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Fishery Bulletin 1 13(1) 
The persistence of these tracks is not without prec- 
edent (Friedlander et al., 1999), yet the ecological ef- 
fects of these scour marks, which we estimated to be up 
to 20 cm wide and 10 cm deep and to extend for many 
meters, are not known. Still, scour marks do represent 
an alteration of the seafloor. They could positively af- 
fect organisms through mobilization of key prey items 
(Shephard et al., 2009) or creation of additional habitat 
structure (Kaiser and Spencer, 1994). Conversely, the 
scour marks could negatively affect organisms depend- 
ing on the nature and extent of their association with 
the seafloor and the substrate type. 
Our expectations also were not borne out with re- 
spect to densities of sessile, structure-forming macro- 
invertebrates. Biogenic structures on the seafloor have 
been shown to be important for demersal and ben- 
thic fishes at multiple life history stages (Baillon et 
al., 2012; Auster et ah, 2003a). However, in our study, 
there were no significant differences between control 
and trawled plots with respect to densities of sessile 
macroinvertebrates, which were already at relatively 
low densities at the start of the study. Further, the 
densities of mobile invertebrates varied considerably 
over the course of our study but did not differ signifi- 
cantly between levels observed at control and trawled 
plots with either low- or high-intensity trawling effort. 
A brief investigation of infaunal organisms conducted 
as part of this study (Kitaguchi, 2011) also revealed no 
differences. 
Most of the invertebrate groups that we assessed 
had low densities but showed high spatial and tempo- 
ral variability. Polychaete worms and ophiuroids were 
especially patchy and variable in their distributions. 
Information on the dynamics of organisms in and on 
unconsolidated sediments of the outer continental 
shelf off the central California coast continues to be 
very limited, despite the fact that unconsolidated sedi- 
ments characterize more than 80% of the continental 
shelf in California (Allen et al., 2006). Indeed, the 
dominant characterization of communities on soft sedi- 
ments worldwide has been one of patchiness at mul- 
tiple scales (Morrisey et al., 1992; Oliver et al., 2011), 
where the distributions of organisms are frequently 
more diffuse than the distributions of species associ- 
ated with shallow-water reefs where habitats are more 
discrete. 
With this considerable variability as a backdrop, we 
detected no anthropogenic impact from bottom trawling 
despite the precisely georeferenced trawling effort and 
post-trawling ROV surveys. However, we expect that 
the time series data on invertebrate communities (both 
sessile and mobile) collected as part of this project will 
ultimately enhance our understanding of the ecology of 
organisms in unconsolidated sediments, including sea- 
sonal and interannual variability in the distribution of 
mobile and epibenthic invertebrates, the patchiness of 
opportunistic organisms, and interannual variability in 
invertebrate community structure. 
The results of any field research project, as well as 
the implications of those results, must ultimately be 
contextualized by a variety of factors. We planned sta- 
tistical analyses to maximize our chances to capture 
moderate-to-large effects on trawling metrics. This rel- 
atively high statistical power strongly indicates that 
moderate to large impacts to the metrics that we ana- 
lyzed would have been detected if they had occurred. 
However, the very limited impacts of bottom trawling 
to the seafloor that we observed must be considered in 
light of 2 primary factors: the use of a small-footrope 
bottom and the location of the study area in unconsoli- 
dated sand sediments. 
The small-footrope gear was used at 2 distinct trawl- 
ing intensities (3 and 8 times per trawled plot) that 
were designed to reflect the low to moderate intensity 
trawling historically seen in the region off central Cali- 
fornia (Mason et al., 2012). Yet, with the heterogeneous 
distribution of trawling effort among fishing vessels, 
and more specifically, regular focusing of that effort on 
favored locations that differ among captains, some ar- 
eas of the seafloor may be impacted more intensively 
(Auster et al., 1996; Mason et al., 2012). Additionally, 
much of the historic effort (trawling before the imple- 
mentation of the federal requirement for small-footrope 
gear along the continental shelf in 2000) in the study 
area was prosecuted with a variety of bottom trawls, 
most of them likely employing larger footrope gear and 
heavier trawl doors than those on the bottom trawls 
that we used in our study. As such, the required small- 
footrope gear may cause less impact than heavier gear, 
and therefore the extent to which the lack of impacts 
that we observed can be extrapolated to other gear 
types is potentially limited. 
Substrate type is also an important factor that must 
be considered in any extrapolation of the results of our 
study, as is the behavior of the organisms found in the 
study area. Our study was located on the outer con- 
tinental shelf in an area characterized by low-relief, 
unconsolidated sandy sediments of relatively low di- 
versity (Oliver et al., 2011). We considered the area 
to be broadly representative of the shelf to the north 
and south of the study area on the basis of prelimi- 
nary exploratory surveys completed before this study 
began and on the basis of additional research that we 
completed elsewhere along the coast. The additional 
reference sites that we sampled in May 2012 (Fig. 1) 
appeared to be similar to the study area with respect 
to the percent cover of microtopographic features (Fig. 
4A). However, the density of sessile invertebrates was 
higher at the north reference site (Fig. 4B), and the 
density of mobile invertebrates was higher at both ref- 
erence sites compared with results in the study area 
(Fig. 40. Again, the patchiness in the distribution of 
organisms on the continental shelf may explain these 
differences, and organism responses to trawling may 
also play a role. A study by Troffe et al. (2005) sug- 
gested that sea whips of the same genus observed in 
our study may have the ability to rebound after be- 
ing knocked over by a passing bottom trawl; however, 
