Juvenile steelhead trout ecology in an intermittent stream

Project Summary: 

The purpose of this study is to assess the role of increasing stream fragmentation during California's summer drought period on traits such as growth, survival and movement of juvenile steelhead and on the food web that supports them.

To assess the effects of seasonal stream fragmentation on available habitat, predator (steelhead) ecology, and ecosystem processes, we studied 12 contiguous riffle-pool habitats (areas of alternating riffle [shallow with higher flow speeds] and pooled [deeper with lower flow] water in a stream) in a small California stream from May through October 2009. We measured stream connectivity by quantifying riffle dimensions weekly. We tagged and tracked juvenile trout over the summer to estimate their movement, growth, and survival rates, and collected algal samples to estimate primary productivity.

Our preliminary results indicate that our study stream became increasingly fragmented as the summer progressed, and that this had effects on predator ecology. All 12 of the riffles and 5 of the pools in our study dried up completely by late summer. Tracking data indicate that there was significant movement of juvenile steelhead among our study pools prior to complete disruption of hydrologic connectivity. Many steelhead moved into large pools that persisted throughout the summer and enjoyed high survival rates. However, of the 38 steelhead that we marked in total, 18 were not detected during the late summer, suggesting that dry season mortality can be high. Next steps include comparing increased stream fragmentation to changes in primary productivity over the course of the summer to see if fragmentation may have impacted productivity.