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The influence of resource subsidies on animal growth, survival and reproduction is well understood, but their ultimate effects on life history have been less explored. Some wild species have a partially migratory life history, wherein migration is dictated based upon threshold traits regulated in part by the seasonal availability of resources. We conducted a large-scale field manipulation experiment where we provided a terrestrial invertebrate subsidy to red-spotted masu salmon. Individuals in stream reaches that received a subsidy had, on average, a 53% increase in growth rate relative to those in control reaches. This increased growth resulted in a greater proportion of individuals reaching the threshold body size and smolting in the autumn. Consequently, 19–55% of females in subsidized reaches became migratory, whereas 0–14% became migratory in the control reaches. Our findings highlight seasonal ecosystem linkage as a key ecosystem property for maintaining migratory polymorphism in partially migratory animals. [ABSTRACT FROM AUTHOR]

We examined variations in age at seaward migration and sea age for the anadromous form of red‐spotted masu salmon (Oncorhynchus masou ishikawae) in two Japanese rivers. The anadromous form of red‐spotted masu salmon expressed only two sea migration patterns in the two rivers: (a) the majority of the salmon (95%, n = 81) were of age‐0, and age‐1 migrants were rare (n = 4); and (b) all the salmon examined (n = 22) made a return migration within a year, with 23% of the salmon exhibiting potamodromy in the river. Owing to low variation in their sea migratory patterns, the anadromous form of red‐spotted masu salmon is likely vulnerable to environmental fluctuations. [ABSTRACT FROM AUTHOR]

Most resource subsidies are temporally variable, dynamically affecting the consumer populations, community structures and ecosystem functions of recipient ecosystems. Temporally variable resource subsidies are characterized by the duration, magnitude, timing and frequency of resource subsidy inputs. These different characteristics may have different mechanisms by which to affect recipient ecosystems.Few studies have examined the duration of resource subsidy inputs on recipient ecosystems, although there exist previous studies focusing on magnitude, timing and frequency.We provide the first experimental test of the effects of subsidy duration on a stream ecosystem by using an outdoor mesocosm experiment, in which we directly manipulated the subsidy duration (pulsed vs. prolonged) of terrestrial invertebrate input into the mesocosm.Given the same overall amount of terrestrial invertebrate subsidy was added, a prolonged subsidy allowed large‐stage fish to effectively monopolize the subsidy over small‐stage fish, which led small‐stage fish to maintain their predation pressure on in‐situ prey, that is, benthic invertebrates. On the other hand, a pulsed subsidy allowed small‐stage fish to increase their feeding rate of the subsidy and to become away from foraging in‐situ prey. Consequently, weaker indirect positive effects on in‐situ benthic prey and leaf break‐down rate were found with the prolonged versus pulsed subsidy. However, these indirect effects varied by the dominant benthic prey species, which differed in edibility for fish. Such predator‐specific vulnerability of benthic prey can be important in mediating trophic cascades in detritus‐based stream food webs.Phenological events that generate temporal subsidies (e.g. salmon spawning run and arthropod emergence) can be synchronized (pulsed) or desynchronized (prolonged) within and among species, depending on the degree of spatial and temporal environmental heterogeneity. The effects of subsidy duration would thus be important to better understand ecological processes in spatially and temporally coupled ecosystems. [ABSTRACT FROM AUTHOR]

Dominance hierarchies and the resulting unequal resource partitioning among individuals are key mechanisms of population regulation. The strength of dominance hierarchies can be influenced by size‐dependent tradeoffs between foraging and predator avoidance whereby competitively inferior subdominants can access a larger proportion of limiting resources by accepting higher predation risk. Foraging‐predation risk tradeoffs also depend on resource abundance. Yet, few studies have manipulated predation risk and resource abundance simultaneously; consequently, their joint effect on resource partitioning within dominance hierarchies are not well understood. We addressed this gap by measuring behavioural responses of masu salmon Oncorhynchus masou ishikawae to experimental manipulations of predation risk and resource abundance in a natural temperate forest stream. Responses to predation risk depended on body size and social status such that larger fish (often social dominants) exhibited more risk‐averse behaviour (e.g. lower foraging and appearance rates) than smaller subdominants after exposure to a simulated predator. The magnitude of this effect was lower when resources were elevated, indicating that dominant fish accepted a higher predation risk to forage on abundant resources. However, the influence of resource abundance did not extend to the population level, where predation risk altered the distribution of foraging attempts (a proxy for energy intake) from being skewed towards large individuals to being skewed towards small individuals after predator exposure. Our results imply that size‐dependent foraging–predation risk tradeoffs can weaken the strength of dominance hierarchies by allowing competitively inferior subdominants to access resources that would otherwise be monopolized. [ABSTRACT FROM AUTHOR]

Migration strategy requires complex life‐history decisions, and how variation in migration strategies can increase or decrease during migration to novel environments is fundamentally important for population persistence and adaptation for migratory animals. An anadromous form of red‐spotted masu salmon exhibited limited sea migratory patterns probably due to high migration costs in its southernmost habitat range. Contrary to this, we here found that the masu salmon have increased and shifted their migratory patterns when they migrate to freshwater lakes (i.e. adfluvial migration). Adfluvial migrants spent 1–3 years in lakes whereas anadromous migrants spend only half a year at sea. In addition, adfluvial migrants returned to the river to reproduce after maturation, whereas all anadromous migrants did so before maturation. Furthermore, males often adopted migratory tactics, shifting the migration propensity from female‐biased to unbiased, in a lake. The diversification of the years of migration and the shifts in return‐migration timing and sex‐ratio of migrants were only found in deep lakes, where migrants can continue migrating in the favour of the cold water of lakes. The present study demonstrated that a migratory salmonid extensively diversified and shifted its migratory life history, often requiring complex adjustments in life history traits with a genetic basis, in novel environments. [ABSTRACT FROM AUTHOR]

Variation in life history is fundamental to the long-term persistence of populations and species. Partial migration, in which both migratory and resident individuals are maintained in a population, is commonly found across animal taxa. However, human-induced habitat fragmentation continues to cause a rapid decline in the migratory phenotype in many natural populations. Using field and hatchery experiments, we demonstrated that despite both migrants and residents being maintained in captive environments, few individuals of the red-spotted masu salmon, Oncorhynchus masou ishikawae , became migrants in natural streams when released prior to the migration decision. Released fish rarely reached the threshold body size necessary to become migrants in natural streams, presumably owing to lower growth rates in natural than in captive environments. The decision to migrate is often considered a threshold trait in salmonids and other animal taxa. Our findings highlight the need for management programmes that acknowledge the effects of the environment on the determination of the migratory phenotypes of partially migratory species when releasing captive-bred individuals prior to their migratory decisions.