BIOLOGICAL POLLUTION - Marine Pollution: What Everyone Needs to Know - page 201

of Brown University found that when green crabs invaded
these marshes they ate and displaced the Sesarma, and actu-
ally promoted recovery of the marsh grass.
What can be done to prevent new invasive species
from arriving?
It is much harder to eradicate an alien species in a marine
environment than on land, but not impossible. If eradication
is not possible some type of control may be achievable, even
though it will need to be ongoing and is very expensive. In all
cases, it is better to prevent the introduction in the first place.
Surveillance and monitoring are essential to detecting a new
arrival in time to deal with it before it turns into a problem.
Prevention through management of ballast water is gaining
much attention, since ballast water discharges are a major vec-
tor of introductions. Because of international regulations, ves-
sels are now supposed to exchange ballast water in the open
ocean before arriving at their destination. Organisms in ballast
water taken on in a port are likely to be adapted to estuarine
or river conditions so they will not survive in the open ocean
when released. The ship refills its tanks with ocean water;
oceanic species should not be able to survive when released
in ports and harbors. However, it is not always possible for
ships to use this method because of safety concerns, such as
in rough seas. Also, emptying the water does not remove the
sediments on the bottom of the ballast tanks. Many organisms
inhabit these sediments, including dinoflagellate cysts, some
of which are toxic, presenting a possibility of harmful algal
bloom (HAB) problems in the new location. Also living on
the bottom of ballast tanks are adult invertebrates including
green crabs, mud crabs, periwinkles, soft shell clams, worms,
and blue mussels, which are not removed by midocean water
exchange and live their whole adult lives cruising on the seas.
While their densities are usually low, invasion risk may still
be significant, especially during reproductive seasons. Gravid