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Purging Ships of Aquatic Invaders

As Asia's clams and crabs take America's Pacific ports by storm, everyone's pointing the finger at ships' ballast where aquatic species often hitch a transoceanic ride. At least 2.4 million gallons of ballast water arrive in U.S. harbors from foreign ports every hour, according to a recent Sea Grant study. The average ship coming into S.F. Bay to tank up on oil or load up on cargo may unload 10,000-50,000 metric tons (3-13 million gallons) of ballast water with an accompanying menagerie of foreign invaders. In the face of this onslaught - which Sea Grant study author James Carlton calls "invasion roulette" - concerned officials and experts are considering control options ranging from simple precautions such as not taking on ballast water at night when more critters are out in the water column to complex on-board ultrasonic treatment measures and tough preventive laws.

First, a few basics on ballast - water pumped in and out of ships to compensate for changing cargo loads. "A ship is basically a big metal box with a point at the front and a propeller in back," explains retired ship's officer Alistair Hamilton of the Nautical Institute. "It's designed to carry say 20,000 tons of cargo but only weighs 4000 tons, so when it's empty it sits high up in the water with the propeller spinning in the wind. From the point of view of getting anywhere, you need ballast."

The current preferred method for purging ballast of unwanted plants and animals is for ships to dump their ballast from the previous port at sea and to then replace it with ocean water. The theory is that the saltwater marine organisms in the replacement ballast are unlikely to survive if discharged in the fresher, more temperate waters of a subsequent estuarine port of call. This prevention method is outlined in 1993 International Maritime Organization guidelines that recommend ballast exchange in waters at least 2000 meters deep.

Though Hamilton sees the need for ballast exchange, and has carried it out himself, he says it's a dicey thing to pull off at high sea. Even in good weather the roiling ocean exerts a lot more force on the ship than the relative calm of a protected harbor. Emptying and filling a ship's 6-30 ballast tanks can create weaknesses and imbalances in the vessel. "Imagine a whole bunch of boxes lightly glued together," says Hamilton. "If you put weight in one and take it out of the other you get one pushing down and the other buoying up, causing a shear at the join." Hamilton says such maneuvers can actually break a ship in half.

Hamilton prefers the more gradual approach of pumping sea water into a full tank and letting the overflow go out the ventilator shafts. This method has its own engineering problems, he cautions, mainly the pressure build up from the fact that the ventilator shafts (designed to convey air not water) are often half the diameter of the pump-in pipe.

This method takes a long time and requires careful supervision, says Hamilton. "It's too easy to turn the pump on and go off to lunch," he says, adding that the decade-long depression in bulk shipping means that many older ships have been poorly maintained. "If you pass rules to just overflow tanks, you're going to see a series of shipping accidents due to blown tanks," he says.

In addition to the engineering challenges, ballast exchange also places a certain financial burden on shipowners. A full exchange, by whatever method, can take a day and a half or more and use 250 gallons of fuel per day for pumping alone, according to Hamilton. Then there's crew labor and time delay on the voyage to factor in.

With all the difficulties and costs, deep sea ballast exchange may not be the silver bullet everyone's looking for. Indeed the Sea Grant shipping study says a combination of different ballast exchange, treatment and management options may be more like it. The study describes and evaluates 32 different control alternatives, including specialized shoreline treatment facilities to provide and accept ballast water; on-board mechanical filtration to prevent organism uptake; on-board extermination of organisms by agitation or salinity alteration, or by chemical, thermal, ultrasonic or ultraviolet treatment, or by oxygen deprivation; passive disinfection via increasing the length of the voyage; micromanagement in which ships refrain from ballasting in places (such as disease hot spots) and at times (such as night) where more organisms may be present; and ballast exchange in calmer waters closer to port.

Many of these measures are long-term, requiring changes in the way ships are designed and ports are equipped. Whatever the approach, shippers are likely to prefer an international standard so that the regs aren't different in every port. Currently, no international law exists, just the maritime organization guidelines. In the U.S., the Great Lakes - ravaged by a European zebra mussel - is one of only two regions that mandate at-sea ballast exchange. As local lore has it, the first time the Coast Guard notified a vessel entering the Lakes region that it was planning to test the salinity of the ship's ballast water, the captain poured table salt into his tanks to comply.

"Spot checks with good enforcement and high penalties bring a pretty high level of compliance," says marine biologist Andrew Cohen, who just completed a major study of exotic species intrusions for U.S. Fish & Wildlife. "There's no reason why the same laws couldn't be applied to the San Francisco Bay, and with great benefit." California law currently does not require at-sea ballast exchange but does require vessel masters to fill out a form describing what's been done with the ship's ballast.

Even though less than 3% of all the exotic species arriving via ballast and other means actually become established in new regions, according to the Sea Grant study, it only takes one species to do great damage - a single species of Asian clam was recently credited with grazing the entire Suisun Bay phytoplankton food supply down to aquatic stubble.

Cohen says one exotic aquatic species has been introduced into S.F. Bay every 24 weeks since 1970.

Contact: Andrew Cohen (510)848-1029 or Alistair Hamilton (707)557-0758

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