Abstract
When inundated by floodwaters, river floodplains provide critical habitat for many species of fish and wildlife, but many river valleys have been extensively leveed and floodplain wetlands drained for flood control and agriculture. In the Central Valley of California, USA, where less than 5% of floodplain wetland habitats remain, a critical conservation question is how can farmland occupying the historical floodplains be better managed to improve benefits for native fish and wildlife. In this study fields on the Sacramento River floodplain were intentionally flooded after the autumn rice harvest to determine if they could provide shallow-water rearing habitat for Sacramento River fall-run Chinook salmon (Oncorhynchus tshawytscha). Approximately 10,000 juvenile fish (ca. 48 mm, 1.1 g) were reared on two hectares for six weeks (Feb-March) between the fall harvest and spring planting. A subsample of the fish were uniquely tagged to allow tracking of individual growth rates (average 0.76 mm/day) which were among the highest recorded in fresh water in California. Zooplankton sampled from the water column of the fields were compared to fish stomach contents. The primary prey was zooplankton in the order Cladocera, commonly called water fleas. The compatibility, on the same farm fields, of summer crop production and native fish habitat during winter demonstrates that land management combining agriculture with conservation ecology may benefit recovery of native fish species, such as endangered Chinook salmon.
Highlights
Seasonal inundation of floodplains drive important hydrologic and geomorphic processes that provide substantial trophic benefits to river ecosystems [1, 2]
Examples of issues to be resolved in the future include: 1) improving salmon floodplain access by increasing connectivity between floodplains and rivers; 2) evaluating use of water retention infrastructure to extend the duration of flood events; 3) determining habitat types that maximize food web production, salmon growth, and survival; 4) determining whether some managed conditions could create adverse water effects or enhance predation; and 5) determining how all four Central Valley runs of wild Chinook salmon, steelhead (O. mykiss) and native cyprinid minnows can benefit from floodplain management
Previous studies in the Central Valley found that rearing in complex off-channel habitats during natural inundation resulted in rapid growth of juvenile Chinook salmon [10, 11, 13]
Summary
Seasonal inundation of floodplains drive important hydrologic and geomorphic processes that provide substantial trophic benefits to river ecosystems [1, 2]. In California’s Central Valley more than 95% of pre-development floodplain habitats have been leveed and drained, primarily for flood control or conversion to agriculture [3]. Interrupt natural flow regimes and sever hydrologic, sediment, nutrient and fish connectivity between river channels and adjacent floodplain wetlands [4]. Reconciling floodplain management for fish and farms in California patchwork of agricultural lands and communities located on former floodplain wetlands which are separated from rivers by high, steep levees [5] and only inundate when floodwaters spill into managed floodways or when levees fail during severe storms. Access to ancestral floodplain habitats by juvenile salmon and other native fishes has been greatly diminished
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