Striped Bass 2023 – Recovery?

When I last updated the status of striped bass in August 2021, it did not look good.  The fall index was at its minimum (Figure 1).  In 2021 and 2022, the index remained unchanged – as expected for drought years.

With 2023 very wet and with somewhat reasonable indices in wet years 2017 and 2019, combined with the fact that stripers start reaching maturity at ages 4 to 6, might we expect some level of recovery in this fall’s index?  I think so.  The numbers of 4-to-6-year-old adult spawning stripers should be up this year.

South Delta export pump salvage of age 0 stripers was low in summer 2022 a drought year (Figure 2).  But it is much higher since the Bureau of Reclamation turned on the CVP’s Delta pumps in late June 2023 (Figure 3), even compared to 2017 (Figure 4), which had moderate indices (see Figure 1).  With the state pumps turning on in early July and exports going to a maximum 10,000+ cfs (20,000 AF/day) there are much higher striped bass age-0 salvage numbers in this wet year.  The fall striper index should indicate at least a modest recovery.

Graph showing Striped bass Fall Midwater Trawl Survey Index (log10[index+1]) versus prior Summer Townet Index (log10). Select years labeled, with color of number showing year type: blue=wet, green=normal, and red=critically dry.

Figure 1. Striped bass Fall Midwater Trawl Survey Index (log10[index+1]) versus prior Summer Townet Index (log10). Select years labeled, with color of number showing year type: blue=wet, green=normal, and red=critically dry.

Graph showing Striped bass salvage in spring-summer 2022.

Figure 2. Striped bass salvage in spring-summer 2022.

Graph showing Striped bass salvage in spring-summer 2023.

Figure 3. Striped bass salvage in spring-summer 2023.

Graph showing Striped bass salvage in spring-summer 2017.

Figure 4. Striped bass salvage in spring-summer 2017.

Are Striped Bass Next to Go? 2021 Update

The prognosis for Bay-Delta and California striped bass could not be much worse. Numbers of stripers salvaged at the south Delta pumping plants in 2021 are down to or below 2014-2015 levels.1 Like many other species that spend all or part of their lives in the Delta, striped bass are on a downward spiral. Good water years are not providing enough population rebound to offset devastating lack of recruitment in dry year sequences.

In my report on the status of striped bass in 2016, stripers had suffered terribly in four years of drought (2012-2015). By 2019, the prognosis was still not good after a sequence of wetter water years (2016-2019) that were below normal, wet, below normal and wet.

Now, the summer-to-fall recruitment relationship (Figure 1), with its strong relationship between the summer index and fall recruitment, shows that dry year 2020 recruitment was as low as during the critically dry drought years 2014 and 2015. In the critically dry drought year 2021, late spring net catches are at historic lows.2 If the summer-to-fall recruitment relationship holds in 2021, striped bass are circling the drain as the next fish species to be lost in the collapsing Bay-Delta ecosystem.

Figure 1. Striped bass Fall Midwater Trawl Survey Index (log10[index+1]) versus prior Summer Townet Index (log10). Select years labeled, with color of number showing year type: blue=wet, green=normal, and red=critically dry.

Phantom Predator – Striped Bass?

In a recent 2020 essay in SAN FRANCISCO ESTUARY & WATERSHED SCIENCE, authors Nobriga and Smith describe striped bass as a “phantom predator” that for a century has been secretly driving down their “naïve prey,” the Delta smelt. The authors hypothesize that Delta smelt were much more abundant that the earliest regular monitoring data would indicate, and that striped bass did most of this damage to the Delta smelt population before there was widespread monitoring of either Delta smelt or striped bass.

The authors’ analyses, interpretations, and conclusions have a major omission. They fail to include the potential role of other native and non-native predatory fish in driving down the population of Delta smelt, regardless of the actual abundance of Delta smelt in the eighty years after stripers were introduced to the Bay-Delta in 1879 and 1882. Dozens of other predatory species also proliferated in the Delta over that past century, especially over the past several decades. Today, those other predatory species are far more abundant than the striped bass, and many are equal if not greater potential predators on young smelt than striped bass. In fact, striped bass are more likely to prey on juveniles and adults of other predator species than on smelt.

