Seasonal use of the hyporheic zone by spinous loach, Cobitis shikokuensis, in an intermittent river, southwestern Japan

Background/Question/Methods

The hyporheic zone is the saturated interstices below the streambed and banks and known as an ecotone between river and groundwater ecosystems. Use of the hyporheic zone as habitat has been documented in many benthic invertebrates. However, little is known about its use by benthic fishes. Here, we show the importance of the hyporheic zone in a lifecycle of an endangered spinous loach (Cobitis shikokuensis) in southwestern Japan. During October 2009-November 2010, we monitored spinous loach density both on the streambed surface by electrofishing and in the hyporheic zone (5-25 cm below the surface in pebble-gravel sediment) by bilge pump sampling in the Shigenobu River.

Results/Conclusions

The loach density on the streambed exhibited a strong seasonality that they disappeared from the bed surface during the winter (December-March). By the bilge pump sampling, the loach was captured from the hyporheic zone (15-25 cm depth) in all seasons, with the peak density being in the summer (their reproductive season). Most individuals captured from the hyporheic zone in the summer were small juveniles (less than 3 cm in total length). Our results suggest that the loach uses the hyporheic zone both at their early-developmental stage and for overwintering. Hydrological connectivity between the streambed surface and the hyporheic zone is critical for conservation of this endangered loach.