THE fishermen of St Petersburg realised something was wrong when their nets
started coming up clogged with stinking, grey sludge. At first they suspected
pollution. But the true culprit was revealed only under the microscope. It was
Cercopagis pengoi鈥攁 water flea that is native to the Black Sea.
The crustacean arrived at St Petersburg in 1995. It found few predators and
there was a population explosion. In some parts of the Baltic Sea, masses of the
creatures鈥攖heir tails hooked together to form clumps鈥攃an make it
impossible to pull a fishing net through the water.
The incident is the most visible example yet of a threat that is increasingly
alarming marine scientists. The Baltic ecosystem, home to an important fishery
and to a huge coastal population, is experiencing a massive invasion of foreign
organisms. Alien invaders have already wrought havoc in the Black Sea. By 1990,
an introduced American comb jelly, Mnemiopsis, had gobbled up so much
of the zooplankton in the Black Sea that fish fry starved. Combined with
pollution and overfishing, this has led to the collapse of Black Sea fisheries
(鈥淗ow the Soviet seas were lost鈥�, 麻豆传媒,11 November 1995, p
38). At a meeting in Llandudno in Wales last week, biologists warned that the
Baltic could suffer the same devastation as the Black Sea.
Jim Carlton, a marine biologist at Williams College in Massachusetts, who in
the mid-1980s was among the first to sound the alarm over alien marine species,
says: 鈥淪o far, the Baltic has been lucky. We can鈥檛 predict which new species
will cause major ecosystem changes.鈥�
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Most of the invaders hitch a lift in the ballast water of ships. Rather than
wait for disaster, he says, 鈥渨e should be limiting the arrival of new species,
by controlling the dumping of ballast water by cargo vessels鈥�. While the US and
Australia have now clamped down on dumping ballast water in their ports, dumping
in the Baltic is largely unregulated.
How much damage are these invasions likely to cause? The Baltic, the world鈥檚
largest brackish sea, is only 10 000 years old. It has shifted from salt to
fresh to brackish several times as land masses have moved. This means the
ecosystem has had to start again from scratch several times, says Sergej Olenin
of the University of Klaipeda in Lithuania. 鈥淚n many ways the Baltic is already
a sea of invaders,鈥� he says.
This means there may be empty niches for new invaders to occupy. So far,
around 70 alien non-microscopic species have set up house in the Baltic,
alongside the 400 natives. Only 25 non-microscopic species successfully invaded
the Black Sea before Mnemiopsis struck.
But the impact on the Baltic so far seems small. No native species has gone
extinct. And some aliens are longtime residents. Mya arenaria, a North
American bivalve, arrived in the 11th century, clinging to the bottom of Viking
longboats. Others were introduced deliberately. In the 1960s, Soviet planners
brought in Pontogammarus, a Caspian shrimp, for local fish to eat. The
shrimps have done well, and fish have changed their diet. Little else
changed.
If most of the invasions have so far been benign, their scale is still
troubling. Piotr Czgruszka of the Agricultural University of Szczecin in Poland
estimates that 97 per cent of the bottom dwellers in the Oder Estuary and 95 per
cent in the Vistula estuary are now Marenzelleria. This bristly
polychaete worm from Chesapeake Bay on the east coast of the US arrived in 1985,
probably in ballast water. The worm seems to have displaced a few aquatic midge
larvae. And it has certainly taken over. But has it done any real harm? The
answer is still not clear.
鈥淚ts burrows are several times longer than any native species make, which may
liberate more nutrients from sediment,鈥� suggests Olenin. This could add to
eutrophication: excessive nutrients cause an explosion of algae, which die, rot
and deplete the oxygen in the water. On the other hand, the worm鈥檚 larvae are a
new food source for fish, says Stephan Gollasch of the Institute for Marine
Research in Kiel, Germany.
Other invaders also have good and bad sides. Besides fouling nets,
Cercopagis feeds the economically important herring, says Gollasch. And
although round gobies from the Black Sea (Neogobius melanostomus) are
competing with native flounder off Gdansk, they taste just as good and might be
controllable if people would start eating them, says Krzysztof St贸ra of
Gdansk University. But scientists are wary of suggestions that some
introductions might be beneficial. 鈥淚t鈥檚 a game of ecological roulette,鈥� says
Carlton.
Some species have already surprised scientists. The shipworm Teredo
navalis, a native of the tropics, started reaching the Baltic on ships in
the 1730s. But it did not breed in the chilly northern waters. At least, not
then: this year, says Gollasch, larvae have been found off Kiel. The species has
adapted. No risk assessment model would have predicted that, he says. 鈥淲e need a
concerted, European approach to this problem. After these species are released,
you can鈥檛 get them back.鈥�
Some newcomers are undeniably dangerous. In a study for the Nordic Council
published last week, Gollasch and Erkki Lepp盲koski of the University of 脜bo
Akademi in Turku, Finland, list the invaders that can travel in ballast, but
have not hit the Baltic yet. They include toxic algal blooms; cholera, thought
to have reached Peru in 1991 in ballast; Pfiesteria, a North American
alga that kills fish and causes human disease; a Japanese seastar that has
rampaged through shellfish beds in Australia and a Japanese snail that destroyed
Black Sea mussel beds.
In 1996, a study of German ports found that each incoming ship carries on
average four million specimens of macrofauna鈥攅verything from zooplankton
to fish鈥攁nd as many as 110 million phytoplankton per cubic metre. Some
7000 ships a year dock in Klaipeda in the Baltic, Lithuania鈥檚 only port. 鈥淭hey
dump 2 to 4 million cubic metres of ballast water here a year,鈥� says Olenin.
Yet there are almost no controls on dumping ballast water in European ports.
In 1997, the UN鈥檚 International Maritime Organization (IMO) recommended that
when ships reach mid-ocean they exchange the ballast water they took on in
harbours鈥攚hich is typically full of organisms鈥攆or relatively
lifeless water. The recommendation is enforced in the US and Australia by
inspectors who check the salinity of ballast water. Similar controls are planned
for some British ports and all Swedish ones.
The IMO鈥檚 guidelines are voluntary, and captains may ignore them if they feel
that the weather makes ballast exchange at sea too dangerous. 鈥淭here is
tremendous resistance in the shipping industry to ballast controls,鈥� says
Gollasch. Captains may also be swayed by the knowledge that frequent exchanges
wear out expensive ballast pumps faster.
But foreign invasions are also expensive. The Australian government
calculates that the accidental introduction of toxic dinoflagellate algae in its
waters would cost as much as 拢80 million. The Caspian zebra mussels that
now foul waterways throughout North America have so far cost 拢300 million
to clean up. 鈥淚 hope we won鈥檛 have to wait for a disaster like that before we
start taking preventive measures in the Baltic,鈥� says Olenin.
