and 15% of the samples. However, none of the other nonindigenous species 
occurred in more than 7% of the samples. 
One similarity between the shelf nonindigenous species and those in 
coastal estuaries and Puget Sound is the predominance of non-native spionid 
polychaetes. Five of the 13 nonindigenous species on the shelf are spionids (L 
cirrata, D. bidentata, D. caulleryi, D. quadrilobata, and P. paucibranchiata), while 
14 nonindigenous spionids have been reported from coastal waters (Lee and 
Reusser 2008). However, the shelf and estuarine assemblages differ in the 
identity of the dominant spionid invaders. In comparison to Laonice and the 
Dipolydora species on the shelf, the most frequently occurring nonindigenous 
spionids in estuaries are Polydora cornuta, Pseudopolydora kempi, 
Pseudopolydora paucibranchiata and Streblospio benedicti. Although P. 
paucibranchiata was found on the shelf, it was reported from only two of the 256 
samples. Another notable difference between shelf and estuarine invaders is the 
absence of the three most widespread estuarine invaders, the amphipods 
Grandidierella japonica and Monocorophium insidiosum and the bivalve Mya 
arenaria. Monocorophium acherusicum is also one of the most frequently 
occurring invaders in coastal estuaries, and though it was found on the shelf, it 
apparently has a very low abundance, since only a single individual was 
reported. 
Future resolution of the taxonomy and native ranges of the shelf fauna will 
reduce the uncertainty in evaluating the extent of invasion along the coast. 
Nonetheless, this preliminary analysis indicates that the shelf benthos is 
substantially less invaded than estuaries along the Pacific Coast when measured 
either by the number of nonindigenous species or by their abundance. 
Additionally, the common and widespread invaders in estuaries are either absent 
or in very low abundance on the shelf. The absence or low abundance of these 
estuarine invaders indicates that, at least to date, the offshore discharge of 
ballast water has not resulted in widespread invasion of the offshore benthic 
assemblages. 
3.5.6 Potential Linkage to Stressor Impacts 
Multi-metric benthic indices are often used as indicators of pollution- 
induced degradation of the benthos (see review by Diaz et al. 2004) and have 
been developed for a variety of estuarine applications (Engle et al. 1994, 
Weisberg et al. 1997, Van Dolah et al. 1999, Llannso et al. 2002a, 2002b). A 
desired feature of these indices is the ability to differentiate impaired vs. 
unimpaired benthic condition, based on a number of key biological attributes 
(e.g., numbers of species, diversity, abundance, relative proportions of sensitive 
vs. dominant species, biomass), while attempting to take into account variations 
associated with natural controlling factors. While a related index has been 
developed for the Southern California mainland shelf (Smith et al. 2001), there is 
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