Multipath ecological influence of an iteroparous fish migration from Lake Biwa to an alluvial stream

Abstract

An increasing number of studies describe the ecological influence of semelparous and iteroparous fish migrations on spawning streams. This influence consists of not only the carcasses of spent adults, but also the nutrients that they excrete subsidising stream communities. Given the complex food‐web structure of stream communities, which includes aquatic insects from various functional feeding groups, resource subsidies provided by migratory fishes should influence stream communities through multiple pathways. We evaluated the ecological influence of an iteroparous three‐lips fish (Opsariichthys uncirostris uncirostris) in a alluvial stream by treating fish weirs as large‐scale manipulative field experiments. Between fish‐bearing and fish‐free areas, we compared concentrations of nutrients and fine particulate organic matter (FPOM), and compared densities and stable isotopes of periphyton and benthic macroinvertebrates. The presence of the three‐lips increased the concentrations of ammonium, soluble reactive phosphorus, particulate organic carbon and nitrogen, and chlorophyll‐a densities by up to 2.7‐fold. The δ15N of FPOM and macroinvertebrates during the migratory season were 2.8–7.0‰ higher in the fish‐bearing area, indicating a greater contribution of fish‐derived organic matter. Three‐lips subsidised stream macroinvertebrates through multiple pathways: consumers fed on (1) periphyton increased by the fish nutrient subsidy, (2) fish‐derived organic matter (such as fish carcasses and eggs), and (3) FPOM with fragmented periphyton and fish‐derived organic matter constituents. Results showed that, given the complex food‐web structures in alluvial streams, migratory fish influence stream ecosystems in complex ways. Further studies of the ecological influence of migratory fishes in alluvial streams are needed.