Movement patterns of Japanese fluvial sculpin Cottus pollux in a headwater stream

Movements of the Japanese fluvial sculpin Cottus pollux were studied over a 2-year period using a mark-recapture technique in two reaches (100 and 300 m) of a small, temperate stream system, the Fujii River, Japan. Of 321 recaptures, 62 fish (19.3%) moved across at least one riffle (i.e., were mobile). Fish moved distances of up to 192 in (median = 4 in). Distance moved (m) by all fish correlated positively with the recapture interval (d) in both years, illustrating the importance of the temporal dimension for movements. Mean daily distance moved (m/d) in the high-flow year was greater than that in the low-flow year. The high-flow year, which had a higher frequency of peak flow events, had a higher frequency of mobile fish than did the low-flow year, suggesting that interannual variation in movements is related to streamflow. Fish size and growth did not differ between mobile fish and residents in the high-flow year; however, mobile fish were larger and grew more slowly than residents in the low-flow year. Patterns of movement from multiple recaptures showed evidence for round-trip movements at channel unit and reach scales, but no fish completed round-trip movements by crossing a small weir (< 0.5 m in height). These results emphasize the importance of maintaining fish passage and pool-riffle sequences throughout the stream course to prevent isolation of subpopulations by weirs, culverts, and road crossings. Physical barriers to Japanese fluvial sculpin movements could be mitigated by placing a ramp of cobbles downstream of the barriers.