Reported by Alison Hawkes

As the sun comes up, lighting a ridge of windmills, I'm traipsing around the State Water Project grounds outside of Tracy trying to puzzle out this battle between farming and fish

The courts are packed with lawsuits on this issue. But one in particular caught my attention. It has to do with striped bass, a voracious predator in the system that was introduced in the 1870s and went on to become a popular sport fish.

Stripers are almost a naturalized citizen to the Delta, but farmers hope to blame the big fish for the plummeting numbers of endangered fish. If they win the case, which is now headed for trial, it could surely lessen the onus on water diversions from the Delta. That means the farmers could get more water, leaving the state to pay for costly mitigation measures to save the endangered fish species like Delta smelt and chinook salmon.

What is the evidence that stripers are wiping out these native species? At Clifton Court forebay, a large reservoir that feeds water to the pumps, stripers are a-plenty. Fishermen line the banks waiting for the gates to the forebay to rise so that the rushing water jostles the stripers out of their slumber. I met one fisherman down there who showed me a photo on his cellphone of a 15-incher he had recently caught. He brought it home for dinner.

It was also easy fishing for a team of state biologists who were on a motor boat in the forebay catching and tagging the stripers for further study. The big question the courts want answered is whether the stripers have a population level affect on the endangered fish species. Meaning, do stripers eat enough of these smaller fish to affect their numbers delta-wide. Here's where the science gets squishy.

In the lawsuit, the farmers and water users argue that stripers do have a population level effect.
They want to see fishing regulations on stripers lifted as a means to bring back endangered fish. But some biologists claim there's not enough research to show a population level impact. UC Davis fish biologist Peter Moyle says the best estimate done, involving a winter run chinook salmon, showed a 9 percent loss to stripers, which he says is “not a big number.” That was a National Marine Fisheries study. Remove the stripers, he says, and you could actually be upsetting a balance in the ecosystem. Stripers may be keeping other predators in check, like the tiny inland silverside, another non-native fish that has an appetite for delta smelt eggs.

Perhaps the best solution is to target striped bass fishing where we know they hang out – at places like Clifton Court Forebay and other structures created by the state and federal water projects. It's here where the endangered fish are most vulnerable, and where the stripers know they can find an easy meal.

Listen to Delta Predators radio story online.

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Every year, millions of fish make a strange and harrowing detour through the Skinner Fish Facility, part of the State Water Project's facilities in the Delta.

In my last post, I wrote about my visit to the Banks Pumping Plant, whose giant pumps slurp water from the Delta to help quench California's thirst. As the volumes of water are sucked up, both resident and migrating fish come along for the ride. The Skinner Facility, in operation since 1968, was built to protect fish from being killed at the pumps–an effort that sadly is not as successful as one would hope (more on that below).

I was amazed to learn there is a whole art and science to fish screens, which range from physical barriers–called positive barriers–like perforated plates or wire mesh, to behavioral barriers like sound, light, or other stimuli aimed at keeping fish away. Well-designed screens minimize both entrainment (fish being pulled into the pump or diversion) and impingement (fish being trapped or injured against the screen itself due to water velocity).

Both physical and behavioral barriers are used at the Skinner Facility. Fish being pulled toward the pumps first encounter a trash rack that diverts many bigger fish, along with floating debris. Next, fish encounter a large, v-shaped array of metal louvers. The louvers create turbulence that functions as a behavioral signal, encouraging the fish to swim away into bypass pipes that function, as our tour guide put it, like "a big vacuum system."

From the bypass pipes fish travel to another set of louvers and pipes, concentrating them into a smaller volume of water, and then into holding tanks in a nearby warehouse. Giant, suspended cone-shaped buckets are used to periodically sample the fish, which are identified, counted, and measured. Some 90 species turn up in the facility, including Chinook salmon, steelhead, white sturgeon, and delta smelt. (I asked our guide if delta smelt really do smell like cucumbers. He confirmed it. In fact, when a school of smelt comes through–an event that has become rare–the warehouse smells "like a salad.") When enough fish have been collected, they are loaded into trucks and driven back to the Delta.

Here's the rub. Many fish caught in the pull of the pumps are lost to predation before even reaching the screening facility. Then, the facility does not effectively screen fish smaller than about 1.5 inches, meaning that littler, less powerful species and juveniles are still vulnerable to the pumps. For the fish that make it to the holding tanks, the process is such a trauma–with big and little fish squashed together in the tanks, buckets, and trucks–it's no surprise there are casualties; in fact, the delicate delta smelt often do not survive. And even for fish that make it through the entire process and out the other end, there's a final, fatal hurdle: the trunks routinely dump salvaged fish at the same locations, where more predators have learned to cluster for a free lunch.

Scientists agree that the loss of fish at the huge state pumps–and other pumps and intake pipes throughout the Delta–is a major contributor to plummeting populations. How much water we use makes a difference: The higher the export rates, the more fish are entrained. There also is broad consensus that more state-of-the-art fish screening facilities are needed. That could come with a hefty price tag. But with our fish disappearing, can we afford not to invest in their survival?

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