786 



Abstract.— The abundance and orien- 

 tation of trawl marks was quantified 

 over an extensive portion (>2700 kni^) 

 of the Eureka, CaHfornia, outer shelf 

 and slope, an important commercial 

 bottom trawling ground for such high- 

 value species as rockfish, sole, and 

 sablefish. Fishing logbook data indicate 

 that the entire reporting area was 

 trawled about one and a half times on 

 an average annual basis and that some 

 areas were trawled over three times 

 annually. High-resolution sidescan- 

 sonar images of the study area revealed 

 deep gouges on the seafloor, caused by 

 heavy steel trawl doors that act to 

 weigh down and spread open the bot- 

 tom trawls. These trawl marks are com- 

 monly oriented parallel to bathymetric 

 contours and many could be traced for 

 several kilometers. Trawl marks showed 

 a quadratic relationship in relation to 

 water depth, with the greatest number 

 of trawl marks observed at -400 m. 

 There was a significant positive corre- 

 lation between the number of trawl 

 marks observed on the sidescan images 

 and the number of annual trawl hours 

 logged within reporting areas. This 

 finding indicates that acoustic remote 

 sensing is a promising independent 

 approach to evaluate fishing effort on 

 a scale consistent with commercial fish- 

 ing activities. Bottom trawling gear is 

 known to modify seafloor habitats by 

 altering benthic habitat complexity and 

 by removing or damaging infauna and 

 sessile organisms. Identifying the ex- 

 tent of trawling in these areas may help 

 determine the effects of this type of fish- 

 ing gear on the benthos and develop 

 indices of habitat disturbance caused 

 by fishing activities. 



Sidescan-sonar mapping of benthic trawl 

 marks on the shelf and slope off 

 Eureka, California 



Alan M. Friedlander 



Pacific Fisheries Environmental Laboratory 



Southwest Fisheries Science Center 



National Marine Fisheries Service, NOAA 



1352 Lighthouse Avenue, Pacific Grove, California 93950 



Present address: The Oceanic Institute 



Makapuu Point, 41-202 Kalanianaole Highway 



Waimanalo, Hawaii 96795 

 E-mail address afnedlandergteligentmail com 



George W. Boehlert 



Pacific Fisheries Environmental Laboratory 



Southwest Fisheries Science Center 



National Marine Fisheries Service, NOAA 



1352 Lighthouse Avenue, Pacific Grove, California 93950 



Michael E. Field 



United States Geological Survey, Coastal and Marine Geology 

 345 Middlefield Road, Menio Park, California 94025 



Janet E. Mason 



Pacific Fisheries Environmental Laboratory 



Southwest Fisheries Science Center 



National Marine Fisheries Service, NOAA 



1352 Lighthouse Avenue, Pacific Grove, California 93950 



James V. Gardner 

 Peter Dartnell 



United States Geological Survey, Coastal and Marine Geology 

 345 Middlefield Road, MenIo Park, California 94025 



Manuscript accepted 7 April 1999. 

 Fish. Bull. 97:786-801 ( 1999). 



Concerns about trawl impacts on 

 benthic fish habitats date back to 

 the earliest use of this gear in the 

 13"^ and 14"^ centuries (de Groot, 

 1984), but the extent and longevity 

 of these impacts have been difficult 

 to quantify. The effects of trawling 

 depend on the size and type of bot- 

 tom trawl, footrope gear, bridles, 

 doors, scope of main wires, trawl- 

 ing speed, duration, and repetition 

 of trawling. Vulnerability of the sea 

 floor to trawling impacts depends on 

 the nature of the bottom type, 

 benthic fauna, sedimentation rates, 

 tidal velocity, and the degree of re- 

 working of sediments caused by 



storms. Seabed disturbance can oc- 

 cur over the entire distance between 

 the doors but is most pronounced in 

 the region scoured by the trawl 

 doors (Messieh et al., 1991), which 

 have been shown to plough to 

 depths greater than 15 cm (Caddy, 

 1973: Churchill, 19891. 



In the short term, disturbance 

 from bottom trawling can cause 

 resuspension of sediments and 

 make benthic infauna more avail- 

 able to scavenging predators (Kai- 

 ser and Spencer, 1994; Kaiser and 

 Ramsay, 1997; Ramsay et al., 1997; 

 Lindeboom and de Groot, 1998). 

 Long-term shifts in abundance and 



