Fishery Bulletin 113(2) 
lock is recognized (De Robertis and Wilson, 2006), al- 
though the quantifiable extent to which it occurs is not 
well understood. 
Research with acoustics has shown that fish behav- 
ioral tactics to avoid vessels and trawls, usually in the 
form of diving, are used by a variety of pelagic and 
semipelagic species in advance of an approaching ves- 
sel (Vabp et ah, 2002) and on through the time that 
a trawl arrives (Ona and Godo, 1990; Handegard et 
al., 2003; Kaartvedt et al., 2012). This diving behavior 
accounts for a considerable increase in the number of 
fish available to the trawl, an increase that can bias 
survey results (Aglen, 1996; Hjellvik et al., 2003; Han- 
degard and Tjostheim, 2009), particularly if the ver- 
tical herding is size- or age-related (De Robertis and 
Wilson, 2006). Auditory stimuli, such as that of vessel 
noise (Mitson and Knudsen, 2003), of vessel induced 
pressure waves (Mitson, 1995), and of warp vibration 
(Handegard and Tjpstheim, 2009), are attributed most 
often to fishes with diving behavior. Additionally, light 
levels at depth (Misund, 1997) and density dependence 
play a role in vertical herding for some species (Hoff- 
man et al., 2009; O’Driscoll et al., 2002). 
As shown in this study, the addition of a restrictor 
line reduces the trawl footprint; however, it also intro- 
duces uncertainty to our current knowledge of the re- 
lationship between fish behavior and trawl sampling 
efficiency. The effect of this tool on vertical herding 
and the resultant catch rates for species of the Ber- 
ing Sea is unknown. To better understand the potential 
of this effect, knowledge of the height of the restrictor 
line above the seabed is critical. Because of instrument 
failure, we were unable to collect such data, but we 
can still estimate the height of the restrictor line, as 
the product of bottom depth and 116 m (the position 
of the restrictor line forward of the doors), divided by 
warp length, assuming that the warps form a straight 
line from the trawl doors to the vessel. In actuality, the 
restrictor heights off bottom would be somewhat less 
than our predicted heights because of a narrow degree 
of natural warp catenary. 
If the above relationship and our standard survey 
scope ratios had been used, restrictor heights would 
have been 18, 44, and 38 m at our 3 depth sites, com- 
pared with a constant 30 m off bottom when the scope 
ratio was modified. Although the average headrope 
height of the 83-112 trawl is 2-3 m during standard 
survey operations, a recent study by Kotwicki et al. 
(2013) reported that the effective fishing height of this 
trawl for walleye pollock that display the diving re- 
sponse was, on average, 16 m. Although our predicted 
restrictor heights were all above the 16-m effective 
fishing height calculated by Kotwicki et al. (2013), the 
vibrations of the restrictor line, along with bringing 
the trawl warps closer together, could contribute to 
fish disturbance, and hence inconsistencies in sampling 
efficiency. 
A better understanding of the effect of a restrictor 
line on fish behavior and catch rates is required be- 
fore it can be considered an accessory to standardized 
survey trawl gear. If it were incorporated, a means for 
correcting our 30-year survey time series (before and 
after the use of a restrictor line) would also have to be 
developed. We are aware of one bottom trawl survey 
during which a restrictor line is regularly deployed: 
Norway’s Institute of Marine Research (IMR) Barents 
Sea Cod Survey. In the case of that survey, the restric- 
tor line was introduced over a 2-year period (one-third 
of tows during the first year and half of the survey 
tows during the next year). In those trials, evidence of 
increased catch rates for smaller Atlantic cod ( Gadus 
morhua) and haddock ( Melanogrammus aeglefinus), but 
not larger fishes, was observed. In additional studies at 
IMR, when a transducer was mounted to the restric- 
tor line, IMR found evidence of pelagic distributions 
of adult Atlantic cod and haddock diving to avoid the 
restrictor line (Aglen 5 ). We suggest that a large-scale 
study should be initiated to examine the changes in 
species-specific catch rates before a restrictor line is 
incorporated as a standard tool for the eastern Bering 
Sea survey. 
Our most promising towing treatment to reduce 
variability in trawl geometry involved a fixed ratio 
of depth to warp length. Given the relatively shallow 
depth range of the eastern Bering Sea survey (<200 m), 
this method proved successful. Surveys where deeper 
waters are sampled may not achieve success with a 
fixed ratio of depth to warp length because the de- 
gree of warp catenary, particularly in the warp’s lower 
half, increases at greater depths, thereby transferring 
pulling forces from upward to horizontal 6 . Too much 
horizontal pull will lead to imbalance as evidenced by 
doors collapsing and warp dragging through sediment. 
Polishing of trawl warps ahead of the doors and on the 
doors in unexpected places are clear indications of ex- 
cess warp ratios. 
Escapement 
We found that the use of a restrictor line allowed us 
to achieve a modest reduction in the effect of depth 
on the distance off the bottom of the relatively light 
weight footrope of the 83-112 trawl. The use of the re- 
strictor line, we believe, would result in a reduction in 
the variability in fish escapement under the footrope 
and a more uniform catching efficiency (Engas and 
Godp, 1989b; Walsh, 1992). That there was still an ef- 
fect of depth with the restrictor line was unexpected. 
Von Szalay and Somerton (2005) described increases in 
the distance of the footrope off bottom to be a function 
of increasing wing spread in which the footrope ten- 
sion increases and, therefore, lifts the rope from the 
bottom. If this were the case, then one would expect 
no differences in footrope height off bottom when wing 
5 Aglen, A. 2013. Personal commun. Inst. Mar. Res., 5817 
Bergen, Norway. 
6 Dickson, W. 1973. Warp length in deep water, 19 p. MIR/ 
UNDP/FAO/Polish/UNSF Highseas Fisheries Research Proj- 
ect, Gydnia, Poland. 