The authors are from the US Fish and Wildlife Service, the federal agency bound to protect the Delta smelt under the Endangered Species Act. The authors used “California Department of Fish and Wildlife fish monitoring data to provide evidence for a ‘phantom predator’ hypothesis: that ephemeral but persistent predation by Striped Bass helped to marginalize Delta Smelt before the estuary was routinely biologically monitored.”, The authors argue against “a misinterpretation that Striped Bass had little contemporary effect on Delta Smelt,” and “contend that the Delta Smelt population has declined steadily since Striped Bass were introduced to the estuary, and that has masked a substantial predatory effect of Striped Bass on Delta Smelt.” The article describes and supports a hypothesis that striped bass remain a problem for Delta smelt, despite the precipitous decline in the production of juvenile striped bass over the past century or so.

To partially address the hypothesis myself, I analyzed some Interagency Environment Program (IEP) data1 collected over the decade of 2009-2018 from one of the remaining Delta smelt strongholds – the lower Yolo Bypass portion of the Cache Slough Complex of the north Delta (Figure 1). Delta smelt were a common seasonal resident of the area (Figure 2). Striped bass, as expected, were very abundant in all age groups over much of the survey periods (Figure 3). But so were many species of invasive non-native catfish, sunfish, crappie, gobies, minnows, tule perch, black bass, and shad, most of which have been present in the Delta as long as striped bass.

Black crappie alone make up an equivalent or greater predator force on Delta smelt (Figure 4). In addition, black crappie as well as many of the other abundant predators compete with Delta smelt for their common zooplankton food supply. Not one of these potential other sources of predation or competition is mentioned in the essay.

Nobriga and Smith do acknowledge: “A generalist predator like Striped Bass, however, could suppress Delta Smelt competitors in addition to Delta Smelt, leading to non-linear and counter-intuitive community dynamics” – a theory that once-abundant striped bass have been suppressing other non-native predators and competitors of Delta smelt. So how long has that dynamic been functioning? Was it functioning in the hypothesized epoch of “phantom” predation? Did striped bass accelerate the decline of Delta smelt or, by eating and consuming other predators, slow it down? And assuming that Delta smelt really were much more abundant than previously believed prior to widespread monitoring in the Delta, to what degree was predation a factor in that decline?

Figure 2. Lengths of Delta smelt collected in fish surveys in lower Yolo Bypass 2009-2018. Note that up to 70% of the smaller young 20-60 mm smelt were later genetically identified as Wakasagi.

Figure 3. Lengths of striped bass collected in fish surveys in lower Yolo Bypass 2009-2018.

Figure 3. Lengths of black crappie collected in fish surveys in lower Yolo Bypass 2009-2018.

Analyzing Fish Population Dynamics in the Bay-Delta

I have been analyzing the declines in Bay-Delta and Central Valley fish populations for over 40 years. Fish population dynamics were the focus of my college education and my 50-year career in environmental impact assessment. I have participated in all the major efforts to understand the Bay-Delta fish population declines. From all of these efforts, it is clear to me what has caused the major fish population crashes.


First and foremost are the well known historic factors, the original sins pre-1970s of diverting water, building levees and dams, urban development, gold mining, cutting forests, polluting rivers, over-fishing, and introducing non-native species. These explain many of the major native fish population declines and extinctions such as the Sacramento perch and San Joaquin spring-run Chinook salmon, and the near extinctions of Delta smelt, green sturgeon, winter-run and spring-run salmon, and steelhead.


Since 1970, there have been dramatic declines in salmon, steelhead, smelt, sturgeon, splittail, and striped bass, often described as “recruitment failure” or failure to reproduce. While some of the blame most certainly is on continuing effects of the aforementioned original sins, the major post-1970 shifts were the consequence of a new array of stresses that hit the whole fish community, especially native fish populations. Most certainly the droughts of 76-77, 87-92, 01-02, 07-09, and 12-15 were a major underlying factor; however, it was the man-made responses to the droughts that caused most of the damage. Asian clam and other non-native aquatic invertebrate invasions to the Bay-Delta in the 80s were another stress, in part brought on by the aforementioned factors. Poor water management response to these new threats has caused further damage. The big culprits of change were the water management stresses described below.

