Risk Assessment in Ballast Water Management - Global Maritime Transport and Ballast Water Management: Issues and Solutions - page 154

unfavourable conditions inside the tank during a voyage, become discharged from
the vessel and to survive the deballasting process. This “chain of events” needs to
coincide with opportunities in the recipient area that they fi nd suitable environmental
conditions and food sources so that they can survive and reproduce. The latter
requirements are termed 'invasion windows'. The survival of species during vessel
voyages has been studied earlier. It was assumed that longer containment inside
ballast tanks negatively affects species survival.
In contrast it was found that species survive several months in ballast tanks.
Resting stages may even be viable for many years (e.g., Hallegraeff and Bolch
1992 ; Gollasch et al. 2000 ; Olenin et al. 2000 ; David et al. 2007 ; McCollin et al.
2008 ). Further, a RA model as the chain of events was prepared (Hayes 2000 ; Bailey
et al. 2011 ). However, the high diversity of potential species in transit with their
stochastic behaviors, e.g., some species have even been found to reproduce in bal-
last tanks (Gollasch et al. 2000 ), it can be assumed that some species will survive a
vessel voyage (see chapter “ The Transfer of Harmful Aquatic Organisms and
Pathogens with Ballast Water and Their Impacts ” ) so that survival en-route is con-
sidered as not robust or reliable enough to be used as a risk quantifying factor.
Noting the above and applying the precautionary principle, this RA model
assumes that all species present in a ballast water donor port which can theoretically
be transported with ballast water will become discharged alive in a recipient port.
However, it is impossible to predict at which point in time this might happen. This
means that, a priori , ballast water discharges from a donor port with a harmful spe-
cies is an undesirable (unacceptable) event.
Quantity and Frequency of Ballast Water Discharges
Other shipping factors such as the quantity and frequency of ballast water discharges
also relate to the risk level (Bailey et al. 2011 ; Chan et al. 2013 ). We assume that
the higher the number of introduced organisms is and also the higher the introduc-
tion frequency is, the greater is the expected probability of a successful species
introduction. However, this is species-specifi c and certainly depends on many
conditions in each new environment where the species is introduced (Briski et al.
2012 ).
We found that the total number of ballast water discharge events and their tem-
poral distribution in the recipient environment are insuffi ciently studied regarding
their possible risk level impact and infl uence, and were therefore not considered in
this RA model. Ruiz et al. ( 2013 ) concluded recently that there was no relationship
between the quantity and frequency of ballast water discharges of foreign vessels
with the number of introduced ballast water mediated species in 16 large bays in the
United States. Furthermore, to our knowledge there is not even a single study to
quantify the minimum number of organisms (propagule pressure) which would
need to be discharged with ballast water to enable a species establishment with a
self-sustaining population which may subsequently become invasive in a new
environment.