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Fishery Bulletin 99(4) 
periments to quantitatively examine the effects of varia- 
tions in bridle length on catch for each area and fish size. 
Reasoning that the changes in catch that accompanied the 
changes in bridle length provided information on the ef- 
ficiency of herding, Dickson (1993b) estimated the bridle 
efficiency coefficients for Atlantic cod and haddock by fit- 
ting a simple linear model of the trawl herding process to 
data from Engas Godp’s (1989a) experiments. Ramm and 
Xiao (1995) conducted a similar trawl herding experiment 
and developed a new type of herding model appropriate 
for cases in which escapement at the center of the trawl is 
zero. 
In our study, we extended Dickson’s (1993a) herding 
model so that it is more specific to the peculiarities of flat- 
fish herding and additionally extended Dickson’s (1993b) 
approach to fitting the model to data by providing a more 
rigorous statistical foundation. The model was then ap- 
plied to herding data for the 83-112 Eastern bottom trawl 
(Armistead and Nichol, 1993) used by the Alaska Fisher- 
ies Science Center (AFSC) to conduct its annual bottom 
trawl survey of the Eastern Bering Sea. These data were 
collected during two herding experiments, patterned af- 
ter those of Engas and Godo (1989a), in which emphasis 
was placed on seven species of flatfish: yellowfin sole ( Li - 
manda aspera ), flathead sole ( Hippoglossoides elassodon), 
rock sole ( Lepidopsetta bilineata ), English sole ( Paroph - 
rys vetulus ), Dover sole ( Microstomus pad ficus), rex sole 
( Glyptocephalus zachirus), and Pacific sanddab ( Cithar - 
ichthys sordidus). 
Materials and methods 
N n = DLW n 
N b = DL(W d -W n ), 
where L = tow length; 
W n = the width of the net path; and 
W d = the width of the door path (Fig. 1). 
( 2 ) 
Combining all terms, the total catch then can be expressed 
as 
N = k n DLW n + k n k b DL (W d -W n ). ( 3 ) 
At this point the model is quite similar to the one proposed 
by Dickson (1993a) to describe the trawl catching process 
for Atlantic cod and haddock. We now modify the model to 
make it more specific to flatfish by considering that herd- 
ing is restricted to the portion of the bridle path in which 
the lower bridle is in contact with the bottom. This can be 
expressed as 
N = k n DLW n + k n hDL ( W d - W n - W off ), ( 4 ) 
where W 0 „ = the width of the path in which the bridle is 
not in contact with the bottom; and 
h - the herding coefficient or the proportion of 
fish in the bridle contact path that is herded 
into the net path. 
Thus, k b is the average efficiency over the entire bridle 
path and li is the average efficiency in the portion of the 
bridle path where herding actually occurs. 
Development of the herding model 
Consider that the area of the bottom swept by a trawl con- 
sists of two components, the area between the wingtips 
of the net (net path), and the area between the wing tips 
and the doors (bridle path; Fig. 1). The catch (AO obtained 
in a trawl can be represented as some proportion, k n , of 
the number of fish in the net path ( N n ) plus some propor- 
tion, P, of the number of fish in the bridle path (N b ). If the 
probability of capt ure for a fish herded into the net path is 
identical to that of a fish initially in the net path, then P 
can be considered equal to k n k b , where k h is the bridle effi- 
ciency 2 or the proportion of fish within the bridle path that 
is herded into the net path. Algebraically this is expressed 
as 
N = k n N n + k n k b N b . (1) 
The numbers of fish within the net and bridle paths are 
equal to the fish density (D) multiplied by the path areas, 
that is, 
2 In Dickson (1993a) this same quantity is referred to as sweep 
efficiency (k s ). We use the term “bridle efficiency” because the 
83-112 Eastern trawl does not have sweeps connecting the bri- 
dles to the doors as does the trawl considered by Dickson. 
Description of the herding experiments 
The strategy that we used in our herding experiments 
was to repeatedly trawl in such a way that the width of 
the net path was held approximately constant, whereas 
the width of the door path was varied among three dis- 
tances by changing the length of the bridles. The first of 
two herding experiments was conducted 25 July-2 August 
1994 aboard the FV Arcturus in the eastern Bering Sea 
and focused on yellowfin sole, flathead sole, and rock sole. 
The second experiment was conducted 14-25 September 
1994 aboard the RV Alaska off the coast of Washington 
State and focused on English sole, Dover sole, rex sole, and 
Pacific sanddab. For both experiments, we used a blocked 
sampling design to minimize the effects on catch of the 
spatial variation in fish density. In each geographic block, 
three nearby, but nonoverlapping, 30-min trawl hauls at 
a speed of 1.5 m/s were made with each of three bridle 
lengths chosen in random order. Bridles measured 27 m, 
55 m (the standard length used on AFSC surveys), and 
82 m in length and were constructed of 16-mm diameter 
steel cable. Tailchains connecting the doors to the bridles 
(Fig. 1) were always constructed of 13-mm diameter long 
link chain but differed in length between vessels (Table 1). 
Trawl doors were always a “V” style measuring 1.8 m x 
2.7 m and weighing 910 kg. On all hauls, spreads of the 
doors were measured at the tops of the wings simultane- 
