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Anadromous brown trout display a continuum of life-history strategies with high intraspecific variability across their distribution range. While many of these strategies have been widely studied, some have received considerably less attention. In this study, we investigated intraspecific differences in two overlooked life-history strategies--outmigration phenology (spring vs. autumn migrants) and the prevalence of straying--in brown trout originating from four rivers flowing into the same fjord system. Juvenile brown trout (Salmo trutta) were tagged with passive integrated transponder (PIT) tags in their natal river, and were subsequently followed during their outmigration and subsequent return to freshwater. We found that patterns of outmigration (spring vs. autumn migrant, day of season) and return to freshwater (return likelihood, strayers vs. nonstrayers) differed between rivers, as did the factors (length, condition) affecting these outcomes. We also found evidence that outmigration phenology was linked to the likelihood of returning in a river-specific way. Our findings suggest that the costs and benefits of autumn migration and straying differ depending on an individual's home river. Key words: phenotypic plasticity, autumn migration, life-history strategies, phenology, dispersal, Introduction Phenotypic plasticity enables organisms to modify their developmental trajectories, activities, and resource allocation, typically in response to changes in environmental conditions (Pigliucci et al. 2006). For example, cyprinids in [...]

The use of Pop‐up Satellite Archival Tags (PSATs) has provided considerable new information about the behaviour of migrating (silver) eels (Anguilla spp.) at sea, with 9 of 19 recognised species or subspecies tagged to date. However, such studies often reported premature tag detachment and relatively high rates of predation. Since 2000, several PSAT‐tagged New Zealand longfin eels, Anguilla dieffenbachii, were released to understand migratory behaviour and location of potential spawning grounds. Of 20 PSAT‐tagged eels released in 2019, all tags detached prematurely, but 7 (58%) were likely eaten or injured by predators 1–81 days after release. Two eels were probably eaten by sperm whales, whereas other tagged eels were likely eaten by lamniform and non‐lamniform sharks, and southern bluefin tuna. Because of the high rate of predation, we recommend that future releases should be from smaller catchments or offshore. [ABSTRACT FROM AUTHOR]

Alternative migratory tactics, like partial migration, are common in many taxa. The proximate and ultimate drivers underpinning these strategies are unclear, though factors like condition and energetic status have been posited as important predictors. We sampled and PIT-tagged 1882 wild brown trout (Salmo trutta) prior to the first so-called decision window and explored the links between migratory tactics (residency, autumn or spring migration) and body metrics (length and condition), lipids (triglycerides and cholesterol), and sex in 150 randomly selected individuals. We found that more females adopted the autumn and spring migration tactic than males, while more males adopted the residency tactic than females, likely reflecting sex-biased benefits in anadromy. We also found that autumn migrants were in poorer condition prior to the presumed first decision window than spring migrants and residents. Lastly, we found that both condition and cholesterol were positively correlated to the timing of migration, such that individuals in poorer condition and (or) with lower cholesterol migrated earlier. Collectively, these results suggest that energy depletion is an important factor in determining migratory strategy, including timing. Differentes tactiques migratoires, telles que la migration partielle, sont repandues au sein de nombreux taxons. Si les facteurs proximaux et ultimes qui motivent le recours a ces strategies ne sont pas bien etablis, il a ete postule que certains facteurs, comme l'embonpoint et l'etat energetique, en constitueraient d'importants predicteurs. Nous avons preleve et muni de radio-etiquettes passives integrees (PIT) 1882 truites brunes (Salmo trutta) avant la premiere << fenetre de decision >> et examine les liens qui existent entre les tactiques migratoires (residence, migration automnale ou printaniere) et des parametres du corps (longueur et embonpoint), les teneurs en lipides (triglycerides et cholesterol) et le sexe chez 150 specimens choisis aleatoirement. Nous avons constate que plus de femelles que de males adoptent les tactiques de migration automnale et printaniere, alors que plus de males que de femelles adoptent la tactique de residence, ce qui reflete probablement un biais selon le sexe de certains avantages de l'anadromie. Nous avons aussi constate que les migrants automnaux presentent un moins bon etat d'embonpoint avant la premiere fenetre de decision presumee que les migrants printaniers et les residents. Enfin, nous avons constate que tant l'embonpoint que le cholesterol sont correles positivement au moment de la migration, les individus presentant un moins bon embonpoint ou une cholesterolemie plus faible migrant plus tot. Collectivement, ces resultats semblent indiquer que l'epuisement des ressources energetiques est un facteur important pour la determination de la strategie migratoire, y compris le moment des migrations. [Traduit par la Redaction], Introduction Life-history strategies are ultimately the means by which an individual can maximize its fitness (i.e., number of descendants and genes passed on; McNamara and Houston 1996; Thorpe et al. [...]

Salmonids are well known for their natal homing behaviour, meaning they return to breed in the same area where they originated. However, not all individuals return to their natal breeding grounds—a behavioural trait known as straying. The prevalence of straying is difficult to explore and therefore quantitative estimates for straying are seldom reported. In this study, otolith microchemistry and genetics were combined to investigate patterns of straying over ecological and evolutionary time, respectively, between neighbouring rivers flowing into Mariager fjord, Denmark. Otolith microchemistry was used to determine the river of origin for sea trout (Salmo trutta) upon their return to freshwater and 288 SNP markers were used to determine genetic structure among the rivers in the fjord. In this system, where the distance between rivers is short, otolith microchemistry achieved 80% accuracy in assigning juvenile brown trout to their natal river, thus allowing us to determine that approximately 43% of the adult sea trout had returned to non‐natal rivers to spawn, with a similar proportion of strayers and natal homers in all of the rivers. Genetic analysis further supported that there was substantial gene flow among individuals originating from different rivers, indicating that sea trout in Mariager fjord make up one population. The findings obtained from otolith microchemistry and genetics complement each other and provide further evidence that sea trout in this system migrate to non‐natal rivers and spawn there, which consequently affects the genetic structure of the population. [ABSTRACT FROM AUTHOR]

During migration, animals are typically limited by their endogenous energetic resources that must be allocated to the physiological costs associated with locomotion, as well as avoiding and (or) compensating for oxidative stress. To date, there have been few attempts to understand the role of oxidative status in migration biology, particularly in fish. Semi-anadromous brown trout (Salmo trutta L., 1758) exhibit partial migration, where some individuals smoltify and migrate to sea, and others become stream residents, providing us with an excellent model to investigate the link between oxidative stress and migration. Using the brown trout, we obtained blood samples from juveniles from a coastal stream in Denmark in the fall prior to peak seaward migration that occurs in the spring, and assayed for antioxidant capacity (oxygen radical absorbance capacity) and oxidative stress levels (ratio of oxidized to reduced glutathione). We found that individuals that migrated had higher antioxidant capacity than residents and that future migration date was negatively correlated with both antioxidant capacity and body length in the fall. This study provides the first evidence that oxidative status is associated with migration strategy and timing, months in advance of the actual migration, and provides insight into the role of oxidative status in animal migration. Key words: brown trout, Salmo trutta, antioxidant, partial migration, resident, migrant, oxidative stress., Resume : Durant la migration, les animaux sont typiquement limites par les ressources d'energie endogenes devant etre affectees aux couts physiologiques associes a la locomotion, ainsi que pour eviter ou [...]