Weinberg et al.: Survey selectivity for Gadus macroceph 
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0 -I ' 1 ' 1 ' 1 ' 1 ' 1 ' 1 
0 20 40 60 80 100 120 
Length (cm) 
Figure 1 
Length-based survey selectivity curve derived from the 
stock assessment of Pacific cod (Gadus macrocephalus) 
for the Bering Sea region in 2013. Lengths are given 
in fork lengths. 
importance of availability (e.g., Do small fish occur at 
depths shallower than those of surveys?) and sampling 
efficiency (e.g., Do small fish pass through trawl mesh?) 
in determining the shape of a selectivity function is 
difficult to determine without additional information. 
The shape of the survey selectivity function is at 
issue for the model used for stock assessment of Pa- 
cific cod (Gadus macrocephalus] Thompson^’^’^) in the 
eastern Bering Sea (EBS). The assessment model, con- 
ducted with the Stock Synthesis package, vers. 3.24q 
(Methot and Wetzel, 2013), is fitted to commercial 
catch data dating back to 1977, as well as to fisheries- 
independent data from the National Marine Fisheries 
Service annual bottom trawl survey of demersal fishes 
in the EBS (hereafter referred to as the survey). The 
survey provides estimates of relative abundance and 
length compositions dating back to 1982 and age com- 
positions from 1994 onward (Lauth and Nichol, 2013). 
The current assessment model accepted by the 
NOAA National Marine Fisheries Service for fishery 
management, in addition to several historical model 
configurations, includes a flexible survey selectivity 
function that, after being fitted to the data, decreases 
at larger (>55 cm in fork length [FL]) fish sizes (Thomp- 
goni-2,3; pjg This dome-shaped functional form has 
rising and descending limbs to either side of the top. 
The descending limb on the right side suggests that 
larger fishes are less vulnerable to the survey in some 
way, perhaps because they are better able to escape the 
trawl or are separated spatially from smaller fishes. 
In contrast, the more traditional asymptotic, survey 
selectivity function implies that the survey is sampling 
a greater proportion of the large fishes in the popula- 
tion. If an assessment model is not well informed hy 
the data, there will be uncertainty about whether the 
shape of the estimated function accurately reflects the 
survey sampling processes or whether it reflects pa- 
rameter confounding in the model (Maunder and Punt, 
2013). The difference between interpretations of the 
shape of the estimated function with regard to these 
2 types of uncertainty may have a pronounced effect 
on the determination of stock size and recommended 
harvest rates. 
Field studies designed to describe survey-gear effi- 
ciency and stock availability provide a source of “di- 
rect” evidence and can be useful in the fitting of the 
selectivity function (Cadrin et al., 1999; Weinberg et 
al., 2004; Clark and Kaimmer, 2006; Nichol et al., 2007; 
Somerton et ah, 2007; Somerton et al., 2013). We pres- 
ent the results from a new study and review results 
from previous works to determine whether direct evi- 
dence from field studies corroborates the dome-shaped 
survey selectivity function estimated by the current as- 
sessment model used for Pacific cod. Although we focus 
on Pacific cod, the concept that field experiments can 
better inform assessment models is applicable world- 
wide for multiple species. 
If it is assumed that the survey covers the entire 
geographic range of Pacific cod in the EBS, a dome- 
shaped selectivity function could result from a progres- 
sive decrease in trawl sampling efficiency for larger fish 
sizes. Sampling efficiency is dictated by 3 processes: 
vertical herding, horizontal herding, and escapement, 
all of which are dependent on trawl design, fishing pro- 
cedures, fish behavior, and swimming endurance. To- 
gether, these processes play an important role in esti- 
mates of abundance and size composition of groundfish 
resources (Godp and Walsh, 1992). 
Although studies on the behavior of Pacific cod are 
scarce, evidence has been collected from various field 
and laboratory experiments on other cold-water gadids 
and various demersal species, clearly showing that fish 
swimming stamina and reactions to trawling are spe- 
cies specific (He and Wardle, 1988; Winger et al., 1999), 
size dependent (Main and Sangster, 1981; He and War- 
dle, 1988; Winger et al., 1999), temperature affected 
(He, 1991; Winger et al., 1999), light responsive (Glass 
and Wardle, 1989; Walsh, 1991), and often density de- 
pendent (God0 et al., 1999; Kotwicki et al., 2014). Not 
all studies have come to the same conclusions for all 
species, or even within the same species in all cases, 
but the most universal observation is the inverse re- 
lationship between swimming speed and endurance. 
The faster a fish swims, the more energy required and 
the less time it is capable of sustaining such speed. If, 
however, a fish is able to swim fast enough and long 
enough to outpace a survey trawl, sampling efficiency 
will be reduced. Likewise, if large Pacific cod, more so 
than smaller Pacific cod, have the strength and stami- 
na to outswim the survey trawl, survey selectivity will 
be reduced for the larger animals. 
In addition to the possibility that larger Pacific cod 
avoid capture by outswimming the trawl, it is also pos- 
sible that larger Pacific cod occur higher in the water 
column and are more likely to swim over the headrope 
of the survey trawl. The presence of fish in the wa- 
ter column can be documented by using acoustic data 
collected at the time of trawling. Analysis of acoustic 
