362 
Fishery Bulletin 114(3) 
data to estimate abundance has not been attempted for 
Pacific cod because of concerns stemming from the con- 
founding of backscatter signals close to the seabed (i.e., 
separating the weaker fish signal from the stronger 
seabed signal), in the area known as the acoustic dead- 
zone (Ona and Mitson, 1996), and from the difficulty of 
separating species-specific backscatter when multiple 
species with swim bladders, such as Pacific cod and 
walleye pollock (Gadus chalcogrammus), co-occur. 
Our objective was to report the results of an ex- 
periment aimed at examining whether survey trawl 
efficiency decreases for large-size Pacific cod because 
they outswim the trawl or because they pass over its 
headrope. If such size-specific trawl efficiency can be 
demonstrated, it would support the application of a 
dome-shaped function in the stock assessment model 
for Pacific cod. 
Materials and methods 
Experimental design 
Our experiment was designed to test the hypothesis 
that a substantial proportion of large Pacific cod avoid 
capture by outswimming the survey trawl under stan- 
dard survey protocols (Stauffer, 2004). Secondarily, we 
were also able to provide a test of the hypothesis that a 
substantial proportion of Pacific cod are unavailable to 
the trawl because they are in the water column above 
the headrope of the survey trawl. A Pacific cod was con- 
sidered large if its FL was >55 cm, a definition based 
on lengths at the right tail of the selectivity schedule 
estimated in the 2013 stock assessment of EBS Pacific 
cod (Thompson^), for which estimated survey selectivity 
was less than 100.0 percent (Table 1, Fig. 1). 
The experiment took the form of paired parallel 
tows: one vessel trawled at the survey standard speed 
of 1.5 m/s (3 kn, slow), while the other vessel towed at 
a faster speed of 2.1 m/s (4.0 kn, fast). Various Bering 
Sea fishermen of Pacific cod have reported tow speeds 
that range from 1.25 to 2.25 m/s (2. 5-4. 5 kn), depend- 
ing on vessel power, mesh size, and other trawl design 
features (senior author, personal commun.). We felt the 
upper limit for towing the survey trawl should be no 
more than 2.1 m/s in order to maintain proper fishing 
configuration (Weinberg, 2003). At such a speed, we 
were 0.15 m/s short of the fastest speeds for commer- 
cial trawling. If the number of large Pacific cod cap- 
tured in the standard slow tows is no different from 
the number caught in the faster tows, we would con- 
clude that Pacific cod did not outswim the survey trawl. 
Field operations 
The experiment was conducted during 3-5 August im- 
mediately following the 2013 NOAA EBS bottom trawl 
survey aboard the 2 trawlers used for the survey. An 
83-112 eastern trawl (standard for the EBS survey) 
was used in this experiment. The 83-112 eastern trawl 
Table 1 
Survey selectivity (rounded to one decimal place) by 
length group based on the length-based schedule of 
the 2013 assessment model used for Pacific cod (Gadus 
macrocephalus) in the eastern Bering Sea. Ranges for 
length groups are provided in fork lengths (FLs). 
Survey selectivity Length group (cm FL) 
1.0 
34-54 
0.9 
55-60 
0.8 
61-65 
0.7 
66-69 
0.6 
70-74 
0.5 
75-79 
0.4 
80-88 
0.3 
89-105 
is a 2-seam flatfish trawl with a 25.3 m (83 ft) long 
headrope and a 34.1 m (112 ft) long footrope (more de- 
tails are provided in Weinberg, 2003; Lauth and Nichol, 
2013). The simple 5.2 cm diameter footrope is weight- 
ed with 75 kg of chain hung in equal loops along its 
length from which the nylon netting is attached. Mesh 
size varies from a maximum of 10.2 cm in the wings 
and throat to a minimum of 3.2 cm for the liner in 
the codend. Each side of the net is attached to a steel 
V-door (1.8x2. 7 m) that weighs approximately 816 kg 
by a pair of 54.9-m-long, 1.6-cm-diameter bare wire 
bridles. Because faster trawling has been shown to ex- 
acerbate inconsistencies in seabed contact of this trawl 
(Weinberg, 2003), an additional 34 kg of weight was 
secured to the footrope, then monitored with a bottom 
contact sensor for all tows in this experiment. 
The major difference between tows of our experiment 
and standard survey tows was towing speed. All other 
trawling procedures followed those used during the 
survey (e.g., straight-line towing, locked winches with 
equal lengths of warp, standard warp length to depth 
ratios, and setting and retrieval methods designed to 
lower the net down on the seabed in fishing configura- 
tion quickly at the start of a tow and to raise it off the 
seabed quickly at the end of a tow). Our balanced-pair 
design called for repetitive parallel towing and vessels 
safely separated by no more than 463 m (0.25 nmi). On 
odd-numbered pairs, one vessel was randomly selected 
to tow at the standard survey speed of 1.5 m/s, while 
the other vessel towed at the faster speed of 2.1 m/s. 
On even-numbered pairs, the vessels switched towing 
speeds. To reduce potential bias from sea conditions, 
the faster boat was randomly appointed to fish either 
the port or starboard side of the slower boat. 
When fishing with 2 boats at different speeds, we 
had a choice of enforcing either consistent tow duration 
(time) or consistent tow length (distance). Because it 
has been shown that variation in tow durations (15.0 
