The Transfer of Harmful Aquatic Organisms and Pathogens with Ballast Water and Their Impacts - Global Maritime Transport and Ballast Water Management: Issues and Solutions - page 43

theoretically some species can generate new populations with low numbers (Bailey
et al. 2009 ). Survival depends on their tolerance to the conditions in the new envi-
ronment and the degree of dispersal following discharge. Very often these windows
of opportunity for establishment may depend on the precise location of ballast
release. The colonization success may depend on the season during which arrival
takes place, in cold climate regions warm water species may only survive discharge
in summer, and might not subsequently survive any winter. Unless a species can
reproduce a colonisation cannot evolve. Once a founder population is formed a
species can then be spread by a wider range of human activity processes but also
by natural processes.
Transfer and Impacts of Non-indigenous Species
Hewitt and Campbell ( 2010 ), Hayes and Gollasch (both unpublished) suggest
>2,000 aquatic non-indigenous species have been introduced world-wide, of which
in minimum 850 are thought to have been introduced by ships (Hayes and Sliwa
2003 ). There are some world regions that have greater numbers of recorded aquatic
non-indigenous species present, these have often been in port regions, in sheltered
bays and estuaries in regional seas (see Fig. 3 ).
In Europe >1,000 non-indigenous species are recorded from coastal and adjacent
waters. The numbers of non-indigenous species in European seas have different pat-
terns to all other world regions, this is because more than 50 % of the introductions
occur in the Mediterranean Sea with more than 650 species records of which, at least
Fig. 3 Hot spots of invasive marine species. Small circles : <150 species, medium circles : 150-250
species and large circle : >250 species (see text for references)