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Home » News/Events » The Journal » Vol. 12 Issue 2 » Floodplain Sanctuaries for Juvenile Native Fish

Floodplain Sanctuaries for Juvenile Native Fish

  • By Andrew Rayburn, Director of Science

Figure 1. Juvenile salmon grew rapidly over a four-week floodplain rearing experiment, nearly doubling in length and tripling in weight.

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.

Under the ideal scenario, floodplains are reconnected to rivers so fish can enter them, fatten up on abundant food, and re-enter the river to continue their journey. River Partners’ previous efforts have focused on creating these frequently activated floodplains within large-scale restoration projects across the state. However, this approach does assume that flows will be high enough to reach floodplains and that the timing of high flows will match the timing of young fish migration downstream. As we have always recognized, this is a challenging scenario given all the modifications to Central Valley rivers. For example, spring flows are often captured behind dams and held for later release – yet these flows match the timing of young fish migration and could otherwise usher fish on and off floodplains. A second challenge arises during periods of drought, when flows are simply too low to reach floodplains altogether.

Conservationists are now exploring more managed solutions for fish, similar to what has been required for other terrestrial and aquatic species. For example, “simulated floodplains” can be created to which young fish could be transported, held for a short period of time sufficient for rapid growth, and then released into the river or transported further downstream. This water-efficient approach is especially relevant during droughts, since a large number of fish can be reared in shallow water on a relatively small area. Given River Partners’ access to riparian sites across the Central Valley, and our willingness to be creative in trying new approaches for species recovery, we are very interested in these kinds of options for native fish.

In 2015, River Partners successfully implemented two pilot fish-rearing projects based on these concepts. The first project was implemented as part of the “Nigiri Project”, a large-scale collaborative study at Knaggs Ranch in the Yolo Bypass involving both farmers and researchers from California Trout, UC Davis, and the California Dept. of Water Resources. The goal of the study is to test the general hypothesis that farm fields can both produce crops in the growing season and also serve as habitat for native fish and wildlife during winter months. In addition to the main project site, several satellite sites were established this year to broaden the study. Dos Rios Ranch, River Partners’ 2,100-acre flagship multi-benefit restoration site in the San Joaquin Valley, was chosen as one of them.

Figure 2. River Partners’ Biologists Trevor Meadows (left) and Jeff Holt (right) monitoring juvenile Chinook salmon at Dos Rios Ranch in Spring 2015.

At Dos Rios Ranch, River Partners flooded a 3-acre grain field on the San Joaquin River floodplain in February. Our biologists and field staff installed six rearing pens into which were placed 20 hatchery-raised fallrun juvenile Chinook salmon that were reared for four weeks (Fig. 1). During this time, staff monitored fish growth, zooplankton development, and both water temperature and dissolved oxygen (Fig. 2). Fisheries biologists from the consulting firm FISHBIO lent their expertise and assisted with fish monitoring. After four weeks, all of the fish were measured a final time and sent to DWR for gut content analysis. The final numbers were highly encouraging and similar to results from the larger Knaggs Ranch project over the past few years: salmon experienced a 1.6-fold increase in length (49 mm to 76 mm) and a 4.8-fold increase in weight (1.3 g to 6.1 g). Survival was also extremely high (98%), which was largely the result of suitable water conditions maintained during the project.

Figure 3. River Partners Biologist Michael Rogner (left) and FISHBIO Fisheries Biologist Michael Hellmair (right) collect juvenile salmon for measurement at the Willow Bend site.

At the same time, a second pilot project took advantage of a natural floodplain swale at the Willow Bend restoration site, north of Colusa on the Sacramento River. After one month of floodplain rearing, several hundred juvenile Chinook salmon experienced a 1.6-fold increase in length (37 mm to 62 mm) and a 5.8-fold increase in weight (0.5 g to 2.3 g). Similar monitoring of zooplankton, fish, and water chemistry was performed on this project (Fig. 3), and overall survival was again extremely high (93%).

Together, these two pilot projects successfully demonstrated the potential for flooded riparian habitat nested within River Partners large-scale restoration projects to serve as rearing habitat for juvenile native fish. We are now exploring options for expanding this approach to help create a network of floodplain sites throughout the Central Valley to improve native fish population resiliency and to support species recovery.

We thank the Volgenau Foundation and the Resources Legacy Fund for their generous support of these pilot projects and appreciate their interest in conserving native California species.

The above article originally appeared in the Spring 2015 issue of the River Partners Journal.