Chapter 1: Impending extinction of salmon, steelhead, and trout (Salmonidae) in California. California contains the southernmost native populations of most Pacific Coast salmon and trout, many of which appear to be rapidly headed toward extinction. A quantitative protocol was developed to determine conservation status of all salmonids native to the state. Results indicate that if present trends continue, 25 (78%) of the 32 taxa native to California will likely be extinct or extirpated within the next century, following the bull trout (Salvelinus confluentus), which was extirpated in the 1970s. California's salmonids are adapted to living in a topographically diverse region with a Mediterranean climate, characterized by extreme seasonal and inter-annual variability in streamflow. Consequently, California salmonids have evolved extraordinary life history diversity to persist in the face of stressful conditions that often approach physiological limits. The spatial distributions of California salmonids vary from wide-ranging anadromous forms to endemic inland forms persisting in only a few kilometers of stream. Eighty-one percent of anadromous taxa are threatened with extinction and 73% inland taxa are either threatened or already extinct. Although specific drivers of decline differ across species, major causes of decline are related to increasing competition with humans for water, human degradation of watersheds, and adverse effects of hatchery propagation. Climate change, interacting with the other causes of decline, is increasing the trajectory towards extinction for most populations. Bringing all of California's salmonid fishes back from the brink of extinction may not be possible. If there are bold changes to management policy, however, self-sustaining populations of many species may be possible due to their inherent ability to adapt to changing conditions.
Chapter 2: Floodplain Farm Fields Provide Novel Rearing Habitat for Chinook Salmon. When inundated by floodwaters, river floodplains provide rich food resources and high quality habitat for many species of wildlife. Globally, many rivers have been leveed and floodplain wetlands drained, primarily for flood control and conversion to agriculture. In the Central Valley of California, USA, where less than 5% of floodplain wetland habitats remain, a critical conservation question is how farmlands located on floodplains can be managed to provide increased habitat benefit to fish and wildlife. In this study agricultural fields located on the Sacramento River floodplain were intentionally inundated after the autumn rice harvest to mimic natural floodplain inundation patterns and determine if they could provide shallow-water rearing habitat for imperiled Sacramento River fall-run Chinook salmon (Oncorhynchus tshawytscha). Approximately 10,000 juvenile fish were reared on two hectares for six weeks when fields were idle between fall harvest and spring planting. Growth rates of juvenile salmon (average 0.76 mm/day) reared in this surrogate floodplain habitat were among the most rapid ever documented in fresh water in California and survival was high (minimum estimated survival 57%). The apparent compatibility of crop production and fish and wildlife habitat on the same ground demonstrates the ecological and economic benefits of reconciling management of agricultural working landscapes with recovery of native species.
Chapter 3: Managed Inundation of Agricultural Floodplains Produces Record Growth in Juvenile Chinook Salmon. Floodplains are important engines of seasonal productivity in river ecosystems. However, in North America and Europe over 90% of floodplains habitats are no longer hydrologically connected to rivers, representing a significant loss of riverine ecosystem food web productivity. Expanding inundated floodplain area increases the volume of the photic zone, the uppermost portion of the water column, where sunlight is converted to carbohydrates by phytoplankton. Greater extent of shallow floodplain augments overall food availability and provides important aquatic habitats for many species. For example, invertebrates are in greater abundance on floodplains than in adjacent river channels, enhancing foraging success of fish that gain access to these seasonally inundated aquatic habitats. Like many large river valleys globally, the Central Valley of California, USA, (encompassing the Sacramento and San Joaquin River watersheds) has lost the majority of its floodplain habitat, primarily to conversion to agriculture. However, remnant floodplains still exist and are managed as flood bypasses which function in the winter wet season as "pressure relief valves," allowing floodwaters to pass safely around vital infrastructure and urban areas. In the summer dry season, bypasses are actively farmed. Managed inundation of agricultural floodplains may offer a means of regaining some of the ecological benefits of floodplains for native fish and wildlife while continuing to support flood protection and production agriculture, thereby creating a landscape where native species and human uses can coexist. An example of such reconciliation is Yolo Bypass, a farmed floodway where two decades of managed winter inundation of post-harvest rice fields—in the agricultural off-season when farm fields are idle—creates wetland habitat for millions of waterfowl using the critically important Pacific Flyway. Recent studies have shown that managed inundation can also provide suitable floodplain foraging habitat for native fish species such as juvenile Chinook Salmon.
In order to better understand how rice farming practices affect the quality of reconciled winter rearing habitat for juvenile fall run Chinook Salmon (Oncorhynchus tshawytscha), we built and experimentally manipulated nine replicated two-acre rice fields. Fields shared a common layout and water source. Each was randomly assigned to one of three post-harvest farming options generally used in the Yolo Bypass; fallow, leaving standing rice stubble, or tilling to incorporate the stubble into the soil. Each field was stocked with approximately 4,500 juvenile salmon. Weekly sampling for water quality, invertebrate abundance and diversity, and fish growth and survival was conducted for 6 weeks, after which fields were drained and fish counted, measured
|Advisor:||Moyle, Peter B.|
|Commitee:||Lubell, Mark, Schwartz, Mark|
|School:||University of California, Davis|
|School Location:||United States -- California|
|Source:||DAI-B 76/10(E), Dissertation Abstracts International|
|Subjects:||Ecology, Agriculture, Natural Resource Management|
|Keywords:||Conservation status, Native fish, Reconciliation, Salmon, Working lands, Yolo bypass|
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