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Predator-prey interactions must be considered when aiming to enhance populations by releasing artificially reared individuals into natural environments. Released individuals create an abundance of prey for predators, and a basic hypothesis of predator-prey interactions suggests that predators select large prey due to the high caloric content. An alternative hypothesis is that small individuals are vulnerable to predation due to their poor predator avoidance. This study tested these hypotheses using stocked chum (Oncorhynchus keta) and masu (Oncorhynchus masou) salmon fry and piscivorous salmonids in marine and riverine habitats in Hokkaido, Japan. Stomach contents were sampled from predators, and fork length of prey fry was measured. Then, their fork length was compared with whole stocked fry (range of mean fork length ([+ or -]SD): 45.8 [+ or -] 2.55 to 49.2 [+ or -] 2.76 mm) for each habitat. As a result, prey fry were ~3%-6% smaller than whole stocked fry, even under a prey-abundant condition (i.e., just after hatchery reared salmon fry were stocked). Piscivorous salmonids pursue schooling fry, and small fry may be easily caught due to their slow speed in avoiding predators. Le renforcement de populations par le lacher dans des milieux naturels d'individus eleves en milieu artificiel doit tenir compte des interactions entre predateurs et proies. Les individus laches creent une abondance de proies pour les predateurs, et une hypothese de base des interactions predateurs-proies veut que les predateurs choisissent des proies plus grandes en raison de leur contenu calorique eleve. Une autre hypothese postule toutefois que les petits individus sont vulnerables a la predation en raison de leur moins bonne capacite d'eviter les predateurs. La presente etude a teste ces hypotheses en utilisant des alevins de saumon keta (Oncorhynchus keta) et de saumon masu (Oncorhynchus masou) et des salmonides piscivores dans des habitats marins et fluviaux d'Hokkaido (Japon). Les contenus stomacaux de predateurs ont ete echantillonnes et les longueurs a la fourche des alevins consommes ont ete mesurees. Ces dernieres ont ete comparees aux longueurs a la fourche de l'ensemble des alevins laches (fourchette de longueurs a la fourche moyennes ([+ or -]ET) : 45,8 [+ or -] 2,55 a 49,2 [+ or -] 2,76 mm) pour chaque habitat. Les alevins consommes etaient d'environ 3 % a 6 % plus petits que l'ensemble des alevins laches, et ce, meme dans des conditions de proies abondantes (c.-a-d., juste apres le lacher d'alevins issus d'ecloseries). Les salmonides piscivores poursuivent les alevins rassembles en bancs, et les petits alevins pourraient etre plus faciles a attraper en raison de leur faible vitesse quand ils essaient d'eviter des predateurs. [Traduit par la Redaction], 1. Introduction Predator-prey interactions play a key role in population dynamics of both predator and prey species (Case 1999). In particular, prey species experience decreased population size, and sometimes threatened [...]

In Japan, stocked chum salmon (Oncorhynchus keta) fry may have become the perfect prey for non-native brown trout (Salmo trutta), which are popular targets of anglers. If this is the case, fry stocking which is intended to boost commercial fishing may be helping to sustain the populations of an invasive predator. We used dietary and biochemical analyses to examine whether brown trout quickly restore their nutritional status following wintertime declines by preying upon chum salmon fry that are stocked in spring. We targeted six rivers in Hokkaido, Japan, three with fry stocking and three without. Changes in brown trout condition factor, triglyceride contents in muscle and serum, serum insulin-like growth factor-1 (IGF-1; an indicator of short-term growth), and docosahexaenoic acid (DHA; an essential fatty acid abundant in fish) content in muscle were examined between before stocking and during the stocking period in the six rivers. Dietary analysis showed that brown trout preyed on fry during the stocking period in all stocked rivers. Their nutritional status tended to be higher during the stocking period than before stocking in stocked rivers, but not in unstocked rivers. These results suggest that the massive stocking of chum salmon fry provides brown trout with the perfect prey to quickly restore their nutritional status and fuel increased growth; this may therefore be a controversial issue among stakeholders. [ABSTRACT FROM AUTHOR]

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]

Nematodes that parasitize salmonids are found in both seawater and freshwater. Unlike seawater species such as those in family Anisakidae, freshwater species have not been well studied. In particular, the influences of these nematodes on the body condition of salmonids remain unclear. We studied the effects of Salmonema cf. ephemeridarum on the body condition of masu salmon Oncorhynchus masou. We found a positive relationship between the number of parasites and fish fork length. In contrast, we found a negative relationship between the body condition (condition factor) of fish and the number of parasites. These results suggest that nematode infection could affect host energy reserves for future growth. [ABSTRACT FROM AUTHOR]

