Spotlight July 2023: Member-Supported Research Examining Role of Invasive Species in Bay-Delta

• by ACWA Staff
• Jul 21, 2023
• Newsletters

Biologists with FishBio take measurements on a black bass as part of research into the impact of invasive predatory fish species within the Sacramento-San Joaquin Bay-Delta.

The Sacramento-San Joaquin Bay-Delta supports a thriving fishery, probably one of the healthiest in the nation – if you’re talking about invasive bass species. 

Prowling the rivers, channels and sloughs within the Bay-Delta, non-native striped and black bass feast on crayfish, minnows, bugs and occasionally even waterfowl. Their diet certainly includes native juvenile salmon making their way to the Pacific, but to what extent, until recently, remained unknown. 

Currently, scientists with FishBio, a fisheries and environmental consulting company, are conducting research in the Bay-Delta and its tributaries that focuses on invasive predator fish species, their abundance, what and how much they eat, and how they move within the Bay-Delta. The research is being funded by the Banta-Carbona Irrigation District, Patterson Irrigation District, West Stanislaus Irrigation District and the San Luis Water District, all ACWA-member agencies. 

Proving the impact of bass predation on juvenile salmon is revealing an important factor behind the alarming slide toward extinction of salmon within the Bay-Delta and its tributaries, a decline some believe is mostly, or even exclusively, linked to decreased flows into and out of the estuary because of water diversions to farms and cities.

That perspective often minimizes or leaves out altogether the role of habitat restoration and the radical transformation that has turned the Bay-Delta into the most engineered water way on earth. The arrival and spread of invasive, predatory fish species occupy a significant role in that transformation, and ongoing research supported by the four ACWA member agencies is defining that role and its impact on endangered native species such as salmon. 

FishBio’s research relies on catching predatory species using traps and electrofishing, or “e-fishing,” a method that employs metal strips lowered into the river to generate an electrical field that temporarily stuns fish. 

Biologists net the fish, use a tube to safely empty their stomachs and record the contents. They also take measurements and tag fish for tracking migration patterns before releasing them back into the water. Anglers who catch and report tagged fish receive $20 rewards. In a few cases, striped bass tagged in the middle reaches of the Bay-Delta were caught in the ocean beyond the Golden Gate Bridge. 

During a recent e-fishing outing in the south end of the Bay-Delta, FishBio scientists captured and analyzed a string of black bass while envious anglers watched from a campground on the shore. 

Crayfish topped the menu that morning, but previous trips on tributaries such as the Stanislaus River added to growing evidence that bass, and striped bass in particular, were consuming juvenile salmon at significant levels. That included identifying the remains of 25 juvenile salmon that were in the bellies of three striped bass. Based on estimated populations of bass and salmon, striped and black bass potentially have the capability to wipe out entire runs of Chinook salmon in tributaries, during some years, before they even reach the Bay-Delta.

“The Delta is one of the most invaded ecosystems in the world,” said Andrea Fuller, FishBio’s Vice President and Senior Biologist. “It appears that we’re beyond a tipping point, where non-native fish outnumber native fish in a severely altered ecosystem, to the point that our native fish don’t really stand a chance.”

Complicating the issue, striped and black bass are prized as game fish within the Bay-Delta. They were originally introduced more than a century ago to replace a salmon fishery in decline from over-harvesting and were stocked in the Bay-Delta as recently as the 1990s before the state put a halt to it. Today, game fishing groups are proposing a “slot limit” that would protect smaller striped bass — the ones that tend to eat the most juvenile salmon — and larger striped bass that are the most reproductive. 

The proposal is fueling an ongoing debate. If striped bass are among invasive species preying on juvenile salmon, how would maintaining and perhaps expanding their population affect endangered salmon? Results from FishBio’s research is intended to clarify that impact, as well as help pinpoint what, in addition to decreased flows, is accelerating the decline of the salmon fishery.

Banta-Carbona General Manager David Weisenberger describes FishBio’s work as filling a data gap that can lead to better fisheries management and help recover the native salmon population. 

“No one is saying flows are not a factor, they are just saying evidence is showing that flows are not the only factor impacting the Bay-Delta’s native salmon fishery,” Weisenberger said. “We think we’re missing a big part of the problem, and we want others to be aware of it and maybe refocus their scientific efforts. Let’s identify and rank the biggest impacts and how to address them. That’s what we’re doing right now.”