1. State Water Project

The addition of the State Water Project (SWP) in the mid-1970s nearly tripled Delta export capacity (4400 to 11,400 cfs pumping rate1) and annual exports (2 million acre-feet to 6 million acre-feet annual exports). The additional Delta exports had huge fish population effects in the mid-70s from salvage mortality and entrainment of young fishes, as well as on fish habitat conditions in the rivers, Delta, and Bay. These stresses resulted in major population declines, which in turn resulted in the imposition of export restrictions in new water quality standards in 1978 (D-1485), and eventually to species listings under the Endangered Species Act in the 1990s.

2. Reservoir Operations

The increase in exports changed reservoir operations, including within-year reservoir release strategies and long-term multiyear reservoir storage patterns. Reservoir storage was depleted faster in droughts because of higher water supply demands. These effects continue today.

3. Water Supply Demands

Ever-increasing water supply demands from agricultural and municipal users have reduced river flows, Delta outflow, and reservoir storage. It’s not only the Delta’s 6 million acre-feet of exports, but the more than 20 million acre-feet from other Central Valley water diversions.

4. Invasive Species

Invasions of non-native clams, shrimp, fish, and zooplankton species since the 1970s have occurred in-part due to changes in Bay-Delta hydrology and water quality, as well as physical and biological habitat conditions. Delta pelagic (open water) habitat is now dominated by low-productivity reservoir water. The low salinity or mixing zone of the estuary became far less productive because of species invasions and reservoir water moving through to the south Delta export facilities, taking productive low-salinity habitat with it. The Delta is warmer from higher warm river inflows from spring through fall to feed water project exports, further favoring non-native warm-water fishes. Turbidity is lower, favoring non-natives. Invasive aquatic vegetation benefits from low turbidity, and the vegetation further favors non-native fishes over native fishes.


Since 1990, there have been steps backward that have undermined effective strategies and actions that had been undertaken beginning in the late 1970s to help depressed fish populations. Below are five examples in a long list of actions/changes.

1. Changes to D-1485

Beginning In 1978, Delta water quality standards in Decision 1485 placed restrictions on Delta exports, improved Delta outflows, and set salinity standards that had benefits for native fishes. Beginning in the 1990s, these post-1970 constraints on water diversions were changed, ignored, or eliminated. For example, new standards in D-1641 (1995 Accord) dropped the D-1485 June-July export restrictions.

2. Eliminating VAMP Export Restrictions and Higher Outflow Requirements in April and May

The Vernalis Adaptive Management Plan (VAMP) from 2000-2009, and its operational precursors under the CVPIA (1991) and the 1995 Accord, sought to protect Central Valley salmon and Delta native fishes by reducing April-May Delta exports and increasing spring Delta inflows and outflows. During the VAMP years, exports were restricted to less than 2000 cfs in April-May to protect fish (Figure 1). In the post-VAMP decade, restrictions were lifted and exports increased, especially in post-drought recovery wet years 2011 and 2017 (Figure 2).

3. Temporary Urgency Change Petitions (TUCPs) and Orders

Temporary urgency change orders during the recent drought allowed April-May Delta outflow to fall to around 5000 cfs in 2014 and 2015, from the normal near-10,000 cfs lower limit (Figure 3). Such low outflows in combination with Delta exports are devastating to Delta native fishes and Central Valley salmon and steelhead.

4. Delta Channel Barriers

The operation of the Delta Cross Channel, Head of Old River, South Delta, and False River barriers helps to keep export salinity down by funneling the fresher Sacramento River water to the south Delta export pumps. This increases the efficiency of exports in taking reservoir water in drier years and seasons. With the exception of the Head of Old River, barrier operation also funnels Delta native fish production (pelagic eggs and juveniles) and migrating young salmon (and their low salinity habitat and food sources) directly to the export pumps instead of to the Bay.

