794 



Fishery Bulletin 97(4). 1999 



sive bioturbation off Eureka (Syvitski et al., 1996). 

 Released gases and fluids from cold seeps are rich in 

 methane and sulfide and support unique productive 

 cold-seep biological communities (Syvitski et al., 

 1996). The magnitude of trawling activity in this area 

 has the potential to modify these communities by 

 direct physical damage or by smothering from resus- 

 pended sedimention. 



There has been increased interest in recent years 

 on the impacts of fisheries on the marine environ- 

 ment (Dayton et al., 1995; Boehlert, 1996; Lindeboom 



124°30' 



124°20' 



124° 10' 



124° 



40°50' 



40°40 



40°40' 



40»30' 



40°30' 

 124° 



10 



IS 20 kilometers 



Figure 6 



Vector plot .showing orientation and magnitudi' of trawl 

 marks along sidescan track lines. Length of arrow is pro- 

 portional to the density of trawl marks in each 10-min 

 interval, and direction is the mean angle of all trawl marks 

 in each time interval. Note that angles are plotted in re- 

 lation to north so that trawls towed north or .south resulted 

 in the same orientation. 



and de Groot, 1998; Watling and Norse, 1998), and 

 an important component is the relationship between 

 exploited fish species and their associated habitats 

 (Schmitten, 1996). With a greater understanding of 

 these associations, concerns have been raised about 

 the adverse impacts that mobile fishing gear may 

 have on the entire ecosystem. The severity and du- 

 ration of disturbance are both factors that affect eco- 

 logical communities (Gislason, 1994; Connell et al., 

 1997; Tuck et al, 1998; Auster and Langton, 1999). 

 Mobile fishing gear, such as trawls, is the most wide- 

 spread form of direct disturbance to marine 

 systems below depths affected by storms 

 (Watling and Norse, 1998). Natural distur- 

 bances to the benthic community off Eureka 

 probably occur from high-energy wave events, 

 floods, and even tectonic activity. The first two 

 processes likely have a greater impact in the 

 shallower water on the shelf, and the latter pro- 

 cess is likely too infrequent to play a major role 

 in impacting the benthos. Thus, in the habitat 

 covered by this study, trawls represent the 

 greatest form of direct disturbance to the 

 benthic habitat — a finding that is consistent 

 with the conclusions of Watling and Norse 

 (1998) for depths below the influence of storms. 

 The extent and location of fishing activity, noted 

 in this study from logbook data and sidescan- 

 sonar records, can potentially have a large im- 

 pact on the local ecological communities, par- 

 ticularly on the slope where the frequency and 

 magnitude of natural disturbance is less than 

 that on the shelf 



Examination of trawl marks was not the pri- 

 mary goal of the sidescan-sonar survey con- 

 ducted off Eureka (Syvitski et al., 1996). Con- 

 sequently, there are aspects of the survey 

 method that could have been conducted differ- 

 ently to improve the quality of these data. Be- 

 cause fishing activities in this area are gener- 

 ally conducted along the north-south bathymet- 

 ric contours, sidescan-sonar tracks oriented 

 east-west tended to resolve slightly fewer trawl 

 marks. Any future study of trawl marks using 

 sidescan sonar should concentrate tracks to ori- 

 entations subparalleling trawling directions in 

 order to maximize the resolution of the survey. 

 There are biases associated with this method, 

 particularly in a dynamic physical location like 

 the northern California continental margin. 

 Estimates of the actual number of trawl marks 

 are minimal because of variability in resolution 

 of the sidescan records, obliteration of older 

 trawl marks by more recent trawling, burial by 

 sedimentation or reworking of the seafloor by 



