Weinberg and Kotwicki: Reducing variability in bottom contact and net width of a survey trawl 
187 
Figure 4 
The effect of depth on mean wing spread during the 
eastern Bering Sea field experiment conducted by the 
Alaska Fisheries Science Center in 2005 to reduce the 
variability in a survey trawl’s geometry. Data were 
fitted with a generalized linear model. Three towing 
treatments were applied: no restrictor line with stan- 
dard scope (circle, solid line), restrictor with standard 
scope (triangle, dashed line), and restrictor with modi- 
fied scope (plus sign, dotted line). 
herding process (Engas and Godp, 1989a; Somerton 
and Munro, 2001), thereby, reducing systematic bias in 
the abundance estimates. For example, bias may occur 
when a particular area produces lower sampling effi- 
ciency than other survey areas, such as may happen 
at different depths where trawl bridle angles of attack 
vary. Changing angles have an effect on fish-size-spe- 
cific reactions to the bridles and on the swimming 
stamina of a fish: the greater the angle, the greater is 
the potential that herded fish will have to swim longer 
and farther to enter the path of the net before tiring. 
As a result, false interpretations of a species’ spatial 
distribution over the whole survey area can occur be- 
cause, in reality, part of the distributional variability is 
caused by differences in the trawl sampling efficiency. 
We found that when the restrictor line was not 
used, mean bridle angles varied from 13° to 20° across 
depths. These angles were reduced to a range from 12° 
to 14° when the restrictor line was used with a stan- 
dard survey scope ratio and to a constant 12° when the 
restrictor was used with a modified scope ratio. There- 
fore, we conclude that the use of a restrictor during 
our bottom trawl survey would ensure more constant 
trawl geometry across the entire survey area, reduc- 
ing the variability in horizontal herding that may be 
associated with changing bridle angles. This deduction 
is consistent with our objective of ensuring that the 
o 
40 60 80 100 120 140 
Depth (m) 
Figure 5 
The effect of depth on mean distance of the center of 
the footrope off bottom during the eastern Bering Sea 
field experiment conducted by the Alaska Fisheries 
Science Center in 2005 to reduce the variability in a 
survey trawl’s geometry. Data were fitted with a gen- 
eralized linear model. Three towing treatments were 
applied: no restrictor line with standard scope (circle, 
solid line), restrictor line with standard scope (triangle, 
dashed line), and restrictor line with modified scope 
(plus sign, dotted line). 
changes in observed trawl catch abundance are repre- 
sentative of actual shifts in abundance. 
Contact of the lower bridle with the seabed also 
contributes to the herding of fish, particularly flat- 
fishes. Whereas our study showed that bridle contact 
decreased as a result of increasing depth and towing 
speed, the addition of the restrictor line did not af- 
fect these relationships and, therefore, likely would 
not contribute to any significant improvement in the 
variability in survey horizontal herding owing to bridle 
contact, at least given the conditions observed during 
this experiment. 
Vertical herding 
Although we did not measure net height directly, our 
survey data show that by keeping net spread constant 
we achieve a more constant net height that, therefore, 
sweeps a more constant volume of water and presum- 
ably stabilizes the effect of the net on vertical herding 
of fish near the bottom. The 83-112 bottom trawl was 
designed to catch flatfish and semipelagic species, such 
as walleye pollock ( Gadus chalcogrammus ) and Pacific 
cod ( Gadus macrocephalus), that have near-bottom dis- 
tributions. The vertical herding of benthic species with 
this trawl is perceived to be negligible. Conversely, the 
vertical herding of semipelagic species like walleye pol- 
