Risk Assessment in Ballast Water Management - Global Maritime Transport and Ballast Water Management: Issues and Solutions - page 152

when ballast water is transported between freshwater ports and higher saline brackish
ports with salinities >18 psu. In these cases a species-specifi c method would be
required in addition to the environmental match taking into account the species
salinity tolerance ranges, especially considering species which have a known salinity
tolerance higher than <0.5 psu and >18 psu.
The salinity limit of 18 psu is based upon the work of Remane ( 1934 ) and
Remane and Schlieper ( 1958 ). They compared the diversity of freshwater, brackish
and marine species along salinity gradients and showed that for many groups of
species the minimum species diversity was found in low salinity conditions. A bor-
derline used in their studies is at approximately 18 psu. It is interesting to note that
the Venice salinity system (Venice System 1959 ) draws the line between polyhaline
and mesohaline also at this psu level and it is found that this relates to a change in
species diversity (den Hartog 1964 ). Paavola et al. ( 2005 ) more recently found the
same trend for native and non-indigenous species. In European brackish seas, most
non-indigenous species are well adapted to salinities with the lowest native species
diversity. Also the non-indigenous species diversity maximum is frequently
observed in the salinity ranges where the native species diversity reached a mini-
mum. Bleich ( 2006 ) compared the macrozoobenthos diversity at different Baltic
Sea sampling stations with different salinities. He found that the species diversity
changed by more than 80 % at ca. 18 psu and concluded that this may be a salinity-
related distribution limit. We therefore assume that the 18 psu salinity limit chosen
is well enough justifi ed.
Species-Specifi c Method
The identifi cation of species-related risk takes into account the potential invasive-
ness of each selected species and the potential harm that it could cause in a new
environment. The selection of target species was based on the IMO defi nition in the
G7 Guidelines using the following criteria: (a) evidence of a prior introduction; i.e.,
where a species has become introduced outside its native range; (b) potential impact
on the environment, economy, human health, property or resources; (c) strength and
type of ecological interactions, i.e., severeness of its impact; (d) current distribution
within the biogeographic region and in other biogeographic regions; and (e) rela-
tionship with ballast water as a vector, i.e., it has been shown to be carried in ballast
water or it has a life-history stage that might be carried in ballast water.
The target species selection process should consider all harmful native, non-
indigenous and potentially harmful cryptogenic species present within the donor
and recipient ports and their surrounding areas. For a species-specifi c RA, an assess-
ment results in an unacceptable risk if it identifies at least one target species
that satisfi es all following criteria: the target species is (a) likely to cause an unac-
ceptable level of harm; (b) present in the donor port, but not in the recipient port;
(c) likely to be transferred to the recipient port with ballast water; and (d) likely to
survive in the recipient port.