Tuesday June 30, 2015

River Partners Newsletter

California’s once-thriving fish populations have dramatically declined, and many of our salmon, steelhead, and other iconic species will be extinct this century if current trends continue. Innovative conservation strategies are urgently required to reverse this rapid slide towards extinction.

One of the most promising new strategies is using Central Valley river floodplains as rearing habitat for juvenile native fish. This “fish on floodplains” concept is based on the straightforward ecology of aquatic food webs. When river water reaches floodplains during high flows, it spreads out and is exposed to increased sunlight that serves as fuel for a highly productive food web. Algae convert sunlight through photosynthesis and are eaten by zooplankton, which rapidly grow and reproduce. A staggering amount of zooplankton are produced on floodplains in only a few weeks after innundation, resulting in a nutrient-rich “soup” that is readily consumed by fish.

Young native fish, often only an inch or two in length, evolved to use these floodplains as rearing habitat when migrating downstream to the ocean. Besides food, floodplains offered these vulnerable fish safe refuge from predators and high flow velocities, thus supporting the large native fish populations once present in the Central Valley and downstream in the Delta.

In modern times, almost all Central Valley floodplains have been disconnected from rivers and flooding, all but eliminating access by native fish. Dams, levees, and other water control structures have constrained young fish to less productive river channels in which they struggle to find adequate nutrition and avoid predators on their journey to the ocean. Conservation agencies and practitioners agree that making floodplains accessible to young native fish is critical, but it remains uncertain how best to do so.

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