Brown trout Salmo trutta L. lives mainly in European rivers and is also bred in hatcheries for fishery purposes. Since the end of 19th century it has been introduced in all other continents. For the present survey most of the known self-sustaining brown trout river populations in Japan have been sampled and analyzed through sequences of the entire mitochondrial DNA control region and twelve microsatellites. In Japan, brown trout are genetically not homogeneous, probably as a consequence of several introductions, one in the Azusa river and at least one other in the remaining territory. The Chuzenji hatchery houses a genetically very distinct strain, probably due to intense manipulation in isolated scientific experimentations over 30 years. Finally, most populations showed high genetic diversity (Mamachi, Kane and Odori streams, Lake Chuzenji) with the exception of the Azusa river samples. This molecular analysis clearly demonstrates a European north Atlantic origin arrived in two distinct ways.

Stocking with hatchery fish is the principal method for harvest augmentation and, recently, restoring endangered populations. However, there is increasing concern about the negative effects of competition between hatchery and wild fish. In this study, enclosure experiments were conducted to evaluate the effects of hatchery chum salmon fry (Oncorhynchus keta) on wild masu (Oncorhynchus masou) and chum salmon fry through density-dependent competition. For masu and chum salmon, density-dependent performance (foraging rate and growth) varied depending on the presence or absence of competitor species. The negative effects on the foraging rate (evaluated by stomach content mass) and growth of wild masu salmon caused by wild conspecifics were greater than those exerted by wild and hatchery chum salmon. Wild chum salmon decreased foraging rate with increasing density of conspecifics and masu salmon and also in the presence of hatchery chum salmon. Although we could not evaluate growth under interspecific competition, wild chum salmon show body mass loss in the presence of hatchery chum salmon. These results suggest that the effects of stocking with hatchery chum salmon vary depending on the interacting species. L'empoissonnement avec des poissons d'elevage est la principale methode d'augmentation des recoltes et, recemment, de retablissement de populations en peril. Les effets negatifs de la concurrence entre poissons d'elevage et poissons sauvages suscitent cependant des inquietudes croissantes. Dans cette etude, des experiences en enclos ont ete menees afin d'evaluer les effets causes par la concurrence dependant de la densite d'alevins d'elevage de saumon keta (Oncorhynchus keta) sur des alevins sauvages de saumons masou (Oncorhynchus masou) et keta. Pour les saumons masou et keta, la performance dependant de la densite (taux d'alimentation et croissance) variait selon la presence ou non d'especes concurrentes. Les effets negatifs sur le taux d'alimentation (evalue a la lumiere de la masse du contenu stomacal) et sur la croissance des saumons masou sauvages causes par des conspecifiques sauvages etaient plus grands que ceux causes par des saumons keta sauvages ou d'elevage. Le taux d'alimentation des saumons keta sauvages diminuait a mesure qu'augmentait la densite de conspecifiques et de saumons masou, et aussi en presence de saumons keta d'elevage. Bien que nous n'ayons pas pu evaluer la croissance en situation de concurrence interspecifique, les saumons keta sauvages presentent une diminution de leur masse corporelle en presence de saumons keta d'elevage. Ces resultats laissent croire que les effets de l'empoissonnement de saumons keta d'elevage varient selon les especes en presence. [Traduit par la Redaction], Introduction For many fish species, stocking has been the dominant method of harvest augmentation and, recently, for conservation and restoration of endangered populations (Molony et al. 2003; Lorenzen et al. [...]

We examined the effect of flow regime on the occurrence of interference and exploitative competition in a salmonid species, white-spotted char (Salvelinus leucomaenis). In the lotic treatment, char showed typical consequences of interference competition (i.e., fish aggressively defended their foraging positions, and individuals occupying the most profitable positions grew fastest). In the lentic treatment, however, char behavior was consistent with exploitative competition; fish cruised over a larger area in search of food resources and showed limited aggression and no evidence of a dominance hierarchy. Our results indicate that white-spotted char showed interference competition more commonly than exploitative competition in the lotic habitat and vice versa in the lentic habitat. Resume: Nous examinons leffet du regime decoulement sur la presence de competition dinterference et dexploitation chez une espece de salmonide, lomble a taches blanches (Salvelinus leucomaenis). Dans les conditions experimentales deau courante, les ombles affichent les consequences typiques de la competition dinterference (cest-a-dire que les poissons defendent avec agressivite leurs sites dalimentation, et les individus occupant les sites les plus favorables croissent le plus vite). En revanche, dans des conditions experimentales deau stagnante, les poissons explorent de grandes surfaces en quete de ressources alimentaires, manifestent peu dagressivite et il ny a pas dindication de hierarchie de dominance. Nos resultats indiquent que lomble a taches blanches presente plus communement une competition dinterference qu'une competition dexploitation dans lhabitat lotique et linverse dans lhabitat lenitique. [Traduit par la Redaction], Introduction Interference and exploitative competition are two of the primary modes of competition (Miller 1967). Interference competition is characterized by interindividual interactions (e.g., aggression) that occur when individuals compete for [...]

