SFEWS: Volume 19, Issue 3
In our September issue new research and commentary provide insights on several topics: how to integrate zooplankton science to inform estuary management; how simulated fishing can avoid missed fish and detect gear bias in the water; why juvenile Chinook Salmon length-at-date criteria don't match genetic run assignments; whether declines in breeding waterfowl population relate to wetland habitat and salinity; and what kinds of food web support can be achieved by use of a managed flow pulse.
Photo: Forster’s Terns at Crown Beach, public domain. Attribution: © Ingrid Taylar, Creative Commons 2.0 Generic license.
Catch as Catch Can
"Catchability" refers to the relationship between catch rate and the true population. Ecological monitoring programs use catch per unit of effort (CPUE) to standardize catch and monitor changes in fish populations; however, CPUE is proportional to the portion of the population that is vulnerable to the type of gear used in sampling, which is not necessarily the entire population. Tobais' simulation combines a module for sampling conditions with a module for individual fish behavior to estimate the proportion of available fish that would escape from the sample. The method is applied to the case study of the well monitored fish species Delta Smelt (Hypomesus transpacificus) in the San Francisco Estuary, where it has been hypothesized that changing water clarity may affect catchability for long-term monitoring studies.
Waterfowl Reproductive Success Depends on High Water, Low Salt
Availability of wetlands with low salinities during the breeding season can influence waterfowl reproductive success and population recruitment. Salinities as low as 2 ppt (3.6 mS/cm) can impair duckling growth and influence behavior, with mortality occurring above 9 ppt (14.8 mS/cm). Schacter et al. used satellite imagery to quantify the amount of available water, and sampled surface water salinity at Grizzly Island, in the brackish Suisun Marsh, at three time-periods during waterfowl breeding (April, May, July) over 4 years (2016–2019). Among their findings was during peak duckling production in May, 81%–95% of available water had salinity above 2 ppt, and 5%–21% was above 9 ppt. Local waterfowl populations would benefit from management practices that provide fresher water during peak duckling production in May and retain more water through July.
Deep Dives Among Waterbird Populations in South SF Bay
In south San Francisco Bay, former salt ponds now managed as wildlife habitat support large populations of breeding waterbirds. In 2006, the South Bay Salt Pond Restoration Project began the process of converting 50% to 90% of these managed pond habitats into tidal marsh. Hartman et al. compared waterbird populations in south San Francisco Bay before (2001) and after (2019) approximately 1,300 ha of managed ponds were breached to tidal action to begin tidal marsh restoration. Study results showed average annual nest abundance declined during 2017–2019 by 53%, 71%, and 36%, for American Avocets, Back-necked Stilts, and Forster’s Terns, respectively. All three species established nesting colonies on newly constructed islands within remaining managed ponds; however, these new colonies did not make up for the steep declines observed at other historical nesting sites. For future wetland restoration, retaining more managed ponds that contain islands suitable for nesting may help to limit further declines in breeding waterbird populations.
Managed Pulse Flows as Food Web Support
While freshwater inflow has been a major focus of resource management in estuaries, including the upper San Francisco Estuary, there is a growing interest in using focused flow actions to maximize benefits for specific regions, habitats, and species. To test this concept, in summer 2016, Frantzich et al. used a managed flow pulse to target an ecologically important region: a freshwater tidal slough called the Cache Slough Complex. Their goal was to improve estuarine habitat by increasing net flows through CSC to enhance downstream transport of lower trophic-level resources, an important driver for fishes such as the endangered Delta Smelt. Simulations using a 3-D hydrodynamic model (UnTRIM) indicated that the managed flow pulse had a large effect on the net flow of water through Yolo Bypass, and between the CSC and further downstream. The managed flow pulse resulted in increased densities of zooplankton (copepods, cladocerans) demonstrating potential advection from upper floodplain channels into the target CSC and Sacramento River regions. Though conducted during a single year, this study may provide an instructive example of how a relatively modest change in net flows can generate measurable changes in ecologically relevant metrics, and how an adaptive management action can help inform resource management.
Length-at-Date Criteria and Genetic Run Assignments
Four distinct runs of Central Valley Chinook Salmon are named after their primary adult return times: fall, late-fall, winter, and spring run. Estimating the run-specific composition of juveniles entering and leaving the Sacramento–San Joaquin Delta is crucial for assessing population status and processes that affect juvenile survival through the Delta. Historically, the run of juvenile Chinook Salmon captured in the field has been determined using a length-at-date criteria (LDC); however, LDC run assignments may be inaccurate if there is high overlap in the run-specific timing and size of juveniles entering and leaving the Delta. In this study, Brandes et al. use genetic run assignments to assess the accuracy of LDC at two trawl locations in the Sacramento River (Delta entry) and at Chipps Island (Delta exit).Across years, there was extensive overlap among the distributions of run-specific fork lengths of genetically identified juveniles, indicating that run compositions based on LDC assignments would tend to underestimate fall-run and especially late-fall-run compositions at both trawl locations, and greatly overestimate spring-run compositions (both locations) and winter-run compositions (Chipps Island). We therefore strongly support ongoing efforts to include tissue sampling and genetic run identification of juvenile Chinook Salmon at key monitoring locations in the Sacramento–San Joaquin River system.
