Weinberg et al.: Survey selectivity for Gadus macroceph 
363 
and 30.0 min) did not affect the size distribution of 
catches for some Atlantic species, including Atlantic 
cod {Gadus morhua; God0 et ah, 1990; Walsh'*), we 
elected to reduce the duration of the faster tows so that 
the distance fished and swept area of tows were similar 
between the 2 speeds (Wileman et ah, 1996). Hence, 
the duration of the slow (1.5 m/s) and fast (2.1 m/s) 
tows were set at 30.0 and 22.5 min, respectively, mea- 
sured from the time the nets were on bottom and the 
winches were locked to the time when trawl retrieval 
was initiated. 
Towing occurred at 2 independent sites, one at a 
depth of 136 m and the other at a depth of 86 m. Ten 
successful pairs of fast and slow tows were made at 
the deep site, and 14 pairs were completed success- 
fully at the shallow site. All captured Pacific cod (sex 
not determined) were measured to the nearest centi- 
meter (FL). 
Data analysis 
Swept area Swept area for each haul was estimated as 
the average net width from data collected with a Mar- 
port^ acoustic net mensuration system (Marport Stout 
Inc., Snohomish, WA), multiplied by the length of the 
tow path, derived from GPS data of vessel locations at 
first and last contact of the footrope with the seabed; 
seabed contact was determined with a bottom contact 
sensor (Somerton and Weinberg, 2001). Outlier mea- 
surements of net width were removed by using a se- 
quential outlier rejection algorithm, and the remaining 
data were fitted with a smoothed spline from which the 
average net width was calculated for each tow (Kot- 
wicki et ah, 2011). 
Measuring the swept area of each tow was compli- 
cated by instrument failure during some tows. There- 
fore, only a subset of all tows produced valid net width 
data. Paired /-tests were used to test for a difference 
in the swept area between the fast and the slow tows 
of each pair where net widths were available for both 
tows. If the difference was found not to be significant 
(P>0.05) in this subset of tows after the data from our 
bottom contact sensors were examined thoroughly for 
anomalies that would indicate the likelihood of high 
variability in net width during a tow, we assumed the 
swept area was not different for any paired tows and 
used the raw catch (counts) from all tows as the depen- 
dent variable in subsequent analyses. 
Effect of towing speed on catch The null hypothesis 
that the catch of large Pacific cod at a fast towing 
speed (Cf) was no different than the catch of large Pa- 
cific cod at a slow towing speed (Cg) was tested by using 
paired-sample tests, against the one-sided alternative 
^ Walsh, S. J. 1991. Effect of tow duration on gear selectiv- 
ity. NAFO SCR Doc. 91/84, 9 p. [Available at website.] 
® Mention of trade names or commercial companies is for iden- 
tification purposes only and does not imply endorsement by 
the National Marine Fisheries Service, NOAA. 
that Cf was greater than Cg. First, the probability of ei- 
ther towing speed being equally likely to obtain great- 
er catch was calculated with a sign test: the binomial 
probability that Cf was greater than Cg in x pairs (suc- 
cesses) out of the total y pairs of tows (trials) observed 
if the null hypothesis of no effect of speed on catch 
was true. A paired /-test was then conducted to fur- 
ther confirm the result of the less-sensitive, but more 
robust, sign test. The null hypothesis of the /-test was 
that there was no mean difference id) between In(Cf) 
and InlCg) of the paired tows (Hq: d=0, i.e. the mean 
ratio Cf/Cg = 1), assuming that the differences between 
pairs were normally distributed. The power (1-P) of 
the /-test was calculated for a 1-sided (H^: d>0) alter- 
native hypothesis on the basis of the /-distribution, ob- 
served standard deviation (SD) of In(Cf) - InlCg), sample 
size in) of 24 pairs of tows, and significance level (a) of 
0.05. The power was calculated for a range of d for 
from 0.1 to 1.0, where e'^=Cf/Cs. 
Finally, we estimated d on the basis of the length-de- 
rived, double normal, survey selectivity schedule from 
the stock assessment model for Pacific cod in the EBS 
(see appendix A in Methot and Wetzel, 2013; Thomp- 
son*). For our study, we assumed that at the fast tow- 
ing speed, no large Pacific cod can escape the net and 
all available fish are caught and that at the slow tow- 
ing speed, the large Pacific cod available can escape the 
net in the proportion indicated by the survey selectiv- 
ity function. To increase our sample size, we pooled the 
numbers of fish caught in this experiment into length 
groups with the same survey selectivity, rounded to the 
first decimal place (Table 1). The total expected catch in 
a tow based on the curve (Cg) was calculated as the sum 
of the catch in each length group in the slow tow (Cs_i) 
divided by the survey selectivity for that length group 
<si); c^ = T,ti Cg_i/si. On the basis of the assumptions, 
Cg would be the expected catch in a fast tow. Therefore, 
the mean ratio of expected catch to the catch of the 
slow tow for the n pairs of tows. 
would 
n 
be the expected mean ratio of catch in the fast tow over 
the slow tow. 
Vertical distribution Simrad ES60 echosounders were 
used in both survey vessels (Kongsberg Maritime AS, 
Kongsberg, Norway) and operated at frequencies of 38 
and 120 kHz with a sampling rate of 1-2 pings/s to col- 
lect acoustic backscatter data. The sampling resolution 
of these data was approximately 0.2 m vertically and 
0.8-2. 1 m horizontally at ship speeds of 1.5 and 2.1 m/s 
(at 3 and 4 kn). Given nominal beam widths of 7° at 
both frequencies, depth of the hull-mounted transduc- 
ers (4 m), and depth of the seafloor at the deep study 
site (136 m), the extent sampled by each ping was a 
circle with a diameter of approximately 16 m (close 
to the 18-m average net width for this depth) and an 
area of 205 m^. These data were analyzed with Echo- 
view, vers. 5.4.90 (Echoview Software Pty. Ltd., Hobart, 
Australia), which afforded us the opportunity to detect 
whether Pacific cod occurred above our net opening at 
