Lindholm et al.: Ecological effects of bottom trawling on fish habitat along the central California outer continental shelf 
83 
in the western Gulf of Maine by silver hake (Merluc- 
cius bilinearis) indicate a strong correlation between 
individual fish size and sand-wave period; smaller 
fishes use smaller sand waves (Auster et al., 2003a). 
Acadian redfish ( Sebastes fasciatus ) have been seen to 
have ontogenetic shifts from physical attributes (piled 
boulder reefs) of the seafloor at early life history stag- 
es to structure-forming invertebrates (erect Cerianthid 
anemones) found in adjacent soft sediment habitats at 
late-juvenile stages (Auster et ah, 2003b). Sea pens (of 
the order Pennatulacea) have been shown to harbor 
fish larvae, potentially playing an important role in the 
early life history of some redfishes ( Sebastes spp.) (Bail- 
Ion et al., 2012). Sea whips have also been observed to 
harbor dense aggregations of Pacific ocean perch (Se- 
bastes alutus ) in the Bering Sea (Brodeur, 2001). In 
California, Hallenbeck et al. (2012), using a remotely 
operated vehicle (ROV), found higher densities of small 
fishes and invertebrate fauna inside naturally occur- 
ring rippled scour depressions than densities in low-re- 
lief habitats outside these depressions. These features 
are now known to occur across the continental shelf 
of California in sandy substrates (Davis et al., 2013). 
Further, structural attributes of the seafloor have been 
shown to enhance survival of postsettlement demersal 
fishes, both in laboratory experiments (Lindholm et al., 
1999) and in field studies (Tupper and Boutilier, 1995). 
Literature on the ecological effects of bottom trawl- 
ing has grown over the past twenty years. There is 
some evidence that low-relief sandy environments re- 
cover more quickly after cessation of trawling than 
higher-relief hard substrates and the fauna associated 
them (NRC, 2002; Barnes and Thomas, 2005). The dy- 
namics of recovery from trawling within soft sediments 
are less clear. Studies in which bottom grabs were used 
to sample organisms in unconsolidated sediments have 
revealed a measurable impact from trawling on an in- 
faunal community in the North Sea from a single pass 
of a beam trawl, even in an environment that had 
been trawled heavily for decades (Reiss et al., 2009), 
but in a study in South Africa, no measurable impacts 
of additional trawling to the epifaunal community in a 
continuously trawled area (Atkinson et al., 2011). On 
Georges Bank, Lindholm et al. (2004) observed trawl- 
ing impacts to sand habitats below the 60-m isobath 
in imagery collected with a video drift camera, but no 
impact was evident at depths shallower than 60 m in 
sand habitats, where grain sizes were similar to those 
in the deeper habitats where regular storm and tidal 
currents re-sorted the sediment. Important questions 
remain, particularly with respect to the effects of bot- 
tom trawling on the structural attributes of seafloor 
habitat in unconsolidated sediments. 
Insight into the ecological effects of bottom trawl- 
ing in unconsolidated sandy sediments is particularly 
important for California, where more than 80% of the 
continental shelf comprises sand (Allen et ah, 2006) 
and bottom trawling has been an important component 
of the groundfish fishery. However, a limitation com- 
mon to the few existing studies of impacts of trawling 
along the West Coast of the United States (Engel and 
Kvitek, 1998; Freese et al., 1999; McConnaughey et al., 
2000; Hixon and Tissot, 2007; de Marignac et al., 2009) 
was the fact that trawling effort was not controlled as 
part of these studies. These studies, although instruc- 
tive, have been either 1) snapshots based on data col- 
lected after trawling, 2) studies with little knowledge of 
the intensity of trawling effort in the area studied, or 
3) both. Available historical data on the distribution of 
trawling rarely exist, especially with the level of preci- 
sion in georeferenced track lines that are critical for 
the accurate quantification of impacts to the seafloor; 
most available historical data occur as averaged esti- 
mates of trawling intensity across large areas (Bellman 
et al., 2005; Mason et al., 2012). 
California has a long history of bottom trawling, and 
many ports have relied on landings of trawl-caught 
groundfishes. However, because of various regulatory 
and socioeconomic factors, trawling effort has shifted 
spatially and has diminished in recent years (Bellman 
et. al., 2005; Mason et. al., 2012). Regulations on the 
design of bottom-trawl gear have resulted in much of 
the effort shifting away from rocky habitats (Bellman 
et al., 2005). One regulation in particular restricted the 
diameter of trawl footrope gear that was allowed for 
use along the continental shelf. This regulation, enact- 
ed by the Pacific Fishery Management Council in 2000, 
required trawl vessels to use small-footrope gear ( <20 
cm in diameter) along the continental shelf, therefore, 
inhibiting the use of large-footrope gear that can pass 
over rocky terrain (Dalton, 1999). The level of trawl- 
ing effort (number of active trawl permits) has also 
been reduced over the last decade because of a federal 
trawl buy back, and private purchase of trawl permits 
in the Central Coast (Gleason et al., 2009). The trawl 
fishery recently transitioned to an individual transfer- 
able quota (ITQ) system, and there is increasing use of 
nontrawl fixed gear that produces consistently higher 
prices for groundfishes and is intended to reduce by- 
catch of overfished species. However, trawling is still 
the primary way to catch flatfish and remains an im- 
portant component of California fisheries (Hilborn et 
al., 2012). 
The type and intensity of impact of bottom trawling 
on the seafloor and associated ecological communities 
are significant for effective management of trawling 
activities. As of this writing, state and federal man- 
agement agencies have implemented closures of broad 
areas; these closures have limited the current trawl- 
ing but also have constrained commercial fisheries and 
reduced landings and local seafood supply (Hilborn et 
al., 2012). A ban on trawling in California state wa- 
ters, with the exception of some halibut trawl grounds, 
was implemented through state legislation that was 
enacted in 2004 (Calif. State Legislature, 2004). The 
Rockfish Conservation Area (RCA), established from 
2002 to 2007 and extending from northern Washington 
to southern California, excludes trawling to help re- 
