Fnedlander et al : Sidescan-sonar mapping of benthic trawl marks off Eureka, California 



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diversity of benthic fauna have been noted in areas 

 where trawling has been conducted for extended pe- 

 riods of time (Reise, 1982; de Groot, 1984; Sainsbury, 

 1987; Hutchings, 1990; Colhe et al., 1997; Lindeboom 

 and de Groot, 1998). Chronic, long-term disturbance 

 can delay or prevent recovery of ecological commu- 

 nities (Collie et al., 1997) and large-scale removal of 

 macrobenthos can lead to permanent changes in the 

 community (Jones, 1992; Collie, 1998; Rogers et al., 

 1998). The removal of benthic organisms and sedi- 

 mentary structures (e.g. sand waves, depressions) by 

 trawls can lead to modifications of the physical com- 

 plexity of benthic habitats (Reise, 1982; Jones 1992; 

 Auster et al., 1996; Collie, 1998; Dorsey and Peder- 

 son, 1998), Schwinghamer et al. (1996, 1998) found 

 that trawling homogenized sediments on the Grand 

 Banks of Newfoundland to a depth of at least 4.5 cm, 

 thus reducing fine-scale complexity of the substrate. 

 Sediment mixing and frequent bottom disturbance 

 from trawling activity may affect resuspension fluxes 

 and produce changes in the successional organiza- 

 tion of soft-sediment infaunal communities (Pilskaln 

 et al., 1998). Reduction or removal of benthic struc- 

 tural complexity can lead to reduced recruitment of 

 benthic organisms and often to reduced production 

 (Botsford et al., 1997). 



The degree of impact on benthic habitat is related 

 to the timing, severity, and frequency of disturbance 

 (Watling and Norse, 1998). The average annual area 

 swept by trawls on Georges Bank from 1976 to 1991 

 was between 200% and 400% of the total area (Auster 

 et al., 1996). Trawling does not occur evenly over this 

 area but is concentrated in locations that produce 

 better catches and fewer obstructions to towing 

 (Dorsey and Pederson, 1998; Auster and Langton, 

 1999). As a result, some areas are subjected to trawl- 

 ing at a much higher rate than these reported aver- 

 ages and other areas may not be trawled at all. The 

 effects of dredging on tidal flats in the Wadden Sea 

 persisted for more than 15 years (van der Veer et al., 

 1985); by contrast, experimental trawl marks in a 

 relatively dynamic intertidal zone in the Bay of 

 Fundy persisted only 2-7 months (Brylinsky et al., 

 1994). The effects of mobile fishing gear on bio- 

 diversity are most severe where natural disturbance 

 is least prevalent, such as at outer continental shelf 

 and slope habitats (Watling and Norse, 1998). An 

 experimental program to examine the impacts of 

 mobile fishing gear on the benthic ecosystems in At- 

 lantic Canada clearly indicated that trawling 

 changed the physical habitat structure on sandy bot- 

 tom at 120-146 m over a three-year period (Gordon 

 et al., 1998). Biomass of epibenthic organisms in the 

 trawl catch decreased with repeated trawling and 

 the total biomass, as sampled by epibenthic sled, was 



lower in trawled areas compared with adjacent habi- 

 tats. Fishing effort is spatially nonuniform, and thus 

 techniques are needed that will allow analysis of 

 trawling activity, and thus trawling impacts, on spa- 

 tial scales appropriate to the large scales on which 

 fisheries operate. In this study, we examine sidescan- 

 sonar records as one approach to this analysis. 



The continental shelf off Eureka (water depths less 

 than -120 m) is relatively flat (<0.5 degree) and topo- 

 graphically very smooth due to large inputs of fine 

 terrigenous sediments from the Eel and Mad Rivers 

 and reworking by storms (Fig. 1; Goff et al., 1999). 

 Bottom photographs and samples from the shelf gen- 

 erally show smooth, mud-covered bottom with small- 

 scale bottom roughness resulting from bioturbation 

 and occasional wave ripples (Wiberget al., 1996). The 

 dominant morphological feature of the shelf is the 

 Eel River delta, a sediment bulge in water depths of 

 20 to 60 m adjacent to the mouth of the Eel River 

 and elongated to the north. Slope deposits are domi- 

 nated by fine-grain sediments that show a transi- 

 tion from sandy silt in the south near the Eel Can- 

 yon to clayey silt in the northern portion of the study 

 area (Syvitski et al., 1996). 



Seafloor depressions 5 to 20 m in diameter and sev- 

 eral meters deep, termed "pock marks," are common 

 on both the outer shelf and slope throughout the re- 

 gion. These depressions are inferred to form by the re- 

 moval of particles during expulsion of gas and fluid fi"om 

 subsurface deposits (Yun et al., 1999). The excavation 

 is thought to occur both at slow rates, inducing par- 

 ticle-by-particle removal, and at rapid rates during 

 sudden release of subsurface gas by earthquake shak- 

 ing or other mechanisms (Field and Jennings, 1987). 



In addition to natural processes that modify the 

 seafloor, such as bottom currents, landslides, and 

 fluid-expulsion features, the sidescan data and bot- 

 tom photographs also show evidence (trawl marks) 

 of fishing activities that modify these benthic habi- 

 tats. The northern California outer continental shelf 

 and slope off Eureka are important commercial fish- 

 ing grounds. The trawl fleet that fishes the area off 

 Eureka presently consists of 36 vessels from Eureka 

 and 29 from Crescent City.^ These vessels range in 

 size from 18 to 24 m (60 to 80 ft), the larger vessels 

 having entered the fleet in recent years in response 

 to a shift of fishing effort into deeper water. Trawl 

 designs vary but currently the most common trawl 

 configuration is a Nor'eastern design with 114-mm 

 (4 1/2 in) diamond stretched-mesh polyethylene net- 

 ting equipped with roller gear. Trawls are currently 



Quirollo, L. F. 1998. California Department of Fish and 

 Game, 619 Second Street. Eureka, CA 95501. Personal 

 commun. 



