Masters Thesis

Extended residency and movement behavior of juvenile steelhead (Oncorhynchus mykiss) in the Russian River estuary, California

A total of sixty-nine wild juvenile steelhead were surgically implanted with acoustic transmitters and tracked within the estuary and main stem of the Russian River, California, for up to 141 days from mid-July to early December, 2005 and 2006. Each implanted transmitter emitted a unique code that allowed identification of individual steelhead. Estuarine use, movement behavior, and extended estuarine residency were documented using an array of fixed telemetry receiver stations and frequent mobile tracking surveys. Continuous water quality monitoring was conducted throughout the estuary and manual water quality measurements were taken at site specific locations of detected acoustic tagged steelhead during mobile tracking surveys. Manual water quality measurements were used to determine the association of physical parameters (i.e., water quality and depth) with estuarine use. Minimum estuary residence time was derived from the duration of detection and ranged from 4 to 121 days with significant growth occurring with longer residency. Accelerated growth rates in the estuary were observed in scale patterns from untagged and tagged wild steelhead and confirmed by observed growth and scale patterns of three acoustic tagged steelhead that were recaptured after more than 100 days at-large. Estuarine use, movement behavior, extended estuarine residency, and scale patterns of these recaptured steelhead were similar to those of other tagged and untagged steelhead utilizing the same areas of the Russian River estuary. Fall upstream movements of acoustic tagged wild steelhead and captures of untagged estuary steelhead observed aggregating with acoustic tagged steelhead provided evidence that long estuarine residency coupled with accelerated growth resulted in life history traits similar to those of more northern populations in which the half-pounder life history is present. Observed, extended estuarine residency, accelerated growth, and occurrence of half-pounder-like steelhead indicate a very productive rearing environment within the Russian River estuary.