Scientists studying sea snails in the Gulf of Thailand
and the Straits
of Malacca have discovered that where there is heavy shipping
traffic,
including some sites off the Eastern Seaboard, every single
female
found displayed male sexual organs, typically a penis
and sperm duct.
The shellfish have gone ''tomboy" as a result of contamination
by a
chemical called tributyltin (TBT), which creates hormonal
imbalances
in certain snail-like species known as gastropods, said
Cornelius
Swennen, a Dutch marine ecologist. Gastropods include
some commonly
eaten species, such as Babylonia (known in Thailand as
hoy khao heuor
hoy roy ru).
TBT is an anti-fouling agent, an ingredient in the paints
used on
ships to prevent barnacles and other marine species from
sticking to
hulls, but also leaks into the sea and damages shellfish
populations.
The tin-based chemical is carefully controlled in developed
countries
small ships, such as yachts, are banned from using
it but not in
Southeast Asia, Swennen said.
The impact of TBT on humans is not known, he added. But
a paper
written by Swennen and his colleagues in the scientific
journal
Wallaceana states that ''its use on ships must be severely
reduced in
the interest of public health and that of the marine environment".
In theory, humans who eat TBT-contaminated seafood could
also be
subject to hormonal imbalances. But nobody has ever actually
seen such
effects in people and ''a human would have to eat an awful
lot of
contaminated seafood to be affected".
The scientist said he is greatly concerned over the marine
ecosystem,
however, as the hoy tom (''tomboy shellfish") phenomenon
known in
more formal circles as imposex is irreversible and
prevents
reproduction in affected species, causing shellfish populations
to
drop drastically.
''TBT is influencing non-targeted areas. It leaks out from
the paint
on ships' hulls and basically kills all the shellfish
larvae in the
area, including crab and lobster," explained Swennen,
who works out of
Prince of Songkhla University (PSU) in Pattani.
The affects of TBT have been well-documented in Europe,
North America
and Japan since the late 1960s, when the anti-fouling
agent was first
used. It has caused significant damage to commonly eaten
species, such
as oysters (hoy nang rom) and mussels (hoy malaeng phu).
But while TBT has an impact on all types of shellfish,
and also
contaminates fish. Swennen said it was found to have a
particularly
disturbing impact on gastropods, inducing females to develop
male
sexual organs in the form of penis-like appendages.
In fact, the relationship between TBT and the imposex phenomenon
has
become so well-understood that gastropods are now considered
a
bio-indicator for contamination by the anti-fouling agent:
scientists
judge the level of pollution by seeing how many of the
shellfish
display penises and vas deferentia (sperm ducts).
But it has only recently been shown that the impact of
TBT is felt out
at sea and not simply close to shore. Furthermore, most
studies have
been carried out in temperate or sub-tropical waters.
So Swennen teamed up with PSU lecturer Nukul Ruttanadakul
and Suraphol
Ardseungnern of Mahidol University to check imposex levels
in the
inner Gulf of Thailand and around Pattani. Malaysian and
Singaporean
researchers helped look for sex-shifting shellfish in
the Straits of
Malacca.
''The results show that TBT pollution is now a global problem,"
Swennen said. ''The contamination levels depend on the
amount of
shipping traffic rather than the size of the boat or the
depth of the
sea. Gastropods collecting near major shipping lanes all
had imposex
characteristics."
The greatest contamination areas found in Thai waters were
off Chon
Buri, where imposex incidences reached 100 per cent at
a couple of
sites. Meanwhile, in the western part of the inner gulf,
off
Phetchaburi, no transexual shellfish were found.
Imposex levels were also found to be quite high in Pattani
Bay (86 per
cent), off the northern shore of Singapore (88 per cent)
and in the
Straits of Malacca, due west of Singapore (100 per cent).
Swennen said he tried to contact each country's representative
to the
Marpol Convention, which is supposed to oversee marine
pollution
issues. Countries that are party to Marpol, including
Thailand, are
supposed to ban the use of TBT on ships smaller than 25
metres and set
controls on leaching rates from marine paints used on
larger ships.
''The Singaporean official acknowledged that it is a major
problem,
but he said they are under a lot of pressure from the
[shipping and
TBT manufacturing] industry," the Dutch researcher said.
''The Thai
representative didn't respond."
Harbour Department officials are supposed to look after
Thailand's
obligations under the Marpol Convention, but a source
from the agency
said that no action has been taken in regards to TBT in
Thailand.
''At a recent Marpol meeting in London, some governments
asked for a
tin-free alternative to TBT that would be less damaging
to shellfish,"
said the Harbour Department official who asked not to
be named. ''But
the European Chemical Manufacturers Federation said there
is no
alternative available that works with such efficiency."
The source said that a lot of the smaller fishing boats
in Thailand
use marine paints that contain copper-based anti-fouling
agents
because they are cheaper. But these chemicals also damage
crustaceans
such as crabs, shrimp and lobster, the official said,
and will affect
the health of people who eat the contaminated seafood.