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the second most invaded. The Northern California Ecoregion includes the San Francisco Estuary,
which has been considered the most invaded estuary in the United States (Cohen and Carlton,
1998). Our comparison does not include marsh plants, which could reduce the difference
between Hawaii and Northern California. Nonetheless, these results suggest that the near-coastal
waters of Hawaii are more invaded than the San Francisco Estuary. The next three most invaded
ecoregions all occur in the NEP; Southern California Bight (213 NIS), Oregon, Washington,
Vancouver Coast and Shelf (179), and Puget Trough/ Georgia Basin (125). The next four most
invaded ecoregions all occur in the NWP (East China Sea, Central Kuroshio Current, Yellow
Sea, and Sea of Japan) but they are considerably less invaded (69 to 87 NIS) than Hawaii or the
more invaded NEP ecoregions.
These observations raise the question as to why the NWP appears to be less invaded than the
NEP and Hawaii. As a preliminary analysis, we consider eight possible causes for the smaller
number of invaders reported from the NWP, listed in approximate order of their potential
importance.
1) More benign environment in the NEP and Hawaii than NWP. This “environmental matching”
argument suggests that the greater temperature ranges on the western sides of oceans compared
to eastern sides contribute to the asymmetrical invasion pattern. Support for this explanation
comes from an ecoregional-scale analysis of near-coastal sea surface temperatures (SST) in the
North Pacific (Payne et al., 2012). This analysis demonstrated a much greater seasonal and
monthly range in SSTs in the NWP than the NEP for ecoregions at approximately the same
latitude. For example, the Northeastern Honshu and Northern California ecoregions have very
similar 29-year mean near-coastal SSTs, 13.4 C versus 13.5 C. However, over the 29 year
record, the Northeastern Honshu displayed a mean monthly annual range of 16.4 C versus 3.4 C
for Northern California. Although not included in this analysis, SSTs in Hawaii also show small
seasonal and among-year ranges (e.g., Keeling et al., 2004). Based on the concept of
"environmental matching" (e.g., Gollash, 2006; Herborg et al., 2007), the greater temperature
tolerances of Asian species should allow more of them to survive in Hawaii and the NEP than
the reverse. A similar argument has been previously suggested by Chapman (2000) and Reusser
and Lee (2005). Although further analysis is needed, we suggest that the occurrence of more
constant temperature regimes in the NEP and Hawaii is an important factor in their susceptibility
to invasionfrom Asia and potentially other parts of the world.
2) Differences in search effort for NIS. It is recognized that differences in search effort can bias
the number of reported invaders (Ruiz et al., 2000). The Pacific Coast of North America may be
the most intensely studied area for marine/estuarine nonindigenous species in the world (e.g.,
Cohen and Carlton, 1995; Ruiz et al., 2000, Wonham and Carlton, 2005, CANOD, 2009),
followed closely by Hawaii which has been extensively surveyed through the efforts of the
Bishop Museum and other researchers, culminating in a synthesis by Carlton and Eldredge
(2009). While there has been an increasing interest in nonindigenous species in Asia (e.g.,
Iwasaki, 2006; Zvyagintsev et al., 2011), the level of effort does not yet appear to be equal to that
on the U.S./Canadian Pacific coast for most taxa. One indication of the difference in the effort
devoted to detecting NIS between these two regions is that at least eight rapid assessment
surveys (RAS) have been conducted on the Pacific Coast of the United States since 1993 (e.g.,
Cohen et al., 1998, 2005a). In comparison, the first rapid assessments surveys in the NWP appear
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