An evaluation of the effectiveness of Fall Outflow Actions for delta smelt

A Fall X2 Action, intended to benefit delta smelt by increasing outflow through the Sacramento-San Joaquin Delta in years with greater precipitation in the watershed that feeds the Delta, wetter years, was included in the USFWS 2008 Biological Opinion as a “reasonable and prudent alternative.” The management action requires increases in outflow through the Delta during September and October of wet and above-normal years. A similar flow-enhancing management action, but with differing outflow targets, was included in the operations analyzed in the 2019 Biological Opinion. Those Fall X2 Actions have been controversial since first being implemented because of their uncertain conceptual foundations, their large water costs, and the difficulty in detecting benefits of the management actions for delta smelt.


We reviewed the basis for the original Fall X2 Action, as well as numerous studies since the release of the 2008 Biological Opinion using publicly available data and following standards consistent with rigorous effects analyses to evaluate the effectiveness of the management action. More than a dozen studies address the influence of flows through the Delta in the autumn on the performance of delta smelt. The original study on which the action relied reported a population benefit to delta smelt from outflow volumes that position the low-salinity zone in more western areas of the upper estuary in the autumn (Feyrer et al. 2007). That study was flawed in that it attributed the population benefits to flows in the fall, without considering flows in the subsequent spring, which have a more significant influence on abundance. In a study that followed, Feyrer et al. (2011) identified a relationship between the position of the low-salinity zone and the abundance of delta smelt, but only because the authors included data prior to the appearance of the invasive Asian clam in the upper estuary. Since that mollusk invasion there is no apparent relationship between the study’s constructed habitat index and the abundance of delta smelt in the autumn.



We identified just one study that appears to support a Fall X2 management action. The other studies we evaluated either did not support the action or did not include analyses applicable to the issue. We also evaluated seven hypotheses – that is, justifications for the management action – using publicly available data and found no evidence of benefits of a Fall X2 Action to delta smelt growth, abundance, survival, recruitment, food supply, or habitat area, nor evidence of benefits to the quality of delta smelt habitat. The weight of evidence, presented here, does not support a Fall X2 Action.


Historically, Suisun Bay provided high-quality habitat for delta smelt in the autumn, but since the invasion of the Asian clam in 1986, food availability in Suisun Bay has diminished. Certain studies have found that enhanced outflow results in diminished food supplies downstream. Increasing outflow in the autumn without enhancing food availability will not increase the extent of suitable environmental conditions for delta smelt in Suisun Bay. Suisun Marsh, the Confluence, and lower Sacramento River provide conditions that are frequently more suitable than Suisun Bay. Importantly, studies that have identified benefits to delta smelt from enhanced outflow in the autumn did not consider the correlating or overlapping influence of spring flows, which have been shown to generate greater benefits for delta smelt.


Best professional practice criteria in an adaptive management require identifying performance measures, setting quantitative management targets, designing monitoring programs to enable the effectiveness of management actions to be evaluated, and periodically reviewing the effectiveness of the actions. The Fall X2 Action in both the U.S. Fish and Wildlife Service’s 2019 Biological Opinion (discussed in detail below) and the State of California’s Incidental Take Permit left those essential investigative areas unaddressed. It is imperative that conservation managers learn from the actions implemented and adjust them to make them more effective; or if they are not effective, to terminate those actions and focus on management actions that are effective.


In summary, the findings presented in this paper indicate that Fall X2 Action was based on studies with technical errors and since its implementation, numerous studies have demonstrated that it lacks empirical support.


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