![]() Although DHA and EPA contents can vary among individuals of the same species owing to differences in genetic background 13, piscivorous fish tend to have high DHA content, and insectivorous/planktivorous fish tend to have high EPA content 10, 11, 12. Recent studies have used these traits of DHA and EPA in fishes to evaluate trophic levels in fish communities 10, 11, 12. Fish accumulate DHA via the bioconversion of EPA contained within food resources or by consuming DHA-rich prey (e.g., other fish), and they accumulate EPA mainly by consuming invertebrates or insectivorous fish 8, 9. Because DHA is an important nutrient for growth, reproduction, and the functioning of sensory organs 5, 6, 7, its availability could affect fish fitness. For example, many fishes have high contents of long-chain omega-3 fatty acids including docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) 4. Nonetheless, the nutritional compositions of several important fish species are well known 4. However, the nutritional contributions of bottom-up effects have not been adequately studied even in these systems. Many studies of bottom-up and top-down effects have focused on aquatic species because of the relative ease which with these effects can be detected in aquatic ecosystems 3. ![]() In particular, physiological benefits such as increased nutritional contributions to predators from prey have been relatively understudied in research on bottom-up effects despite that prey nutritional composition has the key role in predator growth and fitness 2. However, other benefits such as increased growth or foraging amounts can result from bottom-up effects. In general, the benefit is quantified as a population-size increment of an upper-trophic-level species 1. Future attempts to gain a comprehensive understanding of the impacts of fish stocking should consider not only community ecology but also physiology.īottom-up effects are benefits accrued by upper-trophic level organisms because of increased prey availability. The impacts of human activities, such as fish stocking, on freshwater ecosystems are a matter of serious concern for conservation. In addition, DHA increased with increasing body size of white-spotted charr and vice versa for EPA. The dynamics of fatty acid contents in charr inhabiting salmon-stocked and unstocked streams clearly support this hypothesis: fatty acid contents (DHA, EPA, and total fatty acid) increased after stocking in stocked streams, but not in unstocked streams. In this study, we tested the hypothesis that white-spotted charr Salvelinus leucomaenis, a salmonid fish, increases body stores of omega-3 fatty acids, especially docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), by preying upon stocked hatchery-reared masu salmon Oncorhynchus masou fry in streams. ![]() However, these physiological aspects of bottom-up effects have not been considered. For example, the increased abundance of prey resources can trigger physiological (internal) changes in predators, such as improvements in nutritional status. This phenomenon is known as a bottom-up effect. Increases in prey population size can affect the physiology and ecology of upper-trophic level organisms.
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