Abstract
The Salmonidae (the trout and salmon) is one of the most highly recognized groups of anadromous fishes. Anadromous salmonids, such as Chinook Salmon (Oncorhynchus tshawytscha) and steelhead (O. mykiss), migrate long distances in order to optimize feeding and growth opportunities found in ocean habitats at the cost of potentially lower survival rates. In the Central Valley of California, historical freshwater habitat spanned from the upper tributaries of the Sacramento River in the north to the Kings River basin in the south, and included all of the San Francisco Bay-Delta Estuary. Central Valley steelhead populations, which migrate to the Pacific Ocean to rear to adulthood, are currently in decline. The National Marine Fisheries Service listed Central Valley steelhead as threatened under the Endangered Species Act in 1998. The decline of Central Valley steelhead is primarily attributed to water development and management, more specifically habitat loss resulting from dam construction. The construction of dams v on Central Valley rivers and streams has resulted in the loss of roughly 80% of historical anadromous salmonid spawning habitat. The research I conducted on steelhead emigration took place on the lower American River, a tributary to the Sacramento River in California’s Central Valley. The American River watershed is approximately 5,500 km2 in surface area and drains the west slope of the Sierra Nevada. The head waters of the American River begin at the crest of the Sierra Nevada and flow southwest until they enter the Sacramento River at the present location of the city of Sacramento, California. Currently, steelhead are restricted to the lowermost 36 km of the American River downstream of Nimbus Dam. The purpose of this project was to determine the emigration patterns of juvenile steelhead on the lower American River and to determine if there was a difference in patterns between natural- and hatchery-origin steelhead making their migration as smolts to the Pacific Ocean. More specifically, the objectives of the study were as follows: in 2012 and 2013, to implant steelhead smolts of both hatchery and natural origin with acoustic transmitters (tags), release hatchery smolts, and track all tagged steelhead to determine emigration timing, survival, and distributions. The hypotheses tested were as follows: (1) there is a significant difference in emigration success between natural- and hatchery-origin steelhead smolts; (2) there is a significant difference in emigration timing between natural- and hatchery-origin steelhead smolts; (3) there is a significant difference in ocean arrival timing between natural- and hatchery-origin steelhead smolts; and (4) vi there is a significant difference in emigration route selection between natural- and hatchery-origin steelhead smolts. In 2012, 80 hatchery-origin steelhead smolts were surgically implanted with acoustic transmitters and then released into the lower American River. In addition, 12 juvenile steelhead of natural origin were captured via hook and line from the lower American River and surgically implanted with acoustic transmitters in the field. In 2013, the same tagging techniques were followed but sample size was different: 51 hatchery- and 80 natural-origin steelhead were tagged and tracked. Steelhead were tracked on stationary acoustic receivers maintained by the California Fish Tracking Consortium, including seven in the lower American River. Data collected on the receivers were uploaded monthly to an online database called Hydra. This study shed light on little known life-history aspects of juvenile American River steelhead. Overall, there was not a significant difference in survival between steelhead smolts of natural and hatchery origin. However, smolts that travelled farther in-river had a significantly reduced survival rate to the confluence of the American and Sacramento rivers. The highest mortality of smolts, regardless of origin, was within the American River itself. The results also showed that hatchery-origin steelhead smolts began leaving the river as soon as the day they were released, peak emigration occurred in early February, and survivors reached the Pacific Ocean by mid-March. Natural-origin steelhead had a wider emigration window: smolts began to leave as early as January, peak emigration occurred in mid-February after hatchery-origin smolts had emigrated, vii and survivors reached the Pacific Ocean by May. Steelhead of natural and hatchery origin both utilized alternative pathways on their migration to the ocean, and survival within these pathways could not be attributed to smolt origin. Natural-origin smolts were significantly larger than hatchery-origin smolts but smolt size did not explain emigration survival. Swim speeds were not significantly different between hatchery- and natural-origin smolts. However, natural-origin smolts travelled faster than hatchery smolts, most likely due to their larger size. Lower American River steelhead smolts are similar to other steelhead stocks in the Central Valley. Smolt size is similar in the American, Sacramento, Feather and Mokelumne rivers, although smolt age varies. Emigration timing and swim speeds are also similar. All steelhead smolts, regardless of origin and river, experience high mortality in-stream. Results of this study suggest that predation may be a major stressor to juvenile steelhead in the lower American River. Improving the smolt-to-adult survival rate could significantly increase adult abundance and ultimately contribute to species recovery. Current water management operations on the lower American River are a risk to the American River steelhead population. Population risks from operations arise from reduced flow and increased water temperatures, which greatly decrease the amount of suitable habitat for juvenile steelhead on the lower American River. Reduced habitat is associated with reduced juvenile steelhead survival, and may also decrease population diversity. viii Results from this study can be used for management purposes, such as determining timing of water deliveries in such a manner as to lessen the impact on juvenile steelhead. The results of this study have also been used as a baseline to compare the performance of a potential replacement brood stock of steelhead, which were introduced into the lower American River in the winter of 2014-2015.