (PIT) telemetry. Antennae positioned at the inflow and outflow of the lake were used to monitor movements of 197 lake-tagged adults between lake and stream habitats. Across 2 years in both streams, movements were overwhelmingly nocturnal and exhibited distinct seasonality, with a peak in daily detections close to the winter solstice. In both streams, seasonal movement activity of males began and peaked before that of females (protandry). Daily detection probabilities for both sexes increased as the moon waned (decreasing lunar illumination) and as river depth increased, the latter being associated with reduced water clarity. These findings are consistent with fish favouring movement between fluvial and lacustrine habitats when light (both solar and lunar) or hydrological conditions decrease their exposure to visually oriented predators. The observed protandry also suggests a role for intrasexual male competition, whereby earlier male arrival could increase mating opportunities. • Brown trout males move from feeding lakes to spawning streams before females. • While length of activity period is similar, males conduct more frequent movements. • Transitional movements of both sexes are primarily nocturnal. • Movement probability increases as river height increases and the moon wanes. [ABSTRACT FROM AUTHOR]

Migration in animals and associated adaptations to contrasting environments are underpinned by complex genetic architecture. Here, we explore the genomic basis of facultative anadromy in brown trout (Salmo trutta), wherein some individuals migrate to sea while others remain resident in natal rivers, to better understand how alternative migratory tactics (AMTs) are maintained evolutionarily. To identify genomic variants associated with AMTs, we sequenced whole genomes for 194 individual trout from five anadromous-resident population pairs, situated above and below waterfalls, in five different Irish rivers. These waterfalls act as natural barriers to upstream migration and hence we predicted that loci underpinning AMTs should be under similar divergent selection across these replicate pairs. A sliding windows based analysis revealed a highly polygenic adaptive divergence between anadromous and resident populations, encompassing 329 differentiated genomic regions. These regions were associated with 292 genes involved in various processes crucial for AMTs, including energy homeostasis, reproduction, osmoregulation, immunity, circadian rhythm and neural function. Furthermore, examining patterns of diversity we were able to link specific genes and biological processes to putative AMT trait classes: migratory-propensity, migratory-lifestyle and residency. Importantly, AMT outlier regions possessed higher genetic diversity than the background genome, particularly in the anadromous group, suggesting balancing selection may play a role in maintaining genetic variation. Overall, the results from this study provide important insights into the genetic architecture of migration and the evolutionary mechanisms shaping genomic diversity within and across populations., (© 2024 The Author(s). Molecular Ecology published by John Wiley & Sons Ltd.)

The assessment report of the 4th International Panel on Climate Change confirms that global warming is strongly affecting biological systems and that 20-30% of species risk extinction from projected future increases in temperature. It is essential that any measures taken to conserve individual species and their constituent populations against climate-mediated declines are appropriate. The release of captive bred animals to augment wild populations is a widespread management strategy for many species but has proven controversial. Using a regression model based on a 37-year study of wild and sea ranched Atlantic salmon (Salmo salar) spawning together in the wild, we show that the escape of captive bred animals into the wild can substantially depress recruitment and more specifically disrupt the capacity of natural populations to adapt to higher winter water temperatures associated with climate variability. We speculate the mechanisms underlying this seasonal response and suggest that an explanation based on bio-energetic processes with physiological responses synchronized by photoperiod is plausible. Furthermore, we predict, by running the model forward using projected future climate scenarios, that these cultured fish substantially increase the risk of extinction for the studied population within 20 generations. In contrast, we show that positive outcomes to climate change are possible if captive bred animals are prevented from breeding in the wild. Rather than imposing an additional genetic load on wild populations by releasing maladapted captive bred animals, we propose that conservation efforts should focus on optimizing conditions for adaptation to occur by reducing exploitation and protecting critical habitats. Our findings are likely to hold true for most poikilothermic species where captive breeding programmes are used in population management.

The high level of escapes from Atlantic salmon farms, up to two million fishes per year in the North Atlantic, has raised concern about the potential impact on wild populations. We report on a two-generation experiment examining the estimated lifetime successes, relative to wild natives, of farm, F(1) and F(2) hybrids and BC(1) backcrosses to wild and farm salmon. Offspring of farm and "hybrids" (i.e. all F(1), F(2) and BC(1) groups) showed reduced survival compared with wild salmon but grew faster as juveniles and displaced wild parr, which as a group were significantly smaller. Where suitable habitat for these emigrant parr is absent, this competition would result in reduced wild smolt production. In the experimental conditions, where emigrants survived downstream, the relative estimated lifetime success ranged from 2% (farm) to 89% (BC(1) wild) of that of wild salmon, indicating additive genetic variation for survival. Wild salmon primarily returned to fresh water after one sea winter (1SW) but farm and 'hybrids' produced proportionately more 2SW salmon. However, lower overall survival means that this would result in reduced recruitment despite increased 2SW fecundity. We thus demonstrate that interaction of farm with wild salmon results in lowered fitness, with repeated escapes causing cumulative fitness depression and potentially an extinction vortex in vulnerable populations.