The Ecology of Juvenile Steelhead Rearing in an Intermittent Stream: The Influence of Flow Disruption on Growth, Survival, and Movement

Studies assessing the role of habitat fragmentation on biodiversity loss and ecosystem function have focused largely on spatial fragmentation across terrestrial landscapes. Changes due to temporal habitat fragmentation—temporary fragmentation of a habitat across time—are less well understood. Streams in Mediterranean-climate regions are ideal systems for studying temporal habitat fragmentation because the distinct wet and dry seasons result in dramatic seasonal changes in stream flow. During the summer low flow season, streams can become fragmented as hydrologic connectivity is disrupted leading to a series of isolated pools. The main objectives of this study were to illuminate the direct and indirect effects of temporal habitat fragmentation on habitat availability and the ecology of juvenile steelhead rearing in a small, intermittent stream in the Point Reyes National Seashore, California. To do so, we studied a series of riffle-pool habitats from May through October in 2009 and 2010. In both years, fish were sampled (measured and weighed) once in the early summer and again in late summer. During the first capture event, fish were implanted with uniquely-coded PIT-tags. We used a portable antenna to resight tagged fish each week across the summer season, which allowed us to estimate weekly-scale survival and movement rates. Additionally, we calculated growth rates for the subset of fish that were recaptured during our end-of-summer sampling event.

We found rapid reductions in shallow-water, riffle habitat and increasing fragmentation as the summer progressed, with more extreme fragmentation during a dry year (2009) relative to a wet year (2010). Results of our mark-recapture study suggest that fragmentation resulted in high mortality rates for steelhead in 2009, with an average weekly survival rate of 61% and an overall summer survival rate of 32%. In contrast, in 2010, when the stream was less fragmented and conditions were relatively benign, juvenile steelhead enjoyed much higher survival rates, with an average weekly survival rate of 75% and an overall summer survival rate of 65%. Additionally, a larger proportion of steelhead showed negative growth over the summer in 2009 (3 of 4 recaptures) versus 2010 (31 of 77 recaptures). Finally, fish were able to move among different pools much later into the summer of 2010 than 2009 because of wetter conditions in that year. Our findings suggest that stream fragmentation can have strong impacts on fish growth, survival, and movement and that the strength of these impacts is primarily driven by interannual variability in precipitation.