36 
Fishery Bulletin 108(1 ) 
o 
Sea whip length (multiples of 10 cm laser width) 
Figure 4 
Length frequency of sea whips ( Halipteris spp.) mea- 
sured in multiples of the 10-cm interlaser width of a 
remotely operated vehicle at four sites with different 
histories of ocean shrimp trawling at Nehalem Bank, 
Oregon (see Fig. 1). 
portion of the mud-seafloor habitat that receives some 
trawling annually for ocean shrimp, which in waters off 
Oregon can be more than 550,000 ha when ocean shrimp 
stock abundance is high (Hannah, 1995). Densities of 
many invertebrate taxa were very different at the north- 
ern and southern site pairs in this study, indicating a 
need for more spatially extensive research to understand 
the impacts of ocean shrimp trawling on habitat over a 
fishery scale (Thrush et al., 1998; Kaiser, 2003). The 
statistical significance of the interaction terms in the 
two-factor ANOVAs for sea whips and squat lobsters 
shows that the long-term fishery-scale effects of ocean 
shrimp trawling may depend on other habitat-specific 
influences on invertebrate density, in addition to fish- 
ery-related factors. More extensive sampling is needed 
to determine if other heavily trawled areas most closely 
resemble site IB, site 2B, or something else entirely. 
The changes we observed in HT sites at Nehalem 
Bank were less extreme than those attributed to 
groundfish trawling in other U.S. west coast studies 
(excluding Alaska). Engel and Kvitek (1998) studied 
HT and LT bottom trawl sites in more sandy habitats 
at slightly shallower depths in California, and found 
large reductions in most of the large epifaunal species 
or species groups at HT sites (50-90% reductions, Fig. 
4 in Engel and Kvitek, 1998). Hixon and Tissot (2007) 
studied sites of mud habitat with different trawling 
histories off southern Oregon at deeper depths than 
those of our study. They found a large reduction in 
macroinvertebrate abundance on trawled seafloors, in- 
cluding an almost complete elimination of Stylatula 
spp., as well as a major shift in the dominant inver- 
tebrate taxa. Interpretation of their study however, is 
complicated by the fact that the sites they compared 
had nonoverlapping depth ranges (their “trawled” site 
was 274-361 m deep, “untrawled” site was 183-215 m 
deep), confounding depth and trawling-related effects 
on the biota. The diversity of macroinvertebrates was 
generally lower at HT than at LT Nehalem Bank sites 
in our study; however, most taxa were not as severely 
reduced as those in the studies by Engel and Kvitek 
(1998) and Hixon and Tissot (2007). Also, some spe- 
cies or groups that are considered vulnerable to trawl 
effects, like sea whips (Wilson et al., 2002) and orange 
sea pens, were found in both HT sites at Nehalem Bank 
and were abundant in HT site IB (Table 2). Moreover, 
the invertebrate taxa found at all four Nehalem Bank 
sites were similar, except for the notable absence of sea 
cucumbers and unidentified corals at HT sites, a likely 
effect of the trawl fishery. 
Smoothing of the sea bed and related physical reduc- 
tion in habitat complexity noted in some other studies 
of bottom-fishing impacts (Auster et al., 1996; Collie et 
al., 1997; Engel and Kvitek, 1998) were not seen at the 
HT sites at Nehalem Bank. Instead, we found an in- 
creased roughness of surface topography resulting from 
more trawl tracks, similar to the findings of Tuck et al. 
(1998), along with more hagfish burrows, and increas- 
ing habitat complexity. However, our ROV video was 
mostly useful for detecting large topographic features 
like trawl tracks and hagfish burrows and therefore we 
may have missed changes in microtopography caused by 
trawling. Schwinghammer et al. (1996) and Kaiser et 
al. (2002) have argued that detecting the physical ef- 
fects of trawling on habitat is highly dependent on the 
scale of measurement. 
The moderate reductions in macroinvertebrates and 
lack of a reduction in physical habitat complexity from 
ocean shrimp trawling at Nehalem Bank could also be 
related to factors that are specific to the ocean shrimp 
fishery. The effect of trawling on seafloor habitats de- 
pends on local factors, such as gear, the spatial and 
temporal intensity of the fishery, the type of habitat, 
and the types of organisms normally found in that area 
(Jones, 1992; MacDonald et al., 1996). The semipelagic 
trawl gear used to fish for ocean shrimp has the poten- 
tial for lower impacts on habitat and macroinvertebrate 
populations than typical bottom trawl gear because the 
net itself is not in contact with the seafloor and thus 
reduces capture efficiency for demersal fish and benthic 
macroinvertebrates (Hannah and Jones, 2003). The 
intensity of U.S. west coast ocean shrimp and bottom 
