Habitat use by spinous loach (Cobitis shikokuensis) in southwestern Japan: importance of subsurface interstices

We examined habitat use by spinous loach (Cobitis shikokuensis), an endangered benthic fish in Japan, in relation to distance to the stream bank, water depth, current velocity, substrate types and bed-subsurface conditions (fine-sediment volume, vertical hydraulic gradient). In the study reach (Shigenobu River in Shikoku Island), spinous loach exhibited a patchy distribution within the channel, being limited to sloping bed of channel margins. Although high selectivities were detected for three variables (close to the stream bank, pebble-dominant substrates, and low fine-sediment volume) from univariate perspective, decision-tree analysis indicated that their distribution pattern was best explained by the two variables representing subsurface conditions. Locations occupied by spinous loach were characterized by extremely low fine-sediment volume (≤1.5%), or by moderate fine-sediment volume (1.5–9.8%) with positive vertical hydraulic gradient (indicative of upwelling). Our results suggest that subsurface interstices are essential habitat for spinous loach and that prevention of excessive inputs and deposition of fine sediments, which cause interstitial sedimentation, is crucial for conservation of this endangered species.