641 
Abstract— A bottom trawl catch of flat- 
fish is composed of fish that were ini- 
tially in the path of the trawl net and 
fish that were initially in the path 
of the bridles and were subsequently 
herded into the net path. Bridle effi- 
ciency (i.e. the proportion of fish in the 
bridle path that are herded into the 
net path) for seven species of flatfish 
was estimated by fitting a model of 
the herding process to data collected 
during a field experiment. The exper- 
iment consisted of repeatedly making 
trawl hauls with three different bridle 
lengths. The model was then fitted to 
the catch of each species, by length 
class, as a function of the widths of the 
net and door spreads at the three bridle 
lengths. Bridle efficiency was indepen- 
dent of body length for five species 
(yellowfin sole, Limanda aspera\ flat- 
head sole, Hippoglossoides elassodon; 
rock sole, Lepidopsetta bilineata ; Dover 
sole, Microstomus pacificus; rex sole, 
Glyptocephalus zachirus) and ranged 
from 0.22 for rex sole to 0.40 for 
rock sole. Bridle efficiency declined 
with increasing body length for two 
species (English sole, Parophrys vetu- 
lus\ Pacific sanddab, Citharichthys sor- 
didus ), ranging from about 0.35 to 0.10 
over the lengths sampled. 
Manuscript accepted 15 March 2001. 
Fish. Bull. 99:641-652 (2001). 
Bridle efficiency of a survey trawl for flatfish 
David A. Somerton 
Peter Munro 
Alaska Fisheries Science Center 
7600 Sand Point Way NE 
Seattle, Washington 98125 
E-mail address (for D A. Somerton): david.somerton@noaa.gov 
Swept-area estimates of biomass pro- 
duced by bottom trawl surveys are typ- 
ically used as relative indices of stock 
size rather than absolute measures 
because the sampling efficiency of the 
trawls 1 is rarely known. As a means 
to estimate trawl efficiency, Dickson 
(1993a) developed a model describing 
the trawling process and the associated 
fish behavior from an undisturbed state 
before arrival of a fishing vessel until 
passage of a trawl codend. For benthic 
species that remain closer to the sea 
bottom than the head-rope height, this 
model specifies that trawl efficiency is 
primarily determined by two processes: 
escapement of fish out of the net path 
by passing either under the footrope or 
through the mesh (net efficiency) and 
herding of fish into the net path by the 
doors, mudclouds, and bridles (bridle 
efficiency). In our study, we focused on 
fish herding, considering the problem 
of quantitatively estimating bridle effi- 
ciency or the proportion of fish encoun- 
tering the bridles that are herded into 
the net path. 
Underwater observations of trawls in 
operation have revealed that the pro- 
cess of herding differs between semi- 
pelagic species, such as Atlantic cod 
(Gadus morhua) and haddock ( Melano - 
grammus aeglefinus ) that tend to swim 
near the bottom, and benthic species, 
such as flatfish that often remain in 
direct contact with the bottom (Main 
and Sangster, 1981b). Atlantic cod are 
apparently stimulated to herd by the 
sight of the trawl doors and the mud- 
clouds that form in the wake of the 
doors, whereas flatfish are stimulated 
to herd by the close proximity or ac- 
tual touch of the doors and lower bri- 
dles as they sweep along the bottom 
(Hemmings, 1969; High, 1969; Marty- 
shevskii and Korotkov, 1969; Main and 
Sangster, 1981a, 1981b). Flatfish, once 
stimulated to move, tend to swim a 
relatively short distance in a direction 
perpendicular to the bridles where they 
slow down or settle on the bottom un- 
til the bridle again overtakes them. 
This pattern is usually repeated, pro- 
ducing what appears to an observer on 
the trawl headrope as a zigzag path to- 
ward the center of the trawl footrope 
(Main and Sangster, 1981b). While this 
is occurring, however, flatfish may ei- 
ther swim over the bridle or become ex- 
hausted and be overtaken by the bridle 
and thereby escape the herding pro- 
cess. Bridle efficiency is determined by 
the relative frequency of such events. 
Perhaps because the mechanism of 
herding seems more obvious for flatfish 
than other types of fish, the earliest 
mathematical models of fish herding 
were based on the contact-swim-con- 
tact pattern of flatfish herding (Hem- 
mings, 1969; Foster et al., 1981; Fuwa 
et al,. 1988; Fuwa, 1989). These models 
were focused on individual fish and typ- 
ically considered both the physical as- 
pects of the trawl, such as shape and 
speed, as well as the biological aspects 
of the fish, such as size and endurance. 
Although these models provided a more 
structured way of examining fish herd- 
ing and allowed the quantitative eval- 
uation of the relative importance of 
the various aspects of the process, they 
were of limited use in estimating bri- 
dle efficiency of specific trawls because 
some of the required parameters were 
unknown or difficult to estimate. 
The approach to modeling fish herd- 
ing changed markedly after Engas and 
Godp (1989a) conducted trawling ex- 
1 Throughout this paper, we will refer to the 
trawl as an entire fishing gear comprising 
the net, bridles, and doors. 
