Effect of Light Intensity on Predation of Sockeye Salmon Fry by Cottids in the Cedar River

摘要

In the Cedar River, we examined the relationship between light intensity level and migratory behavior of sockeye salmon fry (Oncorhynchus nerka) and predation by cottids. Additionally, light intensity readings were taken to document the location of high artificial light intensity levels and determine what atmospheric conditions affect those light levels. The effect of light intensity on sockeye salmon fry was examined with two methods: 1) comparison of lighted and non-lighted areas in the City of Renton; and, 2) experimental trials with standardized amounts of light added to the river. Within the lower 2.9 kilometers of the Cedar River, there were several locations with highlight intensity levels. Most were next to street bridges. The highest light readings recorded wereat the I-405 Bridge and the Renton Library. In the lower Cedar River, artificial lighting appeared to come from two major sources: direct lighting and reflected lighting off of the clouds. At locations with minimal direct lighting, the highest light intensity levels occurred on overcast nights due to reflected light. The lowest levels occurred during clear, moonless nights. As far upstreamas river kilometer 9.8, we recorded readings during overcast nights that were higher than during a clear nightwith a full moon. Experimental trials were done at two locations away from any lights, Lions Club Park and Elliot Park. Two trials were done at the Lions Club Park, both following a release of hatchery sockeye salmon fry. Most trials at Elliot Park were conducted below a sockeye salmon spawning channel. At the Lions Club Park, light intensity treatments were done in two habitat types, gravelshore and rip-rap shore. At all City of Renton sites examined, the abundance of sockeye salmon fry was substantiallyh igher at sites with high light intensity levels than at nearby sites with low light. Correspondingly, most predation of fry by cottids was observed in the bright light areas. Higher predation rates wereobserved along the shoreline as well as in the mid-channel area. In relation to other samplingconducted by the U.S. Fish and Wildlife Service, the I-405 Bridge lighted area was the only location we have ever seen any significant predation of fry by cottids in a mid-channel area of a riffle in the Cedar River.