Pulsed supplies of individuals or organs can dramatically affect recipient ecosystems by facilitating predation and competition and altering diets and food intake (Yang et al. 2008). One common example of pulsed supply is masting, in which individuals of one plant species synchronously produce large amount of seeds in the same season, and consumers (e.g., rodents) exclusively forage on the seeds in competition with other consumers (Lobo and Millar 2013; Stephens et al. 2019). Outbreaks of the desert locust (Schistocerca gregaria) during great migrations are another typical example: the locust swarms decrease food intake of indigenous herbivores through resource competition, while increasing the food intake of predators (Sánchez-Zapata et al. 2007).
Intraguild (IG) predation occurs when predators (IG predators) prey upon potential competitor species (IG prey) (Arim and Marquet 2004). The occurrence of intraguild predation is affected by the relative density of IG prey and of shared food resources (i.e., the total food availability for IG predators), and pulsed supplies of IG prey or of shared food resources can modify intraguild predation relationships by changing these relative densities (Chacón and Heimpel 2010). For example, Greenville et al. (2014) demonstrated that pulsed supplies of shared food resources (an outbreak of rodents) decreased predation pressure by IG predators (the dingo [Canis dingo]) on IG prey (the red fox [Vulpes vulpes] and feral cat [Felis catus]) in the Australian desert. The effects of a pulsed supply of IG prey, however, are likely to be more complex. Since IG prey function as both competitors and prey for IG predators, a pulsed supply of IG prey may decrease food intake and growth of IG predators through resource competition, while at the same time increasing food intake by boosting the supply of prey resources.
One example of an artificially pulsed supply of organisms is the stocking of hatchery-reared fish into natural environments to boost fisheries resources. The stocking of hatchery-reared salmonid fry, in particular, is frequently conducted in rivers inhabited by other wild salmonids. Because both hatchery-reared and wild salmonids predominantly prey upon aquatic and terrestrial invertebrates, they are often in competition for food resources (Hasegawa et al. 2018). In salmonids, both interspecific competition and competition between hatchery-reared and wild fish are density dependent (Grossman and Simon 2020; Matte et al. 2020). Interference competition, in which dominant (i.e., larger) individuals occupy profitable foraging territories, is the principle competitive mode, and exploitative competition, in which individuals scramble for food resources, are thought to occur simultaneously (Elliott 2002). If multiple salmonid species coexist in the same stream, their dietary niches will be affected by interspecific competition. Diet niche partitioning is sometimes regarded as the outcome of interspecific competition (e.g., one species mainly preys upon drifting terrestrial invertebrates, while another preys upon benthic invertebrates; Nakano et al. 1999; Mookerji et al. 2004). Although high diet-niche overlap is often indicative of intense interspecific competition (Larocque et al. 2021), it may also occur in systems where one type of food item is highly abundant, and fish prey upon them exclusively. In such systems, density-dependent competition may be lacking.
Salmonids of various body sizes also prey on fish to the extent that their gape size allows (e.g., L’Abée-Lund et al. 1992; Daly et al. 2009; Hasegawa et al. 2012). The frequency of this piscivory is determined by various factors. For example, of the three types of habitats inhabited by salmonids (riverine, lacustrine and marine habitats), the occurrence of piscivory is the lowest in riverine habitats due to the high availability of other food items such as drifting aquatic and terrestrial insects (Keeley and Grant 2001; Sánchez-Hernández 2020). Population origins (i.e., genetic backgrounds) also determine whether salmonids become piscivorous or insectivorous (Monnet et al. 2020, 2021), and the two ecotypes differ in some biological traits: for example, piscivores tend to locomote more frequently and have longer gut lengths (Monnet et al. 2020, 2021). However, Hasegawa et al. (2021) reported that riverine salmonids originating from insectivorous populations will still prey exclusively upon stocked salmon fry just after stocking (i.e., when there is an extremely high availability of stocked fry).
Therefore, streams stocked with hatchery-reared salmonid fry are ideal experimental systems in which to test hypotheses regarding intraguild predation and the pulsed supply of organisms. In this study, we conducted field surveys to test two hypotheses: (1) IG prey supplied pulse-wise will compete with IG predators, but (2) intraguild predation will mask the negative effects of competition on IG predators.