Interspecific competition is a mechanism by which native salmonids can be replaced by non-native species. According to the two-species Lotka-Volterra competition model, replacement of the native species would occur when the non-native species has a competitive advantage over the native species and interspecific competition is more intense than competition within each of the two species. However, field observations have implied that visual barriers such as woody debris may slow down the replacement by mitigating interspecific competition. Using an experimental stream with white-spotted charr (Salvelinus leucomaenis (Pallas, 1814)) as the native species and brown trout (Salmo trutta L., 1758) as the non-native species, this study examined aggressive actions within and between species to assess the relative intensities of interspecific and intraspecific competitons within native species and the effect of visual barriers for reducing the relative intensity. In a sympatric and no-barrier treatment where trout was the dominant species, interspecific competition occurred more intensely than intraspecific competition among native charr. However, the relative intensity of interspecific competition decreased in a sympatric and with-barrier treatment. Our results suggest that interspecific competition may contribute to the replacement of native species by more competitive, non-native species. However, restoring visual barriers is a potential method to mitigate interference interactions and may deter the replacement. La competition interspecifique est un mecanisme par lequel des salmonides indigenes peuvent se voir remplacer par des especes non indigenes. D'apres le modele de competition a deux especes de Lotka-Volterra, le remplacement de l'espece indigene se produit lorsque l'espece non indigene possede un avantage competitif sur l'espece indigene et que la competition interspecifique est plus intense que la competition a l'interieur de chaque espece. Cependant, les observations de terrain laissent croire que des barrieres visuelles, comme par exemple les debris ligneux, peuvent ralentir le remplacement en mitigeant la competition interspecifique. En utilisant un cours d'eau experimental avec l'omble a taches blanches (Salvelinus leucomaenis (Pallas, 1814)) comme espece indigene et la truite brune (Salmo trutta L., 1758) comme espece non indigene, notre etude examine les actes d'agression a l'interieur de chaque espece et entre les especes afin d'evaluer les intensites relatives des competitions interspecifique et intraspecifique chez l'espece indigene et les effets des barrieres visuelles sur la reduction de l'intensite relative. Dans des conditions experimentales de sympatrie sans barriere, dans lesquelles la truite est l'espece predominante, la competition interspecifique est plus intense que la competition intraspecifique chez les ombles indigenes. Cependant, l'intensite relative de la competition interspecifique diminue dans des conditions experimentales de sympatrie avec des barrieres. Nos resultats indiquent que la competition interspecifique peut contribuer au remplacement des especes indigenes par des especes non indigenes plus competitives. Cependant, le retablissement de barrieres visuelles est une methode potentielle pour mitiger les interactions d'interference et elle peut freiner le remplacement. [Traduit par la Redaction], Introduction Interspecific competition frequently causes a decline in many taxa through the replacement of native species by non-native species (e.g., mussel: Ricciardi and Whoriskey 2004; mosquito: Juliano 1998; squirrel: Gurnell [...]

Abstract: This study aimed to confirm the mitigation effect of structurally complex habitat on interspecific interference competition between native and non-native salmonid species. We evaluated the effects of habitat complexity [...]

Native species may show invasiveness toward a recipient ecosystem through increases in abundance as a result of artificial stocking events. Salmonid species are typical examples of native invaders whose abundance is increased after stocking with hatchery fish. This study evaluated the effects of hatchery chum salmon fry on sympatric wild masu salmon fry, benthic invertebrate prey, and algae, after a single stocking event in Mamachi stream, Hokkaido, northern Japan. The results suggested that the stocked hatchery chum salmon fry decreased the foraging efficiency and growth of the wild masu salmon fry through interspecific competition, and depressed the abundance of Ephemerellidae and total grazer invertebrates (Glossosomatidae, Heptageniidae, and Baetidae) through predation. Also, the hatchery chum salmon fry may increase algal biomass through depression of grazer abundance by predation (top-down effect). These results suggested that a single release of hatchery chum salmon fry into a stream may influence the recipient stream ecosystem. [ABSTRACT FROM AUTHOR]