Large contribution of pulsed subsidies to a predatory fish inhabiting large stream channels

Resource subsidies exert critical influences on recipient habitats with relatively higher perimeter-to-area ratios, such as headwaters in watersheds. However, little is known about how those subsidies contribute to the energy sources in recipient habitats where the perimeter-to-area ratio is low, such as large stream channels. Here, we show that the diet of small Japanese eels (Anguilla japonica) <500 mm in total length inhabiting natural shoreline areas in large stream channels consists largely of terrestrial earthworms (Metaphire spp.). Stable isotopic analyses showed that the earthworms were the prey animal that contributed most to the eels’ diet (45%–47%). Earthworms constituted the largest portion of the eels’ stomach contents (7%–93%). Eels ingested earthworms within 2 days after rainfall during spring, summer, and autumn, and their consumption increased as the precipitation increased. These findings indicate that the pulsed earthworm subsidy that is driven by rainfall could temporarily bias the eels’ diet toward this allochthonous resource, which may explain the large contribution of the subsidy for consumers inhabiting large stream channels. Furthermore, diverse earthworm species could drive multiple pulsed subsidies across seasons and provide the predators with a prolonged subsidy, enhancing the long-term contribution of the subsidy to the predators’ diet.