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Crab Conniptions
Thousands of hairy-clawed crabs native to Chinese waters brought critical Delta fish salvage facilities to a halt this past summer, sending engineers and biologists on a frantic search for ways to separate the crabs from the fish, and to prevent future shut downs and impacts on endangered species.
Ordinarily, the state and federal fish salvage facilities in the Delta act as giant screens that capture fish to prevent them from becoming trapped and killed in the downstream water export pumps. If the salvage facilities are not operating, water cannot legally be exported. But this summer, so many crabs piled up on the trash rack at the Bureau of Reclamation's Tracy facility that flows backed up and threatened to collapse the structure. "The crabs were clogging everything up. Nothing was functioning," says U.S. Fish & Wildlife's Kim Webb, one member of an interagency group studying the crabs.
Up to 20,000 crabs ended up in fish holding tanks at the state and federal facilities each day during the crab crisis - a big jump up from the dozen or so seen just a few years ago. The mittened monsters from another continent-probably brought to California via a suitcase by someone with a taste for delicate crabmeat-have clearly spread throughout the Delta and Sacramento and San Joaquin River systems. With each female mitten crab producing between 250,000 and one million eggs, downstream areas are becoming overcrowded, and the crabs have begun moving farther and farther upstream, says Webb.
The crabs hang out in upstream fresh waters until they begin to reach sexual maturity. They then head downstream for the Delta to spawn in saltier water between late August and mid-October, where in 1998 many were pulled into the fish facilities.
Salvage operators first tried to control the crabs in the holding tanks, where huge cylindrical strainers are lifted in the center of the tanks, allowing fish to flow back into the "loadout buckets." But this fall, masses of crabs either blocked the openings between the screens and buckets, leaving fish high and dry in the bottom of the salvage tanks as all the water rushed out, or - worse yet - ended up in the buckets with the fish.
To separate the crabs from the fish, operators at the Tracy facility studied the crabs' behavior and decided to force them to scuttle up the sides of the tanks by manipulating the water speed. "When they put the intake valves up really high, the crabs would start to climb the tank sides, but in such a confined space the high flows weren't necessarily good for the fish," says Webb. Still, compared to being bruised, bumped or clawed, not to mention poked by the sharp spines of the crabs, higher flows seemed the better option for the fish.
The next salvage step loads the bucketed fish onto tank trucks, which transport them far downstream of the pumps and salvage facilities for release. But this summer, when salvage operators tried to flow the fish from the buckets into the truck tanks, crabs again blocked openings. Both fish and crabs had to be pushed through the holes onto the trucks, which ended up killing many of both, says BurRec's Scott Siegfried. When it came time to release the fish into the Delta through the 12-inch hoses on the backs of the tank truck, crabs got in the way yet again. Salvage operators resorted to making more frequent hauls to release sites and netting fish from crab-laden buckets.
Finally, operators took to trying to catch the crabs in the channels (called "secondaries") that initially convey the fish into the salvage system. "The crabs are easier to remove there because they are confined and concentrated in an open rectangular channel. The holding tanks are round and difficult to work in," explains Siegfried.
All told, crab-control efforts included traps, pumps, and a "traveling" screen positioned across the channel. Of all of the methods, only the traveling screen was at all successful, says Siegfried, since it stopped many of the crabs and but allowed most of the fish to swim through the mesh.
Biologists in Germany, where the mitten crab was very numerous in the 1930s, developed their own control methods. According to the Department of Fish & Game's Kathy Hieb, they trapped juveniles migrating upstream by placing baskets on the upstream sides of dams to collect climbing crabs as they fell from the dam tops. On steeper dams, they built tile-lined trenches along river banks to catch crabs trying to go around.
Similar trapping efforts at the Delta fish salvage facilities may have failed for several reasons, says Siegfried. Crustacean invaders at large in the massive Delta can easily miss the crabpots and traps; outmigrating crabs eat very little, and may thus not be interested in trap bait; or flows in front of the salvage facilities may be too high for successful trapping.
One bright spot in the crab gloom and doom is that endangered salmon runs usually don't arrive at salvage facilities until Thanksgiving. By mid-November this year, mitten crab numbers had tapered off to only 1,000 per day. Biologists say that by spring, most of the adult crabs will have died, and juveniles will be migrating upstream. While a few could end up at the salvage facilities with spring fish, biologists are not anticipating the same kind of mayhem. No one is breathing easy, however. "Next year, in late summer and autumn, we expect to collect millions or tens of millions of crabs," says Siegfried. "Those numbers would overwhelm the control efforts we carried out this year."
In an attempt to stay one step ahead of the crabs, BurRec's Technical Services division in Denver is testing new traveling screens for the conveyance channels. Using a model, as well as actual test crabs, BurRec is trying to perfect the screens in time for next year's onslaught. In the meantime, an interagency project team has been proposed for mitten crabs, and a workshop for all interested parties will be held next March, where a comprehensive mitten crab management plan may be born. "There's really no way to gauge what the crabs will do next year," says Webb. "Endangered species runs shouldn't overlap with the crabs, but it all depends on rainflows, water temperatures and other natural conditions. We have to hope that doesn't happen. In the meantime, we're all just trying to cooperate on solutions."
Despite these reassurances, a few winter-run and steelhead were found at the state fish facility as late as mid-October. What implications would an increased "take" of endangered fish have for pumping and salvage operations? According to Fish & Game's Deborah McKee, "The whole process depends on being able to do a meaningful subsample of fish coming into the salvage tanks. Take limits are based on accurate counts of fish coming through the facilities and the survival of salvaged fish. But instead of counting fish, we're counting mitten crabs. We may need a whole new way of monitoring fish survival in the Delta if the salvage facilities cannot function."
Contact: KimWebb (209)946-6400
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