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A Look at the Delta's Tastiest Invasive Species

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Lucky angler, unlucky striped bass | Photo: Chesapeake Bay Program/Flickr/Creative Commons License

An explanatory series focusing on one of the most complex issues facing California: water sharing. And at its core is the Sacramento-San Joaquin Bay Delta. Stay with for all the project's stories.

Modern-day California has lots of species living here that weren't here 500 years ago. Settlers brought them in deliberately or accidentally, in large numbers or as a single individual that spread like mad.

Some of those new introductions, like tomatoes and daffodils, have mainly played nice with the other species. Others, like these plants currently wreaking havoc in the Delta? Not so much.

But one introduced fish that has thoroughly changed the Bay Delta ecosystem might well be the most popular of all the state's new species. And while scientists differ over the degree of harm this fish might be causing native wildlife, many Californians -- at least those who don't lean vegetarian -- are happy to have a chance to see this species. Especially steamed with ginger and lemongrass.

In other words, striped bass, one of California's most popular sport angling fish and a popular menu item, is an invasive exotic species. The bass, Morone saxatilis, was planted in California waters in 1879 for pretty much the same reason people plant fish anywhere: so they could be caught and eaten. The anadromous striped bass took to the Bay Delta like, well, a fish to water: swimming out to saltwater to mature and then back up into freshwater to spawn.

Unlike Pacific salmon, striped bass tend to avoid the open ocean during their saltwater phase, though one can find larger adult stripers at sea wandering the coast between Monterey and Mendocino, especially during warm El Niño conditions. In general, the fish prefer protected estuaries where juveniles can hunt in conditions a bit less exposed than on the high seas. It's hard to imagine an environment better suited for the species than the Bay Delta, with the expansive and protected San Francisco Bay providing saltwater habitat.

Also unlike Pacific salmon, striped bass don't necessarily die after spawning. Males reach sexual maturity in 2-3 years, females in 4-6, and individuals of either gender can live for 30 years or more, though typical life expectancy seems to be somewhere around 10 years.

That means each individual striped bass can swim upstream to spawn at least four times before they die, and the fish seem to get more fertile the older they get.

Spawning is a bit of a haphazard affair. Adults swim at the surface of the spawning stream in mixed groups dominated by males. Females release their eggs, males release sperm -- "milt," in fisheries jargon -- and the semibuoyant eggs drift downward through a cloud of the males' mixed milt. Fertilized eggs hatch in about two days. For the next week or so the new juveniles drift downstream, feeding on their attached yolk sacs, until they reach the spot where brackish and fresh water mix.

There, the baby fish eat zooplankton such as the tiny crustaceans called copepods and amphipods. As they grow, they slowly add fish to their diet, tiny ones at first and then larger ones. By adulthood, the stripers subsist exclusively on a diet of fish.

There are some scientists who suspect stripers were really able to get a secure foothold in the Bay Delta ecosystem due to the catastrophic floods of 1862, which flooded the Delta and adjoining areas of the Central Valley under 20 feet of water or more. That flood, combined with the huge amount of sediment washed down into the Central Valley from the hydraulic mines of the Mother Lode, likely hurt existing fish populations by silting up their habitat in some parts of the Bay Delta, and scouring it out in other places.

That reduced the number of fish that might have competed with striped bass, which helped the bass establish themselves once they were planted 17 years later. Or so some researchers conjecture.

Regardless, the bass are very well established in the Delta now, contributing to a sport fishery that brings millions of dollars each year for local businesses. The bass are essentially a top predator in the Delta, with adults reaching in excess of four feet in length and averaging between 10 and 30 pounds. There were about 2.5 million adult stripers in the Bay Delta in the mid-1970s; the fish's numbers declined to about a million by 2003 due to many of the same factors that threaten other fish in the system, including the massive pumps of the Central Valley Project and the State Water Project.

That "top predator" thing is potentially a big problem. The Delta is home to seven species of fish that are listed under the U.S. Endangered Species Act, including the Delta smelt, two of the Delta's four runs of Chinook salmon, and the Central Valley population of steelhead. And all of them spend at least part of their life cycles being small fish in the Delta. Do they get eaten by striped bass?

