790 



Fishery Bulletin 97(4), 1999 



1 24''20' 



124°10' 



124° 



41°10' 



40°50' 



40°40' 



40°40' 



40°30' 



40°30' 



124°30' 



124°20' 

 5 



124° 10' 

 10 nautical miles 



10 15 20 l^ilometers 



Figure 2 



Track lines for high-resolution sidescan-sonar surveys. The 

 reporting blocks from California Department of Fish and 

 Game that are used for logbook data entry are superim- 

 posed on the sur\'ey area. Blocks are 10 minutes of longi- 

 tude and 10 minutes of latitude on a side with block num- 

 ber in the center. 



intervals from 10 different sidescan track lines 

 (Fig. 4). From these results, it was determined that 

 two random perpendicular lines were needed to prop- 

 erly sample each 10-min interval and minimize 

 double-counting of trawl marks. Using a sample of 

 50 randomly selected 10-min time intervals from 10 

 sidescan track lines, we counted the actual number 

 of trawl marks and compared this number with the 

 mean number counted with the perpendicular line 

 intercept method described above. The mean propor- 

 tion of trawl marks counted with this sampling 



method to actual number of marks was 0.84 

 (SD=0.11). Trawl mark density in kni'^ was 

 calculated from each time interval and incorpo- 

 rated into a Geographical Information System 

 (GIS) for analysis and contouring. 



The angle of each trawl mark was measured 

 in relation to the two perpendicular lines used 

 to quantify trawl marks in all 10-min inter- 

 vals. These angles were adjusted for the course 

 of the sidescan track and computed in rela- 

 tion to magnetic north. Mean angles for each 

 time interval were then calculated and plot- 

 ted on a polar plot in an r = /! 6) format, where 

 r is the number of trawl marks and theta (6) 

 is the angle relative to 0°. The grand mean 

 angle and 95% CI were computed with the 

 mean values from each time interval (Zar, 

 1984). 



Fishing effort 



Trawlers provide information on catch and 

 effort of individual trawl hauls to the Califor- 

 nia Department of Fish and Game (CDF&G). 

 Hauls are spatially recorded in blocks that are 

 10 minutes of latitude by 10 minutes of longi- 

 tude (Fig. 2). Blocks within the Eureka 

 STRATAFORM survey area were extracted 

 from the NMFS Tiburon Laboratory ground- 

 fish database and used in these analyses. 

 Trawling is prohibited by state law within 5.6 

 km (3 nm) of the coast. All reporting blocks 

 adjacent to the coast were truncated for all 

 estimates and comparisons. 



Pacific whiting form extensive midwater ag- 

 gregations during the day, and the fishery is 

 conducted almost exclusively with midwater 

 trawls (Dorn and Saunders, 1997). Because 

 fishermen did not report whether their trawl 

 hauls were bottom trawls or midwater trawls, 

 all trawl hauls with catches of Pacific whiting 

 greater than 454 kg ( -1000 lb) were eliminated 

 from these analyses. Mean annual fishing ef- 

 fort (trawl hours) was calculated for each re- 

 porting block within the Eureka area for the years 

 1990 to 1994. Depths from the logbook data were used 

 to obtain mean fishing depths for each reporting block 

 for comparisons with fishing effort. 



The reporting block grid was incorporated into a 

 GIS and the mean density of trawl marks was calcu- 

 lated for each block (Table 1). Water depths from a 

 high-resolution multibeam mapping sui"vey (Goff et 

 al., 1999) were binned to obtain mean bottom depth 

 in each block for comparison with mean number of 

 trawl marks. Effort, expressed as mean annual trawl 



