The precocious maturation of some male Atlantic salmon (Salmo salar L.) has become a textbook example of alternative mating tactics, but the only estimates of reproductive success available so far are either the collective contribution of precocious males to reproduction in the wild or individual reproductive success in oversimplified experimental conditions. Using genetic parentage analysis on anadromous and precocious potential spawners and their offspring, we quantified components of individual reproductive success of both tactics in a natural population. On average, precocious males produced 2.24 (variance 67.62) offspring, against 27.17 (3080) for anadromous males. For both tactics, most of the variance in reproductive success was due to mating success, with 83% of precocious males having no mate, against 50% for anadromous males. Body size increased reproductive success of anadromous males and tended to decrease precocious males' reproductive success. Although these results do not solve the coexistence of alternative reproductive tactics (ARTs) in Atlantic salmon, their inclusion in comprehensive models of lifetime reproductive success should shed light on the evolution of precocious maturation in Atlantic salmon and its effect on the selection of phenotypic traits.

We combined habitat mapping, radio tracking of adults, redd mapping, and genetic parentage analysis on juveniles for 2 consecutive years to test eight predictions on the reproductive payoffs and individual features associated with space use tactics of anadromous Atlantic salmon (Salmo salar) in a natural population. Regarding payoffs, we found that (1) males settled in better habitat, visited more redds, and tended to get more mates than males settled in poorer habitat; (2) they also sired more offspring; (3) the latter effect was stronger in the first year, when redds were more aggregated; and (4) female reproductive success increased with habitat quality in their home range, especially in the first year. For individual features, (5) individuals that settled earlier in their home range had better habitat for juvenile production, but only in the first year; (6) females were less mobile than males, especially in the second year; (7) larger males did not settle in better habitats than smaller males; and (8) smaller males were not more mobile than larger males.

Understanding the geographical scales at which environmental variables affect an individual's body size, and thus their mortality risk, can inform management strategies to help conserve wild populations under climate change. Yet, our current understanding of these relationships is based on studies done at different scales that report inconsistent findings. We predicted that temperature‐related variables (e.g. winter temperature) influence body size at a “regional” scale, that is, affecting individuals in geographically independent catchments similarly, whereas non‐temperature‐related variables (e.g. conspecific competitor density) exert a “local” influence, that is, affecting individuals in geographically independent catchments differently. We developed statistical models to test our predictions using body length measures of a large and long‐term sample of juvenile Atlantic salmon (Salmo salar) from three rivers in the U.K. and France. We developed mixture models to predict the individual juvenile salmon ages objectively from their body length. We then developed linear mixed models to describe inter‐annual changes in mean length of the youngest (age 0) cohort of juvenile salmon from river‐specific seasonal variables, and tested whether they exerted their influence at a “local” or “regional” scale. All models accounted for spatio‐temporal differences in sampling protocols and individual reproductive strategy. We estimated and interpreted coefficients using Bayesian theory. Our findings supported our predictions. Juvenile salmon were longer in years of higher overwinter water temperature and this effect was best parameterised as a single “regional” coefficient applicable to all three rivers. Similarly, spring mean temperature was best parameterised with a single “regional” nonlinear coefficient. In contrast, juvenile salmon were shorter in years of high densities of competing conspecifics and their interaction with total mean discharge and these effects were represented by “local” river‐specific coefficients. Summer droughts had a negative effect on juvenile salmon length but was best parameterised as a single “regional” coefficient, contrary to our expectations. We show that environmental variables affect biological processes at different but predictable geographical scales. Temperature‐related variables affect body sizes of exothermic animals at a regional scale, whereas non‐temperature variables, such as the density of conspecific competitors and water abstraction, exert their influence at a local scale. These findings highlight the importance of integrating local and regional management plans to mitigate the impacts of climate change on the body size, and ultimately the conservation, of exothermic species.

We investigated the use of Sr:Ca, Ba:Ca, and 87 Sr: 86 Sr ratios as natural tags for determining the natal origins of juvenile and adult Atlantic salmon (Salmo salar) from 12 tributaries in the Adour basin (southwestern France) and estimated homing on a tributary scale. Geochemical signatures from core regions of the otolith were also used to identify fish from hatchery or naturally spawned sources. Quadratic discriminant function analysis (QDFA) was on average 80% successful at classifying juveniles according to their natal rivers. Adults of unknown natal origin were assigned to their natal rivers using the juvenile fingerprints from QDFA approach. Only 18 adults originated from streams not included in the juvenile database. Although most of the adults showed a marked homing instinct, homing was not perfect, and some wild fish strayed into non-natal spawning areas. Returns of hatchery-reared fish as adult spawners represented 10% of the total sampled fish. Allocation of fish to natal tributaries or hatcheries illustrated the abundance and relative contributions of natal sources, important for the recovery of Atlantic salmon in this area.Résumé : Nous nous sommes penchés sur l'utilisation des rapports Sr:Ca, Ba:Ca et 87 Sr: 86 Sr comme marqueurs naturels des origines natales de saumons atlantiques juvéniles et adultes provenant de 12 affluents du bassin de l'Adour (sud-ouest de la France) et avons estimé le retour à l'échelle des affluents. Les signatures géochimiques des régions centrales d'otolithes ont également été utilisées pour distinguer les poissons issus d'écloseries de ceux issus du frai en milieu naturel. Le taux de succès moyen de l'analyse discriminante quadratique (QDFA) pour ce qui est de classer les juvéniles selon leurs rivières natales était de 80 %. La détermination de la rivière natale des adultes d'origine natale inconnue reposait sur les empreintes de juvéniles tirées de la QDFA. Seuls 18 adultes provenaient de cours d'eau ne figurant pas dans la base de données des juvéniles. Bien que la plupart des adultes présentaient un instinct de retour marqué, le retour n'était pas parfait, certains poissons sauvages se retrouvant dans des régions de frai non natales. Les poissons élevés en écloserie retournant comme géniteurs adultes représentaient 10 % du total des poissons échantillonnés. La répartition des poissons entre affluents natals et écloseries illustre l'abondance et les contributions relatives des sources natales, qui jouent un rôle important dans le rétablissement du saumon atlantique dans la région. [Traduit par la Rédaction]

This study assesses whether the effective number of breeders (N ) can be estimated using a time and cost-effective protocol using genetic sibship reconstruction from a single sample of young-of-the-year (YOY) for the purposes of Atlantic salmon Salmo salar population monitoring. N was estimated for 10 consecutive reproductive seasons for S. salar in the River Nivelle, a small population located at the rear-edge of the species distribution area in France, chronically under its conservation limit and subjected to anthropogenic and environmental changes. Subsampling of real and simulated data showed that accurate estimates of N can be obtained from YOY genotypes, collected at moderate random sampling intensity, achievable using routine juvenile electrofishing protocols. Spatial bias and time elapsed since spawning were found to affect estimates, which must be accounted for in sampling designs. N estimated in autumn for S. salar in the River Nivelle was low and variable across years from 23 (95% C.I. 14-41) to 75 (53-101) and was not statistically correlated with the estimated number of returning adults, but it was positively correlated with the estimated number of YOY at age 9 months. N was found to be lower for intermediate levels of redd aggregation, suggesting that the strength of the competition between males to access females affects reproductive success variance depending on redd spatial configuration. Thus, environmental factors such as habitat availability and quality for spawning and YOY development predominate over demographic ones (number of returning adults) in driving long-term population viability for S. salar in the River Nivelle. This study showcases N as an integrated parameter, encompassing demographic and ecological information about a reproductive event, relevant to the assessment of both short-term effects of management practices and long-term population conservation status.