Somerton and Munro: Bridle efficiency of a survey trawl for flatfish 
647 
Table 2 
Relationship of mean fish length to increasing bridle 
length. The total number of fish measured, the slope of the 
regression line fitted to mean length at each tow versus 
bridle length, and the probability that the slope equaled 
zero is shown for each species. Only English sole and 
Pacific sanddab are significant. Note that a negative slope 
indicates that smaller fish are caught with longer bridles. 
Species 
Number of 
fish measured 
Slope 
P(slope=0) 
Rock sole 
23,586 
-.075 
0.23 
Yellowfin sole 
7231 
0.008 
0.83 
Flathead sole 
2927 
-0.311 
0.31 
Rex sole 
22,774 
-0.052 
0.56 
Dover sole 
12,507 
-0.036 
0.75 
English sole 
21,968 
-0.277 
<0.01 
Pacific sanddab 
34,209 
-0.383 
<0.01 
Estimating W off 
The degree of contact of the lower bridles with the bottom 
varied with increasing distance from the doors. Contact 
never occurred in the first 28 m (Fig. 4). At greater dis- 
tances, contact became intermittent and was focused pri- 
marily at the tops of sand ripples. Contact reached 50% 
at 36 m and nearly 100% at 40 m (Fig. 4). The estimated 
value for L off was 36.0 m (95% confidence intervals; ±1.4 
m). This value was assumed to be constant for all three 
bridle lengths and both vessels. 
Fit of the model 
Of the seven species examined for length-dependent herd- 
ing, only Pacific sanddab and English sole had significant 
slopes in the regressions of k b on fish length (Table 3, Fig. 
5). In both cases the slope was negative indicating that 
herding efficiency decreased with increasing fish length. 
For the five species displaying no significant length-de- 
pendent herding, the estimated herding efficiency in the 
bridle contact path (h) varied from 0.50 for rex sole to 
0.84 for rock sole, whereas the herding efficiency in the 
entire bridle path (k h ) varied from 0.22 to 0.40 (Table 4). 
For the two species showing significant length-dependent 
herding, English sole and Pacific sanddab, k b varied from 
about 0.35 at the shortest lengths to about 0.08-0.10 at 
the longest lengths (Fig. 5). 
Test of model assumptions 
For six of the seven species, the fit of a modified herding 
model, with h parameterized as a linear function of bridle 
length, produced a larger penalized sum of squares than 
the fit of the original model with a constant h . This find- 
ing indicates that the fit was not improved by the specifi- 
cation of a linear variation in h, and that the assumption 
Table 3 
Regression of k b on the midpoints of fish-length classes. 
The number of length classes, the slope of the regression 
line, and the probability that the slope equals zero are 
shown for each species. Only English sole and Pacific 
sanddab have significant (P<0.05) slopes. 
Species 
Number of 
size classes 
Slope 
P(slope=0) 
Rock sole 
10 
-0.006 
0.088 
Yellowfin sole 
10 
0.001 
0.767 
Flathead sole 
10 
-0.004 
0.249 
Rex sole 
10 
-0.004 
0.354 
Dover sole 
9 
-0.003 
0.564 
English sole 
8 
-0.027 
0.006 
Pacific sanddab 
10 
-0.016 
0.008 
of constancy of h at all bridle lengths is valid. However, for 
Pacific sanddab the fit was significantly improved by the 
modification. Because the sign of the slope parameter of 
the linear change was negative, it indicates that for this 
species herding was less effective at longer bridle lengths. 
Discussion 
Bridle efficiency 
Herding by the bridles of the 83-112 Eastern trawl sub- 
stantially contributes to the catch of flatfish. For rock sole, 
as an example, approximately 49% of the catch comprised 
fish that were herded into the net path. 3 This contribution 
is ignored when swept-area estimates of abundance are 
based on wing spread rather than door spread. Although 
such an assumption could lead to an overestimate of abun- 
dance, herding is only one component of the trawl catching 
process and the gains from herding could be offset by the 
losses due to net escapement. 
Bridle efficiency did not vary with fish length for five 
of the seven species examined (Table 3); consequently, for 
these species, herding by the bridles is not length selec- 
tive. A similar lack of length selection was observed by 
Bridger (1969) in a study examining the herding of lemon 
sole ( Pseudopleuronectes americanus). For English sole 
and Pacific sanddab, however, bridle efficiency clearly de- 
clined with increasing fish length and therefore herding is 
length selective. 
Such length selectivity should lead to a change in mean 
fish length when the average duration of herding experi- 
enced by an individual fish is increased by an increase in 
bridle length. Without exception, the changes in mean fish 
length that we observed with increasing bridle length are 
3 From Equation 4, it can be shown that the herded proportion in 
the catch is equal to hw on Kw n + hw on ), where w on = w d - w n —w 0 ^. 