The answer to that question would necessarily seem to be "of course." In fact, according to some biologists, striped bass may eat as much as a quarter of each year's crop of juvenile salmon; the rough scientific consensus would put the minimum "take" of endangered or threatened salmon by striped bass at at least 5 percent.In 2008, the group Coalition for a Sustainable Delta, along with the Berrenda Mesa Water District, the Lost Hills Water District, the Wheeler Ridge-Maricopa Water Storage District, and an individual, sued the California Department of Fish and Wildlife -- then named the Department of Fish and Game -- in an attempt to loosen restrictions on legal fishing of striped bass. By improperly enforcing fishing regulations that protected striped bass, the plaintiffs charged, the Department was causing harm to Central Valley runs of Chinook salmon and steelhead that are protected under the federal Endangered Species Act. In expert testimony offered by the plaintiffs, biologist Charles H. Hanson wrote:

Striped bass predation in rivers tributary to the Delta appears to be the largest single cause of mortality of juvenile salmon migrating through the Delta. The high rates of striped bass predation within the Sacramento River are supported by, inter alia, striped bass diet studies and recent survival studies that have shown high mortality of salmon and steelhead -- approximately 90 percent -- before they reach the Delta.

Note that Hanson doesn't mean to imply that 90 percent mortality is entirely due to stripers: lost of things kill juvenile salmon trying to reach the sea, from great blue herons to accidental motor oil spills. But the bass do their part; they're voracious and aggressive. Stripers have been observed on many occasions parked at the downstream end of passages designed to allow juvenile salmon to go around obstacles on the Delta's tributary streams. Kind of like the aquatic version of grizzlies at the McNeil River Falls in Alaska.

Then again, if you're wondering why those three water districts, all of them serving the southern San Joaquin Valley, took such an interest in striped bass predation on salmon, you've got good California water politics instincts. All three districts rely on State Water Project deliveries, which are another major contributor to mortality of listed salmon and steelhead runs as juvenile fish are sucked upstream toward the pumps. If something were to be done about striped bass, goes the logic often expressed by State Water Project customers, that might ease pressure on the water projects for their contribution to the plight of endangered fish.

For its part, the National Marine Fisheries Service (NMFS), the federal agency charged with enforcing the Endangered Species Act for most marine species, has decided that something has to be done about striped bass numbers. In 2010, NMFS asked the state Department of Fish and Game to lift any restrictions on anglers catching striped bass as a way of encouraging reduction in the species' numbers. "NMFS has concluded that striped bass predation is a significant mortality factor for Central Valley salmon and steelhead," wrote the agency in a document published that year.

The agency cited one 2003 paper as suggesting stripers may eat about nine percent of the federally endangered Sacramento River winter run Chinook each year; certainly sufficient reason to explore the possibility of controlling striper populations.

And in an April, 2011 settlement of the Coalition for a Sustainable Delta lawsuit, that's exactly what the Department of Fish and Game agreed to do: to consider a management plan for stripers that took into account the fate of Chinook, steelhead, and other troubled populations of native fish. The good news from that settlement was short-lived. In 2012, the Department proposed an environmental review of the striped bass sport fishery that would look at effects on salmon and steelhead. In February of that year, the state Fish and Game Commission voted not to go ahead with that environmental assessment.

And of course it might be more complicated than just "striped bass are bad for salmon." In 2011, respected fisheries biologists Peter Moyle and William Bennett suggested that removing striped bass from the picture might not help the Delta's native fish all that much. Moyle and Bennett pointed out that despite popular assumptions that stripers were eating up the few remaining Delta smelt, very few of those fish have ever showed up in the stomachs of the bass, which would make them one of the few fish stripers apparently don't eat.

And though no one disputes the notion that striped bass do eat salmon and steelhead, Moyle and Bennett said that the bass also eat fish such as the invasive exotic Mississippi silverside, a smaller but still voracious predator on both those fish as well as the Delta smelt -- and which 1-3 year-old stripers eat like no one's business. Removing striped bass, they wrote, would very likely release pressure on predatory fish like the silverside, with unpredictable effects down the road.

It's also possible, said Moyle and Bennett, that the majority of striper-related salmon and steelhead mortality involves hatchery fish, which are generally held to be less cautious and wily than their wild-born counterparts. "Predation on hatchery-reared juveniles may even buffer wild fish from such predation," they wrote, "given that wild fish are warier and less conspicuous than the more abundant hatchery fish."

Ultimately, Moyle and Bennett suggested that removal of the striped bass would be removal of the ecosystem's top predator, a practice rarely recommended. "Reducing striped bass and other predator populations is unlikely to make a difference in saving endangered fishes, and will serve only to distract attention from the real problems," they wrote.

As respected as Moyle and Bennett are, it's unlikely that humans watching the Delta will agree on the benefits and drawbacks of removing the striped bass any time soon. In the meantime, for those of us Californians who eat fish, it's nice to know there's at least one species we can have for dinner with a clear conscience.

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