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



795 



270 



90 



Figure 7 



Polar plot of mean angle and density of trawl marks for each time inter- 

 val. Polar plot in an r = /IS) format where r is the density of trawl marks 

 and theta (6) is the angle relative to = . The grand mean trawl direction 

 was 352.6" OSVf CI +/-5'). 



large waves, particularly at shallower depths. Trawl 

 marks may persist for longer duration in deeper 

 water where sedimentation rates are lower and storm 

 waves have a lessened or neghgible effect. Shelf sedi- 

 ment presently accumulates 2 to 10 times faster on 

 the shelf than on the slope ( 1-3 mm/yr on the slope; 

 6-10 mm/yr on the shelf based on -lopB activities: 

 Alexander and Simoneau, 1999; Sommerfield and 

 Nittrouer, 1999). At present rates, and assuming 

 trawl track depths of -15 cm, it would take 50-75 

 years to fill in a typical trawl mark, but evidence 

 shows that small natural gullies on the slope are 

 being draped and are infilled at present and thus 

 maintain their shape and relief even though they are 

 in the process of being buried (Field et al., 1999). 

 The sedimentation rates on the shelf are higher than 

 on the slope (the flood deposits from the 1964 Eel 

 River flood now lies -45 cm below the seafloor) 

 (Sommerfield and Nittrouer, 1999), and annual storm 

 waves resuspend sediments at depths up to 100 m. 

 Resuspension and redeposition of silt particles may 

 lead to draping of trawl gouges and thus preserve 

 their shape and relief on the seafloor. In the pres- 

 ence of bottom currents, which usually are present 

 from tides, storm waves, or boundary flows, the re- 

 suspended particles may fill in the trawl gouges and 

 thus slowly erase them over time. Since fishing ef- 

 fort in the study area was shown to be concentrated 

 in water deeper than 100 m, these effects are possi- 

 bly compounded by the longevity of trawl marks in 

 this habitat. Research is needed to address the lon- 

 gevity of trawl marks in various environments as well 

 as to determine the impacts on (and recovery of) di- 

 versity and production in benthic communities. 



3000 



1C00 - 



200 400 600 



Mean depth (m) 



800 



Figure 8 



(A) The nonlinear relationship between mean den- 

 sity of trawl marks per 10-nim interval by CDF&G re- 

 porting block depth ( m ). Mean trawl mark block depth 

 is calculated from the mean depth of 10-min intervals 

 along the sidescan track in each reporting block. (B) 

 Mean annual fishing hours per reporting block and block 

 depth (m). Mean fishing hour block depth is calculated 

 from the depths reported from logbooks in each report- 

 ing block. Error bars are standard error of the mean. 



