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In mixed fisheries where multiple species are caught, to manage resources sustainably, knowledge about the species-specific vulnerability to fishing is equally or even more important than knowledge of size selectivity of the gear. We compared the vulnerability to bait recreational angling in four salmonid species in natural streams in Japan. The ranking of species-specific angling vulnerability was as follows (from highest to lowest): rainbow trout (Oncorhynchus mykiss), masu salmon (Oncorhynchus masou), white-spotted char (Salvelinus leucomaenis), and southern Asian Dolly Varden (Salvelinus curilus). In all species, larger individuals were more vulnerable to angling, but there were differences in the size dependence between species. In rainbow trout and Dolly Varden (which have a nonanadromous life history in the study area), the probability of being caught monotonically increased with body size, while the vulnerability to angling in masu salmon and white-spotted char (which have an anadromous life history in the study area) showed a domed-shaped pattern. We found that across the species the catch per unit effort showed a hyperstable relationship with population density. Therefore, diminishing local populations are prone to collapse, and this collapse would be hard to foresee based on catch rate data alone. Pour une gestion durable des ressources des peches mixtes dans lesquelles plusieurs especes sont pechees, les connaissances sur la vulnerabilite des differentes especes a la peche sont tout aussi, sinon plus importantes que les connaissances sur la selectivite selon la taille des engins. Nous avons compare la vulnerabilite a la peche sportive a la ligne avec appat de quatre especes de salmonides dans des cours d'eau naturels au Japon. Le classement de la vulnerabilite a la peche selon l'espece s'etablit comme suit (en ordre decroissant) : truite arc-en-ciel (Oncorhynchus mykiss), saumon masou (Oncorhynchus masou), omble a taches blanches (Salvelinus leucomaenis) et omble malma (Salvelinus curilus). Chez toutes les especes, les individus plus grands sont plus vulnerables a la peche a la ligne, mais cette relation avec la taille varie d'une espece a l'autre. Chez les truites arc-en-ciel et les omble malma (qui sont non anadromes dans la region a l'etude), la probabilite de capture augmente de maniere monotone avec la taille du corps, alors que la vulnerabilite a la peche a la ligne des saumons masou et des ombles a taches blanches (qui sont anadromes dans la region a l'etude) presente une distribution en forme de dome. Nous constatons que, pour toutes les especes, les prises par unite d'effort montrent une relation hyperstable avec la densite de la population. Les populations locales en declin sont donc susceptibles de s'effondrer, et un tel effondrement serait difficile a predire a la lumiere des seules donnees sur les taux de prises. [Traduit par la Redaction], Introduction Vulnerability to fishing varies widely by species, body size, and life history (Lewin et al. 2006; Johnston et al. 2013; Nash et al. 2013). Multiple species with different intrinsic [...]

The relationships between body size and density of salmonids in natural streams have been well studied, and density‐dependent growth (somatic growth) is interpreted as the principal mechanism responsible for the relationship. Moreover, the body size–density relationship is known to vary over ontogeny. However, the relationship has been studied mostly by experimental procedures (e.g. stocking fish into streams), and knowledge about the relationship in natural populations (e.g. no human‐induced input of fish) is still inadequate. In this study, we compared the relationships between body size and density of young‐of‐the‐year (YOY) masu salmon, Oncorhynchus masou, between different ontogenetic stages, that is, fry (spring) and juveniles (autumn). We used the monitoring data of YOY body sizes and densities across three study sites collected from Horokashubuto stream in Hokkaido, northern Japan, from 2014 to 2022. In the juvenile stage, YOY body size correlated negatively with increasing density. This pattern is typical of the density dependence of salmonid growth demonstrated in many previous studies. In the fry stage, however, YOY body size correlated positively with increasing density. This study suggested that the patterns of density dependence vary as functions of the ontogenetic stages of salmonids. [ABSTRACT FROM AUTHOR]

