Abstract
Managing endangered species is challenging when increased rarity leads to an inability to detect their responses to environmental conditions. In the San Francisco Estuary, the state and federally listed Delta Smelt (Hypomesus transpacificus) has declined to record low numbers, elevating concern over entrainment at the State Water Project (SWP) and Central Valley Project (CVP) water export facilities. The objective of this study was to: (1) revisit previous work on factors that affect adult Delta Smelt collected at the SWP and CVP fish collection facilities using updated conceptual models and a new statistical approach; and (2) to determine factors that affect salvage at time-scales of interest to management. Boosted Regression Tree (BRT) models were applied to salvage data at the SWP and CVP, aggregated into two response categories: a “first flush” response that represented daily salvage from the start of the entrainment window to the 50% midpoint of observed salvage, and a “seasonal” response that included daily salvage from the entire entrainment window. Precipitation, sub-adult abundance, Yolo Bypass flow, and exports best explained first flush salvage at both the SWP and CVP. The seasonal models included a similar set of influential variables, but the relative influence of precipitation was lower compared to the first flush models., Yolo Bypass flow was more influential for seasonal salvage at the SWP, compared to the CVP; Old and Middle River flow was more influential for seasonal salvage at the CVP. Although the rank of variable importance that explains salvage differed slightly between first flush and seasonal time-scales, this study suggests that salvage is most influenced by hydrodynamics, water quality, and population abundance. The application of BRT models to predict salvage is limited, because salvage has been low since federal protections were implemented in 2008. Forecast models that integrate real-time variables with fish behavior models may improve Delta Smelt management.
Highlights
Since the turn of the 21st century, fisheries management has redirected its focus from individual species to broader ecosystem objectives to address inherent complexities of aquatic environments (Link 2005; Hall and Mainprize 2004; Pikitch et al 2004)
Correlation analysis of potential explanatory variables indicated that only Old and Middle rivers (OMR) and San Joaquin River flow exceeded the threshold of R2 = 0.6, so OMR and San Joaquin River flow were not included in the same data set
This study reinforces previous work that adult Delta Smelt salvage is largely explained by hydrodynamics, water clarity, precipitation, and sub-adult abundance (Grimaldo et al 2009)
Summary
Management objectives that focus on restoring ecosystem functions are considered desirable because they emphasize mechanisms that influence species survival and growth rather than counts of individuals, which may be difficult to detect as population numbers decline. For species listed under the federal Endangered Species Act (ESA), the law allows for recovery actions to be carried out through robust adaptive management plans that include consideration of habitat quality and quantity, reduced exposure to predators and contaminants, and improved access to rearing habitats. When Delta Smelt are located near the SWP and CVP (hereafter referred to as the “water export facilities”), water-diversion restrictions under state and federal ESAs are implemented to minimize entrainment losses (Reis et al 2019; USFWS 2019; CDFW 2020).
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