Invasion of the Chinese Mitten Crabs

It was like a scene out of the X-Files. Tens of thousands of crabs swarming in holding tanks at State and federal fish salvage facilities. Their sheer numbers made normal operations impossible and killed fish that were to be saved and returned to the Delta.
But where did they come from and why in such numbers?
These are not crabs familiar to the American appetite. They were first identified in the early 1990s by their furry claws that resembled mittens, thus the name mitten crabs. Even then biologists warned that the proliferation of an exotic species such as these crustaceans would adversely impact the Bay-Delta's already challenged ecosystem.
Last year's "invasion" was the first real sign of how abundant the crab's population had become. Neither State nor federal officials had a plan to deal with the dilemma. But it was clear that one was needed so officials created a new interagency team to study the mitten crabs, their growing numbers, and the potential problems they present.

The Chinese Mitten Crab Profile
The Chinese mitten crab (Eriocheir sinensis) is a palm-sized grayish brown crustacean with small white pinchers sticking out of puffy brown claw coverings. The crabs reproduce in salt water in late fall or early winter. The young move upstream in early spring and reside there for one to three years, then migrate to saltwater to spawn. Their presence at the State and federal South Delta fish salvage facilities was part of that seaward migration.
This crustacean originates in China, where it is considered a delicacy. But if not cooked properly, crabs infected with the oriental lung fluke could transmit the fluke to humans, which produces symptoms much like tuberculosis. (This fluke has not been detected among crabs in the estuary.)
Whether the crab was introduced by accident or design is not known. Some biologists suspect the crab was intentionally introduced to the San Francisco Estuary. The crab has been illegally transported on airplanes in carry-on hand luggage and sold in San Francisco fish markets. It is also possible that the crab was accidentally introduced via exchange of ships’ ballast water.
But from its initial discovery in the San Francisco Bay in 1992, the crab's presence at the federal fish salvage facility in Tracy (CVP) and the State's Skinner Fish Salvage Facility (SWP) grew from around 30 in 1996 to about 20,000 in 1997 to over 25,000 crabs a day at the Tracy facility last year.

A Salvage Obstacle
The mitten crabs’ growing numbers at the two fish salvage facilities took facility operators and biologists by surprise. They had expected a population boom but not at the magnitude experienced, especially at Tracy.
DWR's Skinner Fish Facility was a bit more fortunate, averaging around 10,000 crabs a day. The lesser amount, some biologists say, may be due to Clifton Court Forebay, a natural barrier to the crabs who follow the water as it is diverted out of the main river system. According to DWR Environmental Specialist Jeff Janik, the H.O. Banks Delta Pumping Plant pumps water mainly during off-peak hours which may also contribute to the smaller number of crabs found at the facility. "The crabs follow the water flowing into the facility. The Tracy operation pumps 24 hours a day, and draws water directly from the river channel, thus has more crabs coming into their system," says Janik.
The crab swarmings impacted fish salvage operations at both the CVP and SWP facilities by clogging up systems throughout both facilities' infrastructures -- in particular the fish holding tanks. The multitudes of crabs created long delays in what is normally a standard process.
"The crabs caused a problem when they ended up in the same holding tanks as the fish we are trying to salvage," says Tanya Veldhuizen, Environmental Specialist and DWR's lead mitten crab biologist.
Usually the fish are filtered out and transported for release in the Delta away from the pumps. But with thousands of crabs clinging to the bins, the process was delayed and half the fish suffocated in concrete holding tanks while workers tried to separate them from the crabs.
Those that survived and were placed in transport trucks were further traumatized by the clawing crabs trying to escape. Fish are released through a small 12-inch hose from the back of the tank truck, but the crabs clogged up the opening, leaving many of the remaining fish to suffocate.
Because the crabs killed so many fish, they posed a secondary impact to the Department's requirement to salvage endangered fish from the pumping plants. According to Department of Fish & Game's Deborah McKee, "take" limits are based on accurate counts of fish coming through the facilities and the survival rate of salvaged fish.
"Instead of counting fish, we were counting mitten crabs," says McKee. The implications of an increased take of endangered fish for pumping and salvage operations are yet unknown, but setting new monitoring standards may be in the future if the salvage facilities cannot function.

