3.5.3 Abundance and Dominant Taxa 
A total of 99,135 individual specimens were collected across the 256 
stations (259 0.1-m 2 grab samples) throughout the region. Densities ranged from 
540 to 22,980 rrf 2 and averaged 3,788 rrf 2 (Fig. 3.5.8, Table 3.5.3, Appendix 
Table 4). On a spatial basis, about 50% of the shelf area had densities > 3,080 
rrf 2 and about 10% of the area had densities > 7,250 rrf 2 (Fig. 3.5.9). In 
comparison, the NCA-West estuarine data (Nelson et al. 2004, 2005; U.S. EPA 
2004; Hayslip et al. 2006) show much higher densities of benthic infauna in 
estuaries along the West Coast (e.g., mean of 10,653 rrf 2 and range of 0 to 
415,820 rrf 2 ) (Table 3.5.3). Flowever, the higher mean and maximum densities in 
the latter case are due to a greater frequency of high-density patches in these 
shallower estuarine systems. Spatially, while 10% of the estuarine area along 
the West Coast had high densities >15,100 rrf 2 , 50% of the area had lower 
densities < 4,100 rrf 2 , which is only moderately higher than that estimated for the 
corresponding percentage of shelf area (3,080 rrf 2 ). Densities on the shelf in 
excess of 10,653 rrf 2 , the estuarine mean density, were limited to about 2% of 
the shelf area. Densities of benthic fauna in the present offshore survey, 
averaged by state, were highest in California (mean of 4,351 m' 2 ) and lowest in 
Oregon (mean of 2,310 rrf 2 ) (Fig. 3.5.8, Table 3.5.3). Mean densities were 
slightly higher at NMS stations vs. non-sanctuary stations for both the California 
and Oregon/Washington regions. 
The 50 most abundant taxa found in shelf waters throughout the region 
are listed in Table 3.5.4. The 10 most abundant members on this list include the 
polychaetes Mediomastus spp., Magelona longicornis, Spiophanes 
berkeleyorum, Spiophanes bombyx, Spiophanes duplex, and Prionospio jubata ; 
the bivalve Axinopsida serricata ; the ophiuroid Amphiodia urtica ; the decapod 
Pinnixa occidentalism and the ostracod Euphilomedes carcharodonta. 
Mediomastus spp. and A. serricata were the two most abundant taxa overall. 
There are clear differences between these dominant shelf fauna and those found 
in estuarine habitats along the West Coast. As an example, with the exception of 
Mediomastus spp., none of these 50 shelf species also appear on the list of 
dominant (10 most abundant) estuarine fauna reported by Nelson et al. (2005). 
The latter estuarine list (based only on 1999 data from the NCA-West database, 
thus excluding Puget Sound, the San Francisco estuary, and the main stem of 
the Columbia River) included the amphipods Americorophium spinicorne, A. 
salmonis, and Eogammarus confervicolus complex; oligochaetes; and the 
polychaetes Streblospio benedicti, Mediomastus sp, Mediomastus californiensis, 
Pygospio elegans , Pseudopolydora paucibranchiata, and Neanthes limnicola 
(Nelson et al. 2005). Thus, while estuaries have been found to be dominated by 
polychaetes, amphipods, and oligochaetes, the shelf environment was 
characterized by a broader range of taxonomic groups, including the occurrence 
of bivalves, ophiuroids, decapods, and ostracods as dominant members in 
addition to polychaetes. Another notable characteristic of these dominant shelf 
fauna is their relatively low densities. Average densities of the 10 most abundant 
94 
