  The Mediterranean Sea is one of the most heavily invaded marine regions. This
work focuses on the dynamics and potential policy options for ballast
water-mediated nonindigenous species to the Mediterranean. Specifically, we (1)
estimated port risks in years 2012, 2015, and 2018, (2) identified hub ports
that connect many clusters, and (3) evaluated four regulatory scenarios. The
risk results show that Gibraltar, Suez, and Istanbul remained high-risk ports
from 2012-2018, and they served as hub ports that connected several spread
clusters. With policy scenario analysis, we found that regulating the high-risk
hub ports can disproportionately reduce the overall risk to the Mediterranean:
the average risk to all ports was reduced by 5-10% by regulating one high-risk
hub port, while the average risk to all ports was only reduced by 0.2% by
regulating one average-risk Mediterranean port. We also found that only
regulating high-risk ports cannot reduce their risks effectively.
