Ecosystem response to spawning bed enhancement in a Yuba River tributary in Northern California

Background/Question/Methods

As the last tributary of the Yuba before the impassable Englebright Dam, Deer Creek provides critical habitat for Central Valley Spring-run, Fall-run, and late Fall-run Chinook salmon (Oncorhynchus tshawytscha) and Steelhead (Oncorhynchus mykiss).  There is a lack of suitable spawning materials in Deer Creek due to the presence of upstream dams. Since 2010, Sierra Streams Institute (SSI) has been undertaking projects to augment the supply of spawning material in Deer Creek, including spawning bed enhancement efforts in 2012 and 2013 . In total, SSI placed approximately 700 tons of spawning material into Deer Creek at three locations spanning 1400 feet of creek. To evaluate how the placed spawning materials are incorporated into the aquatic system, we monitored O. tshawytscha and associated mammal  and avian visitation, benthic macroinvertebrate and algae colonization, water quality, and habitat conditions in three enhanced areas and three unenhanced areas. The number of redds in the stream reach was compared to past years to evaluate salmon use of newly placed spawning material. Non-metric multidimensional scaling and multi-response permutation procedures  were used to evaluate the differences in BMI communities between unmodified and modified sites, and randomization tests were used to compare richness and diversity metrics between sites.

Results/Conclusions

Pebble counts indicate that the median pebble size (d₅₀) in each of the enhanced areas was reduced to a size suitable for salmon spawning, from 77mm to 22mm in work area 1, 100mm to 15mm in work area 2, and 100mm to 13mm in work area 3. Over 75% of observed spawning activity occurred in enhanced sites, with three times more salmon redds in 2012 than 2011, and five times more in 2013 than 2011.  Mammal and bird surveys revealed an impact on trophic interactions as a result of increased salmon activity. We found no significant difference in benthic communities between modified and unmodified sites by week 4 (p<0.05), and no significant difference in algae cover by week 10. Sorenson distance between modified and unmodified sites decreased to <0.5 by week 8. 75% of food source macroinvertebrates sampled were more abundant in modified sites (p<0.05).  The data suggest that the stream’s spawning capacity had been severely limited by lack of suitable habitat materials, that spawning gravels can be quickly incorporated into the aquatic system by benthic organisms, and that salmon spawning bed enhancement can elicit an ecosystem-wide response.