We examined the relationship between environmental factors of a spawning habitat and body shape at maturity in 16 pink salmon (Oncorhynchusgorbuscha) populations. The divergence vector (indicator of body depth and head size development) in both sexes was significantly correlated with distance from the sea and was described by a hump-shaped relationship. An exaggerated body shape was most developed at mid-distance from the sea and less developed both near and far from the sea. The observed frequency of bears decreased significantly with distance from the sea, implying that bear predation on salmon is most intense near the sea. Therefore, our results imply that shape-selective predation by bears affects the shape of pink salmon in rivers where the spawning habitat is near the sea and that migration costs inhibit development of an exaggerated body depth and head size in salmon in rivers where the spawning habitat is far from the sea. Nous avons examine le lien entre des facteurs ambiants de l'habitat de frai et la forme du corps a la maturite dans 16 populations de saumons roses (Oncorhynchus gorbuscha). Le vecteur de divergence (un indicateur du developpement de la profondeur du corps et de la taille de la tete) chez les deux sexes presentait une correlation significative avec la distance de la mer et etait decrit par une relation en forme de bosse. Le developpement d'une forme du corps exageree etait maximum a mi-distance de la mer et moindre plus pres et a plus grande distance de cette derniere. La frequence d'ours observee diminuait de maniere significative plus la distance de la mer etait grande, indiquant que la predation des saumons par les ours atteint un maximum pres de la mer. Nos resultats indiquent donc que la predation selective selon la taille par les ours a une incidence sur la forme des saumons roses dans les rivieres ou l'habitat de frai est pres de la mer, et que les couts de la migration empechent le developpement d'une profondeur du corps et d'une taille de la tete exagerees chez les saumons dans les rivieres ou l'habitat de frai est loin de la mer. [Traduit par la Redaction], Introduction Sexual selection favors exaggerated phenotypic traits, as these traits increase mating success (Andersson 1994). However, exaggerated traits influence survival rates, as natural selection often acts on the traits to [...]

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Animal populations are spatially structured in heterogeneous landscapes, in which local patches with differing vital rates are connected by dispersal of individuals to varying degrees. Although there is evidence that vital rates differ among local populations, much less is understood about how vital rates covary among local patches in spatially heterogeneous landscapes. In this study, we conducted a nine‐year annual mark–recapture survey to characterize spatial covariation of survival and growth for two Japanese native salmonids, white‐spotted charr Salvelinus leucomaenis japonicus and red‐spotted masu salmon Oncorhynchus masou ishikawae, in a headwater stream network composed of distinctly different tributary and mainstem habitats. Spatial structure of survival and growth differed by species and age class, but results provided support for negative covariation between vital rates, where survival was higher in the tributary habitat but growth was higher in the mainstem habitat. Thus, neither habitat was apparently more important than the other, and local habitats with complementary vital rates may make this spatially structured population less vulnerable to environmental change (i.e. portfolio effect). Despite the spatial structure of vital rates and possibilities that fish can exploit spatially distributed resources, movement of fish was limited due partly to a series of low‐head dams that prevented upstream movement of fish in the study area. This study shows that spatial structure of vital rates can be complex and depend on species and age class, and this knowledge is likely paramount to elucidating dynamics of spatially structured populations. [ABSTRACT FROM AUTHOR]

The degree of iteroparity in stream‐resident forms of masu salmon Oncorhynchus masou was examined using mark–recapture studies in natural streams. In a partially migratory population, at least 10% of resident males survived after maturation and repeatedly matured for up to 5 years. In the landlocked amago salmon subspecies, the post‐maturation survival rate was at least 7% and repeat maturation was observed for up to 3 years for both males and females. [ABSTRACT FROM AUTHOR]

The article discusses aspects of the study which examines the relationship between salmonid densities and water temperature in the Kushiro river system in Japan in winter. It is critical of the lack of spatial replication in the study and the authors' comparison of seasonal changes in population densities between two tributaries. Also investigated is the collection of the fishes to conduct fish sampling.