Levee and Ecological Concerns
DWR staff not only have their eyes fixed on the mitten crab's impact on fish salvage facilities, they are watching the levees as well.
The crab's habit of shallowly burrowing into levees may cause some levee erosion. Except for some minor erosion and sloughing in south San Francisco Bay tributaries, there have been no detected impacts, according to Tanya Veldhuizen.
In a statement prepared by the Department during the mitten crab invasions, Keith Luster, Field Operations Chief under the Division of Flood Management, confirmed this observation, but continues by noting that other deep burrowing animals like beavers and ground squirrels that leave potentially deep voids into or under the earthen levee structures continue to be a greater immediate concern.
But as Veldhuizen explains, "If the crab population continues to grow, their burrows could become dense enough to present a problem in the next 10 to 15 years."
Environmental specialist Jim Martin of DWR’s Delta Flood Protection Branch would rather not wait until there is a problem. He and other Branch staff have been actively coordinating with reclamation districts and their engineers to monitor mitten crab activities that could affect the levee’s integrity. As leadperson of the Levee Subcommittee under the interagency task force, he says they’re investigating the crabs’ potential impacts and measures to control their population.
Another concern is their ecological impact on the Bay-Delta estuary. Mitten crabs are just one among many exotic species, such as the Asian clam (Potamocorbula amurensis) and the Gammarus daiberi, an amphipod or crustacean native to the East Coast. These alien invaders were introduced into the ecosystem with potentially damaging effects on the aquatic food chain.
The mitten crab is omnivorous, meaning it eats both plants and animals. Juveniles are mainly vegetarians, consuming aquatic plants and alga. As they mature, they graduate to consuming small animals, usually benthic (bottom-dwelling) invertebrates such as shrimp and aquatic insects, in addition to vegetation. They are not however discriminatory in their diet, having been caught with a variety of baits from ghost shrimp to shad.
With their population explosion, no one knows how adversely it will alter the existence of other estuary residents. In fact, the crab's ecological impact is the least understood of all potential impacts.

Economic Impacts
These little creatures are also known for their fondness for rice crops. In their native China, the crabs consume rice shoots. However, the crabs are routinely stocked in rice paddies and reared for later consumption.
So far, no crops have been damaged here, but no one really knows the toll they could inflict. Studies have shown that rice fields in tidally influenced areas apparently are most subject to damage.
"It is possible that California rice fields, which are dry for a portion of the year, may suffer less damage by the mitten crab than in Asia, where water reportedly is on many fields year-round," says Veldhuizen.
Mitten crabs have also affected commercial fishing operations. Many are caught in trawl nets and must be removed, a time-consuming task. For commercial Bay shrimp trawlers in South Bay, the crabs are more than a nuisance. The numbers caught in their nets can damage the netting and kill the shrimp caught, making them unsalable as bait.

Adept Movers
Though it is illegal to transport live mitten crabs, these hairy-clawed crustaceans are quite adept at transporting themselves. There have been reports of the crabs on roadways in Manteca, in parking lots in San Jose, and in backyards in Stockton.
During migration, they readily move across banks or levees to bypass obstructions, such as dams or weirs. Kathy Hieb, a biologist for the California Department of Fish and Game, predicts the crabs will migrate via waterways throughout the Central Valley. Just recently (early February 1999) mitten crabs were found as far south as the Tulare Basin along the California Aqueduct.
According to Hieb, populations of exotic species typically level off on their own after time. Biologists however are not sure whether or when that will happen with the mitten crab. "I have no reason to think the population would be any smaller next year," says Hieb.
Crab populations are expected to climb substantially by late summer and autumn.

Looking for Answers
Preventing their proliferation is just as much a problem as how to dispose of the crabs. So far, there is no efficient method of getting rid of the pests. Biologists have received reports of largemouth bass, sturgeon, bullfrogs, river otters, raccoons, and wading birds feasting on them. During the fish facility invasion, salvage operators collected, crushed, and buried the crabs. A San Francisco businessman has even offered to harvest the crabs and export them to China for sale (a notion that has been put on hold by the Department of Fish & Game).
Researchers say more information is needed about the crabs. Through the Interagency Ecological Program (EIP), representatives from DWR along with other State and federal biologists and engineers will start finding answers as they begin their work under the newly formed Chinese Mitten Crab Project Work Team. This group will begin the process to learn more about this species' life history and develop methods of control.
"The meetings will cover topics such as study updates and results, program planning, results from outside research, and technical input," explains Veldhuizen. "This team will allow us to coordinate our research. We need more information to develop strategies and technologies that will lead to effective methods of dealing with the crab’s impact on the estuary."

SIDEBAR
Stemming the Invasion
Because their populations appear difficult to control, learning to deal with the mitten crabs may be the only option. Corraling and collecting the crabs before they enter the fish salvage facilities could improve fish handling. The USBR has been experimenting with low profile, underwater electrical fences that appear to be very promising in guiding the crabs to collection traps.
President Clinton has created an Invasive Species Council, a coalition of federal agencies charged with finding solutions to protect the nation’s ecosystems from invading species. The California Legislature is also considering a bill to stop unregulated ballast dumping by requiring that ships obtain permits from the State Water Resources Control Board.