The effects of stream fragmentation on the movements of juvenile salmonids in a Mediterranean-climate stream

Background/Question/Methods

Stream fragmentation, the division of a once continuous stream into smaller disconnected reaches, can severely restrict movement of stream-dwelling organisms. The John West Fork (JWF), in Point Reyes National Seashore, CA, is a first-order Mediterranean-climate stream characterized by high flows during the winter season and low flows during the summer months. During the summer, the stream loses hydrologic connectivity as shallow habitats dry, resulting in a series of isolated pools. Stream connectivity is restored with the onset of rain during the wet winter season. This study focuses on the influence of stream fragmentation on movements of juvenile steelhead trout and coho salmon. During the summer of 2011, we captured fish from an ~1km stretch of the stream at the beginning of the summer, measured their body size, and implanted each individual with a uniquely-coded identifier (PIT tag), and then released them back into their original pools. We then tracked the location of individual fish using a portable PIT-tag antenna across the summer and fall to examine species- and season-specific movements.

Results/Conclusions

Our results indicate that JWF became increasingly fragmented as the summer progressed. By late September, 25 of 29 riffles had dried completely. From early to mid-July, we observed significant among-pool movement for both species, with 35% of tagged steelhead and 33% of tagged coho moving at least one pool. Beginning in late July, movement ceased as riffles started to dry. Due to low levels of precipitation in the fall, the stream remained in a fragmented state until late November and fish movements were limited (proportion of fish moving at least one pool: steelhead = 0.05%, coho = 0%). However, flow was reestablished after a large storm event in late November and as a result, significant movement was observed. Following this storm event, a period of little precipitation led to the stream once again fragmenting and limiting the movements of fish. Our findings indicate that movement of salmonids is affected by habitat contraction, and that movements cease prior to complete fragmentation suggesting a minimum threshold level of water required by fish to successfully move across riffles. We plan to expand this work to future seasons and years to develop a better appreciation for how changing water levels alter movements of stream fish.