5. Suisun Marsh Salinity Control Gates

Since the installation of the Suisun Marsh Salinity Control Gates (SMSCG) in Montezuma Slough in 1989, the Slough and Marsh no longer function as critical low salinity habitat in drier years and seasons. Without high freshwater inflow, the Slough and Marsh no longer maintain the high biological production the once contributed to the Bay. The following excerpt from a DWR 2019 blog post inadvertently describes how limited the benefits of Suisun Marsh have become in the absence of flow:

DWR launched a pilot project last year that directed more fresh water flow into Suisun Marsh. The action involved opening salinity control gates in the summer months instead of during fall and winter, as is customarily done to reduce salinity in the marsh for migrating ducks and other waterfowl. The Delta smelt relies on low-salinity water – opening the salinity control gates allowed the smelt to enter the marsh from the Sacramento River, where it can access greater amounts of food and shelter.

Extinction looms so closely over the Delta smelt population that the project could have been considered a success even if it didn’t lure any countable Delta smelt to the marsh, said DWR Lead Scientist Ted Sommer. Just creating the conditions that allow smelt to thrive – that is, low salinity levels, lots of food, and high turbidity or muddy water that magnetizes smelt – would have been a cause for celebration.


There are many, many other examples of adverse changes that have put fish population dynamics in the Delta in a perpetual downward spiral. Since 1970, almost of all them involve reduction of Delta inflow and outflow, elimination of measures to mitigate the effects of reduced Delta inflow and outflow, and/or the biological response to reduced Delta inflow and outflow.

Figure 1. State south Delta exports (Harvey Banks pumping plant) in spring 1997-2010.

Figure 2. State south Delta exports (Harvey Banks pumping plant) in spring 2011-2019.

Figure 3. Delta outflow April-May 2007-2009 and 2013-2015 droughts.




  1. Initially exports were even higher with the new 11,000 cfs export capacity of the State Water Project. Total exports reached 12,000-14,000 cfs

Striped Bass Status – Summer 2019

I last reported the status of striped bass in 2016.  The prognosis was not good after four years of drought (2012-2015).  Today, after a sequence of water years (2016-2019) that were below normal, wet, below normal and wet, the prognosis has not improved, notwithstanding the remarkable resilience of striped bass.

After improving in below-normal 2016 and wet year 2017, the below-normal 2018 fall index equaled that of below-normal 2010, the lowest since 2000 (Figure 1).  A similar pattern occurred in the 2018 summer index (Figure 2).

The summer-to-fall recruitment relationship (Figure 3) shows a continuing strong relationship between the summer index and fall recruitment.  Furthermore, the relationship continues to show a positive response in wet years (2011 and 2017), but a poor response in the below normal years (2010, 2016, and 2018).  This indicates that summer conditions in these below-normal water years is depressing  the fall recruitment of striped bass.

Early indicators for wet year 2019 give me pause and further concern for striped bass recruitment this fall.  First, numbers and densities in the 2019 late spring 20-mm survey were way down as compared to 2017.  Second, despite high south Delta exports in July 2019, juvenile striped bass salvage is also lower compared to July 2017 (Figure 4), consistent with the lower 20-mm survey results.  The summer and fall indices for 2019 will help complete the story.

Figure 1. Striped bass fall index 2000-2018.

Figure 2. Striped bass summer index 1959-2018.

Figure 3. Striped bass Fall Midwater Trawl Survey Index (log10[index+1]) versus prior Summer Townet Index (log10). Select years labeled, with color of number showing year type: blue=wet, green=normal, and red=critically dry.

Figure 3. Striped bass Fall Midwater Trawl Survey Index (log10[index+1]) versus prior Summer Townet Index (log10). Select years labeled, with color of number showing year type: blue=wet, green=normal, and red=critically dry.

Figure 4. Striped bass salvage at south Delta export facilities with export levels (acre-feet) summer 2017-summer 2019. Note near maximum export levels in July 2017 and 2019.