Diet composition was slightly more diverse in the fallow treatment with an average of 87.3% ± 2.6% SE percent of prey items composed of cladocerans compared with an average of 97.4% ± 1.2% SE in the disced treatment and 97.3% ± 1.0% SE in the stubble treatments. A chironomid midge hatch in the southernmost field was responsible for the increased prey diversity resulting in diets composed of an average of 69% cladocera and 30% diptera. The other two fallow replicates had an average diet composition of 96% cladocera.Depth treatments did not have a significant effect on survival . Depth treatments, did however have a significant effect on daily volitional emigration of fish before draining . A post-hoc Dunn’s test revealed that the two trenched treatments had significantly more daily volitional outmigration compared to the no trench treatment , but that the two trench treatments did not significantly differ from each other . The average cumulativevolitional outmigration before field drainage in the two trenched treatments was 15.4% ± 5.3% SE compared to the trenchless treatment, pipp racks which had 3.3% ± 1.2% SE, indicating the trenches may have functioned as a migratory pathway aiding in volitional outmigration prior to field drainage.
A relatively high rate of initial volitional emigration was seen in the first week across all fields, followed by a much lower rate of emigration in the second week , and steadily increasing emigration in weeks three through five . Manual fish recovery with a seine at the end of field drainage in the trenchless fields ranged between 5 to 20% of the total surviving fish compared to less than 0.5% of survivors from the trenched fields which indicated a more efficient drainage procedure in trenched fields. Again there appeared to be functional equivalence between the 0.5m and 1.0m trench treatments.Rearing of juvenile Chinook Salmon within winter-flooded rice fields shows strong potential for reconciling agricultural floodplain land use with habitat needs of an imperiled and economically important fish. Winter-flooded rice fields demonstrated high production of naturally occurring fish food leading to high growth rates of salmon reared in these environments. As with past fish conservation studies in altered environments, our adaptive research approach enabled us to successfully answer experimental questions despite unpredictable winter hydrologic and temperature regimes in the Central Valley. In our studies, post-harvest field substrate did not have a statistically significant effect on the composition or abundance of zooplankton species, nor on growth rates of rearing juvenile Chinook Salmon.
Overall, fish growth across all treatments was extremely fast and much greater than previously documented in the Sacramento River channel environments over the last century. Accordingly, we do not recommend a specific post-harvest straw management practice. Instead, we feel that field preparation should be left to the farmer. However, we encourage future research that explores other approaches for enhancing in-field habitats to decrease predation risk for rearing fish. There is currently limited means of cost effectively providing avian predation refugia for fish on winter-flooded rice fields. We investigated the potential of in-field trenches to provide depth refuge from avian predation, but direct benefits to survival were found to be insignificant in this study. Fields containing perimeter trenches connecting the inlet and outlet structures did however, show higher rates of volitional emigration of salmon and reduced rates of stranding following draining. We speculate that fish used the trenches as migration corridors when emigrating from the fields. Increased rates of volitional egress would further diversify timing of emigration which has been identified as a key component of population stability via the portfolio effect. Additionally, the trenches buffered water temperatures from the daily maximums observed in the middle of the fields, expedited field drainage, and reduced the number of fish stranded during field draw down.
In floodplain river ecosystems, fishes often respond strongly to hydrological dynamics of ascending and descending flood conditions. Juvenile Chinook Salmon in the Central Valley have evolved physiological and behavioral strategies for the use and egress from winter flooded floodplain habitats. Accordingly, the rate of field drainage and inflow conditions in winter-flooded rice fields may provide important cues for rearing juvenile Chinook Salmon. In our study, extending the drainage period and manipulating inflow conditions in the slow drain treatments had a detrimental effect on survival and the best method was a fast drain where fields were drained in a single day. This was likely the result of increased vulnerability to predation and reduced thermal buffering due to a prolonged period with shallower water depths in the slow drain treatments. Again, these results provide a relatively simple management recommendation for farmers in that simple opening of outlet water control structures with rapid drainage appears to be the best method. We encourage exploration of other drainage methods, and production of other species in winter-flooded rice fields may require different draining practices. An initial mortality of approximately one third was observed in the first week of the 2016 salmon survival through time experiment. The cause of this initial mortality is unknown and could have resulted from a combination of factors, including a stressful transport and acclimation stress under new physical water conditions. Additionally, because we know of no other experiments that have been able to track and assess post-release mortality of hatchery fish through time, we cannot rule out the possibility that the high rate of initial mortality observed immediately after release into the “wild” is a potentially common phenomenon. Transport is a known stressor on many fishes, including juvenile Chinook Salmon. In our study, fish were captured from hatchery raceways, coded wire tagged, allowed to recover for several days and then placed in a fish hauling tank at high densities and delivered to the fields in early February. Exposing naïve hatchery salmon to a new environment in the flooded agricultural fields may have increased stress as it necessitated behavioral adaptations of prey switching and predator avoidance as well as rapid acclimation to the new physical water quality parameters. After the high rate of initial mortality, survival stabilized in week two and remained high for the remainder of the experiment. Without accurate assessment and accounting of initial post-release mortality there is potential for fishery managers to be chronically overestimating habitat-specific mortality rates determined by recapture of hatchery fish transported and released into natural habitats. We therefore recommend that future research examine effects on initial post-release mortality of transporting, acclimatizing, and releasing hatchery fish into the wild.While farmers have incentives to prepare their fields for a new rice crop as early in the spring as possible, fish conservationists may theoretically prefer to keep the rice fields inundated as late as possible to maximize fish growth and survival before release into the river. However, in practice, when weather conditions on the floodplain are good for fish in the late winter and early spring, they are generally not conducive to agricultural field preparation. The inverse is also true, as spring conditions become dry and hot and generally suitable for agricultural field preparation, water quality conditions rapidly become unsuitable for juvenile Chinook Salmon . Thus, given proper timing and coordination within an adaptive management framework, farmers and fish conservationists can collaborate to promote threatened fisheries without impacting crop yields.