Pelagic fish in the San Francisco Estuary are harder to catch in recent decades. Over the past thirty years, Delta Smelt catch in the Fall Midwater Trawl Survey has declined by 99%, Longfin Smelt catch has declined by over 95%, and even the notoriously hardy Striped Bass have declined by over 75%. To manage the system and reverse these declines, we need a better understanding of the “bottom-up” processes that exert control on these populations—we need to study fish food. In other words, in addition to studying fish directly, we need to increase our understanding of what pelagic fish eat: zooplankton. In this essay, Hartman et al. break down not only what fish eat (zooplankton) and why they are important drivers of species abundance in higher trophic areas of the food web, but also how scientists and natural resources managers can communicate better to understand which zooplankton data can inform and develop management-relevant questions.
Volume 11, Issue 1, 2013
Various schemes are often suggested to reverse the subsidence of lands below sea level in California’s Sacramento—San Joaquin Delta, an area protected by levees (dikes) that have significant probabilities of failure. Elementary modeling is used to estimate the probability distribution of land elevations at time of failure for 36 of these subsided islands, assuming a reasonable potential subsidence reversal rate. Given estimated annual probabilities of levee failure, elevation gains at this rate are not expected to exceed 1 to 2 m before flooding, which would be insufficient to restore most subsided islands to mean sea level (msl). However, under some circumstances 1- to 2-m gains are significant. A framework is introduced for evaluating islands as promising candidates for subsidence reversal based on elevation goals other than msl, as demonstrated though a hypothetical aquatic habitatexample. Here, we recommend relevant subsidence reversal strategies by comparing an elevation goal with each island’s anticipated flooded depth, and we prioritize islands for investment based on trade-offs between anticipated outcome and lost agricultural revenues. This approach might help integrate subsidence reversal activities into long-term Delta planning under a range of flooding, land use, and habitat management scenarios.
Ecosystem-scale Selenium Model for the San Francisco Bay-Delta Regional Ecosystem Restoration Implementation Plan
Environmental restoration, regulatory protections, and competing interests for water are changing the balance of selenium (Se) discharges to the San Francisco Bay–Delta Estuary (Bay–Delta). The model for Se described here as part of the Delta Regional Ecosystem Restoration Implementation Plan (DRERIP) draws both from the current state of knowledge of the Bay–Delta and of environmental Se science. It is an ecosystem-scale methodology that is a conceptual and quantitative tool to (1) evaluate implications of Se contamination; (2) better understand protection for fish and aquatic-dependent wildlife; and (3) help evaluate future restoration actions. The model builds from five basic principles that determine ecological risks from Se in aquatic environments: (1) dissolved Se transformation to particulate material Se, which is partly driven by the chemical species of dissolved Se, sets dynamics at the base of the food web; (2) diet drives bioavailability of Se to animals; (3) bioaccumulation differs widely among invertebrates, but not necessarily among fish; (4) ecological risks differ among food webs and predator species; and (5) risk for each predator is driven by a combination of exposures via their specific food web and the species’ inherent sensitivity to Se toxicity. Spatially and temporally matched data sets across media (i.e., water, suspended particulate material, prey, and predator) are needed for initiating modeling and for providing ecologically consistent predictions. The methodology, applied site-specifically to the Bay–Delta, includes use of (1) salinity-specific partitioning factors based on empirical estuary data to quantify the effects of dissolved speciation and phase transformation; (2) species-specific dietary biodynamics to quantify foodweb bioaccumulation; and (3) habitat use and life-cycle data for Bay–Delta predator species to illustrate exposure. Model outcomes show that the north Bay–Delta functions as an efficient biomagnifier of Se in benthic food webs, with the greatest risks to predaceous benthivores occurring under low flow conditions. Improving the characterization of ecological risks from Se in the Bay–Delta will require modernization of the Se database and continuing integration of biogeochemical, ecological, and hydrological dynamics into the model.
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Migration Patterns of Juvenile Winter-run-sized Chinook Salmon (Oncorhynchus tshawytscha) through the Sacramento–San Joaquin Delta
The decline of Sacramento River winter-run Chinook salmon (Oncorhynchus tshawytscha) remains one of the major water management issues in the Sacramento River. Few field studies have been published on winter-run, leaving gaps in our knowledge about their life history. This is especially true in the Sacramento–San Joaquin Delta, which provides essential rearing and migratory habitats for winter-run, and serves as the center of water operations for California. Using long-term monitoring data that identified winter-run-sized fish (“winter-run”) using length-at-date criteria, we examined patterns of juvenile migration in terms of geographic distribution, timing, numbers, and residence times. We analyzed the role of flow, turbidity, temperature, and adult escapement on the downstream movement (“migration”) of winter-run. Winter-run passed Knights Landing (rkm 144 or 51 rkm upstream of the Delta) between October and April, with substantial variation in peak time of entry that was strongly associated with the first high flows of the migration season. Specifically, the first day of flows of at least 400 m3 s-1 at Wilkins Slough (rkm 190) coincided with the first day that at least 5% of the annual total catch was observed at Knights Landing. While the period during which winter-run left the Delta spanned several months based on Chipps Island (rkm 29) catch data, the median catch typically occurred over a narrow window in March. Differences in timing of cumulative catch at Knights Landing and Chipps Island indicate that apparent residence time in the Delta ranges from 41 to 117 days, with longer apparent residence times for juveniles arriving earlier at Knights Landing. We discuss the potential importance of the Yolo Bypass floodplain as an alternative rearing and migratory corridor, contingent on the timing, duration, and magnitude of floodplain inundation. These results carry implications for habitat restoration and management of Sacramento River flows.
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