The Science of Sustainability

A fishy odyssey through the delta

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Talk about a wild ride.

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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Category: Biodiversity, Biology, Environment, Partners, Sustainable Food, Water

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Ann Dickinson

About the Author ()

Before moving to California almost five years ago, Ann served as Sally Brown Fellow in Environmental Literature at the University of Virginia, where she taught undergraduate seminars on literature and the environment and coordinated an ongoing reading series featuring nationally prominent nature writers. Prior to that, she spent a year as a research assistant at the Smithsonian Tropical Research Institute's field station on Barro Colorado Island, Panama, studying how young leaves defend themselves against herbivores.
  • steve

    The solution may not have to be to redesign the fish screens. The answer may be simpler and less costly. Hatcheries to raise smelt to increase and off set the smelt population that is lost by the pumps. 10% of the farm raised fish could be released above the pumping stations and 90% past the pumping stations. Everyone would be happy.

  • ann dickinson

    Thanks for your comment, Steve. Though it might seem like hatcheries would be the solution, unfortunately they are not. Captive-raised delta smelt have never been successfully released into the wild. Even if it could be done successfully, it may not be a good idea: For example, evaluations of the effects of hatchery-reared salmon released into the wild suggest they may be doing more harm than good to the species and the ecosystem.

    And of course hatcheries wouldn’t do anything to address the other problems impacting delta smelt and so many other species in the Delta: poor water quality and lack of freshwater flows due to excessive water exports.