We therefore encourage the continued development of adaptive frameworks for the integration of floodplain fish habitat into farm operations that reconcile the needs and timing for both fish and farm operations. Incentive programs for farmers may be needed to promote these activities to their fullest potential. Land manager and farmer involvement has generally exceeded expectations in our projects, and we are optimistic about continued stakeholder involvement. Given issues with water scarcity in the Central Valley, pipp rack the dual-use of rice fields for agriculture and rearing juvenile salmon could establish stronger water security for farmers. Additionally, off-season inundation of rice fields promotes rice straw decomposition while approximating the natural long-duration inundation patterns that fuel a productive aquatic food web. When compared to concurrent samples in the adjacent Sacramento River channel habitat, the winter-flooded rice fields had 150x zooplankton abundance in 2013 and approximately 53x zooplankton abundance in 2016. Resultantly, the juvenile salmon growth rates we observed in winter-flooded rice fields were 2-5x higher than previously or concurrently observed in the adjacent Sacramento River. By creating high quality habitat on their fields, farmers can help bolster fish populations by rapidly turning small fry into large, healthy smolts during mid-winter when water temperatures are low, river flows are high and when predators are less active, thus improving salmon survival rates during outmigration. Managed inundation of rice fields in winter and early spring appears to mimic historical Sacramento Valley floodplain processes, re-exposing salmon to an approximated version of the hydrologic selection regime under which they evolved and to which they are adapted. The exceptional productivity and resulting rapid rates of salmon growth documented on the managed agricultural floodplain lead us to conclude that winter inundation of rice fields creates high-quality rearing opportunities for juvenile Chinook Salmon. Although these studies suggest that agricultural landscapes can function as high-quality rearing habitat for juvenile Chinook Salmon, our results should not be interpreted to diminish the conservation need for restoring naturally functioning floodplains where feasible or to suggest that suitable natural habitats are not essential to establishing self-sustaining runs of naturally produced Central Valley Chinook Salmon. Rather, these data demonstrate the potential to reconcile management of agricultural floodplain landscapes with the conservation of wild Chinook Salmon populations through slight modification and reoperation of existing agricultural infrastructure. Managed agricultural floodplains are likely to become another important means for fishery managers to produce ecologically functioning off-channel habits for imperiled native fish, especially during times of low water when remaining natural floodplain habitats do not inundate and are therefore inaccessible to salmon populations confined to leveed stream channels.The Special Feature on Diversified Farming Systems is motivated by a desire to understand how agriculture designed according to whole-systems, agroecological principles can contribute to creating a more sustainable, socially just, and secure global food system. “How to feed the world” is an increasingly urgent and looming concern voiced by many people, from local community groups to national and international governing bodies. By 2050, the world population is projected to rise to 9+ billion and food demands to double from current levels. At the same time, climate change, interacting with increasingly uneven access to declining oil, water, and phosphorus supplies, will greatly exacerbate the year-to-year unpredictability of agricultural production, potentially undermining the entire agricultural enterprise . Meanwhile, industrialized agricultural techniques are exacting a huge toll on surrounding environments, polluting waterways, creating dead zones in the oceans, destroying biodiverse habitats, releasing toxins into food chains, endangering public health via disease outbreaks and pesticide exposures, and contributing to climate warming . Moreover, industrial agricultural methods are inherently unsustainable in mining soils and aquifers far more quickly than they can be replenished, and in their high use of fossil fuels . These numerous environmental and social externalities create a huge economic cost that industrialized food producers seldom pay. For instance, pesticide use alone causes up to $10 billion in damage to humans and ecosystems in the United States every year . Finally, although the agricultural sector currently produces more than enough calories to feed humanity, one billion people remain hungry and an additional one billion have micronutrient deficiencies . This paradoxical situation occurs because many people still lack access to sufficiently diverse and healthy food, or the means to produce it, which is primarily a problem of distribution rather than production . As further evidence of this paradox, global obesity rates have more than doubled since 1980 , reflecting an overproduction of food in industrialized countries that creates strong incentives for agrifood companies to absorb excess food production into processed foods and to market and distribute them to customers in supersized portions . This series of articles examines the proposition that diversified farming systems, with their focus on local production, local and agroecological knowledge, and whole systems approachesreduce negative environmental externalities and decrease social costs associated with industrialized monocultures, enhance the sustainability and resilience of agriculture, and contribute significantly to global food security and health.We refer to a farming system as “diversified” when it intentionally includes functional biodiversity at multiple spatial and/or temporal scales, through practices developed via traditional and/or agroecological scientific knowledge. At the plot scale, diversified farming systems may include multiple genetic varieties of a given crop and/or multiple crops grown together as polycultures, and may stimulate biodiversity within the soil through addition of compost or manure .