Energy and nutrients flow in diverse pathways across heterogeneous landscapes and tightly link the discrete food webs in local habitats. However, parasitism that enhances allochthonous resource input has not been previously documented. In a well-known example of parasite manipulation of host behaviour, crickets infected by mature hairworms (Nematomorpha) seek and jump into water when the worms reach the free-living stage. We found that a large number of trout (22%–61%), an aquatic predator, preyed on camel crickets (genera Diestrammena Brunner von Wattenwyl, 1888 and Tachycines Adelung, 1902) in September in five Japanese mountain streams where this host–parasite system exists. Trout (Kirikuchi charr, Salvelinus leucomaenis japonicus (Oshima, 1961); red-spotted masu salmon, Oncorhynchus masou ishikawae Jordan and McGregor, 1925) that preyed on crickets frequently ingested hairworms, whereas trout that did not prey on crickets did not ingest hairworms. Our results strongly suggest that hairworms enhance stream salmonid predation on camel crickets. This is the first documentation of parasitism enhancing allochthonous resource input in nature. Trout ingested a greater mass of crickets than other prey species in September, and this energy influx may play an important role in food-web dynamics in headwater streams.
The changes in physical habitats and responses of fish, e.g. the Kirikuchi charr Salvelinus leucomaenis japonicus, were examined by placing instream structures in a stream on the upper drainage of the Totsu River system of the Kii Peninsula, Japan, over a two-year period. Instream structures created pools below the structures, and these pools have maintained their functions as fish habitats over two years. In two treatment sections, sections A and B (some red-spotted masu salmon were removed in advance in section B), mean water depth increased soon after placing the structures, but decreased over two years. Mean water velocity slowed after placing the structures in both sections. Total fish abundance has been relatively high at the two treatment sections compared with a control section after placing the structures. Age 1 Kirikuchi charr, however, increased only in section B. These results indicate that instream structures can serve as habitat enhancement for fish, including Kirikuchi charr, in the study area if other appropriate habitat factors for respective species are taken into consideration.
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