Somerton and Munro: Bridle efficiency of a survey trawl for flatfish 
643 
ously and continuously with an acoustic trawl mensura- 
tion system. Tow length was measured as the straight line 
distance between the GPS positions of first and last foot- 
rope contact with the bottom determined with a time-depth 
recorder attached to the headrope. Bottom temperature 
was recorded at two second intervals with a bathyther- 
mograph and averaged 3.0°C during the eastern Bering 
sea experiment and 7.0°C in the West Coast experiment. 
Bottom depth averaged 76 m during the eastern Bering sea 
experiment and 67 m in the West Coast experiment. The 
catch from each haul was first sorted to species; then the 
catch of each species was weighed in the aggregate and fish 
of each species were individually measured for total length 
in centimeters. Both experiments were conducted during 
daylight hours to best approximate the sampling protocol 
used on most AFSC bottom trawl surveys. 
Estimating W off 
The herding experiments provided data on N, L, W d , and 
W n , but obtaining data to estimate W oft required a differ- 
ent type of experiment. We obtained the necessary data by 
directly observing the contact of the lower bridle with the 
bottom on an experimental cruise conducted 9-12 Septem- 
