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Vectors of Introduction
The last topic we address is an evaluation of the major vectors by which invaders were
transported into the NEP, NWP, and Hawaii (see Table 2). The six vectors analyzed were ballast
water discharges, hull fouling, intentional stocking, aquaculture escapees, aquaculture associated
species, and aquarium/plant trade. Aquaculture escapees are species that are intentionally
stocked but that escaped into the wild. Aquaculture associated species live on or in association
with an aquaculture species and are accidently transported along with them (e.g., species
transported along with oyster stocks).
One or more of these six vectors were considered as a possible transport mechanism for 92% of
the invaders in the NEP, 93% in the NWP, and 86% in Hawaii (Figure 60). The lower percentage
in Hawaii is primarily due to the large number (40) of nonindigenous insects, many of which
were introduced via other vectors such as from airplanes, cargo, or solid ballast. In evaluating the
results, it is important to recognize that many, if not most, invaders are polyvectic (Carlton and
Ruiz, 2005b) and can be transported by more than one vector. Thus, the values in Figure 60 can
sum to more than 100%.
Hull fouling was the major vector in all three locations, and was considered a possible vector for
47% of the invaders in the NWP to 61% in Hawaii (Figure 60). Ballast water discharges were the
second most important vector in all three locations, with its potential importance ranging from
about 40% of the invaders in the NWP to 53% in the NEP. One major difference among the
locations was the greater importance of intentional stocking and aquaculture escapees in the
NWP compared to the NEP and Hawaii. These two vectors combined were potential routes of
introduction for about 46% of the invaders in the NWP compared to about 9-10% in the NEP and
Hawaii. The high percentage of these two aquaculture related vectors in the NWP reflects the
active aquaculture efforts in Asia, though we caution that for many of these nonindigenous
aquaculture species it is not known if they have established wild populations. Another difference
among the locations was the importance of aquaculture associated species, which was a potential
vector for approximately 42% of the invaders in the NEP compared to 15% to 29% in Hawaii
and the NWP, respectively. The greater importance of this vector in the NEP reflects the large
number of invaders historically introduced via the extensive importation of the Atlantic oyster
(Crassostrea virginica) from the U.S. East Coast and from the Pacific oyster (Crassostrea gigas)
from Asia. The importance of this vector in NEP has declined both with a reduction in the
number of oysters imported from outside the NEP and with the institution of more stringent
quarantine procedures.
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