A number of species did not occur naturally in the Belgian part of the North Sea, but were intentionally or unintentionally introduced by man. Some of these species behave "invasively", that is to say that these species multiply very fast within a short period and occupy the space of the naturally occurring (indigenous) species. Many of these invasive species have entered our waters as "stowaway", along with the ballast water Ballast water is used to improve the draught, stability and strength of the ship when the ship is not (fully) laden. Mariculture (or aquaculture on sea) too is a potential problem when species from other areas are introduced and carry germs of unwanted species.
Known invasive species are the American jackknife clam (Ensis directus), the common slipper shell (Crepidula fornicata) and the Japanese oyster (Crassostrea giga). In some places like for instance in seaports, more than 60% of the species are non-indigenous!
American jackknife clam - Misjel Decleer
The Ballast Water Convention is an international agreement to prevent, restrict and ultimately ban the movement and introduction of harmful aquatic organisms and pathogens in the marine environment. This happens through the control and management of ballast water from the ships when they sail from one port to another.
The Protection of the Marine Environment Law, better known as the MMM –Law also forbids the deliberate introduction of non-indigenous species in the Belgian part of the North Sea. There is also a procedure to license the introduction of non-indigenous species by mariculture (Royal Decree on the protection of species).