368 
Fishery Bulletin 1 14(3) 
(i.e., tags were deployed in the absence of vessel noise 
or oncoming trawl gear). They postulated that large Pa- 
cific cod swim above the survey-wide average height of 
the headrope (2.5 m) approximately 53% of the time 
and within 10 m of the seabed 95% of the time. Al- 
though their study was based on an interpretation 
of estimated tidal activity, their work has had a pro- 
nounced impact on the current stock assessment model, 
such that the catchability coefficient was fixed so that 
the average product of catchability and selectivity size 
range (60-81 cm FL) equaled 47% (Thompson^’^’^). 
We agree with Nichol et al. (2007), in that it seems 
unlikely for the survey trawl to catch 100% of the Pa- 
cific cod in its path 100% of the time; however, we cast 
doubt on the conclusion that more than 50% of large 
fish swim above the trawl in the presence of trawling 
activity. Nichol et al.’s study was based on a very small 
sample, and one could argue that our study similarly 
lacked broad geographical range, over areas with vary- 
ing habitat complexity, light intensity, and tempera- 
tures that (although never shown) may all have an 
effect on Pacific cod vertical distributions or perhaps 
even swimming speeds (Ferno et al., 2011). Additional 
experiments focusing on these factors would shed ad- 
ditional light on the matter. 
Survey selectivity functions in stock assessment 
models are designed to be a parsimonious representa- 
tion of the relative size dependency of the survey sam- 
pling process. However, stock assessment models can 
be quite complex, often including hundreds of param- 
eters that must be estimated when the models are fit- 
ted to data (Maunder and Punt, 2013; Methot and Wet- 
zel, 2013), and such complexity can lead to parameter 
correlation and confounding during model fitting. One 
example of this confounding is the correlation between 
survey selectivity parameters and the natural mortal- 
ity rate (Thompson, 1994), a relationship that can lead 
to ambiguity in ascribing unexpectedly low catches at 
a particular fish length to either reduced survey selec- 
tivity or to an underestimated natural mortality rate. 
We are, therefore, unable to corroborate the dome 
shape for the selectivity function of the survey of Pa- 
cific cod in the EBS by using direct evidence from this 
and other field studies in which trawl sampling effi- 
ciency has been examined. If the estimated survey se- 
lectivity function determined from the model is indeed 
correct, then the mechanisms that explain the steep de- 
scent of the right-hand tail must consist of something 
other than sampling efficiency. Four possible explana- 
tions for this steep descent of the right-hand tail are 
that 1) large fish migrate out of the survey grid, hence 
becoming unavailable to the survey; 2) sampling effort 
in preferred habitat of large fish embedded within the 
EBS survey area is not sufficient; 3) large fish prefer 
the small areas of rough, untrawlable bottom embedded 
within the EBS survey area; and 4) the relationships 
between availability and efficiency, on the one hand, 
and between catchability and selectivity, on the other, 
are complicated enough that studies of availability or 
efficiency alone are insufficient to explain catchability 
or selectivity (see Suppl. Text [Online]). If something 
is misspecified in the assessment model (e.g., perhaps 
the natural mortality rate is too low or varies with fish 
size), the selectivity of the survey for large Pacific cod 
would be closer to unity and could lead to a change 
in the harvest quotas. Therefore, further research on 
these subjects is needed to clarify the mechanisms re- 
sponsible for the selectivity of the survey. 
Acknowledgments 
Funding for this project was provided by the National 
Marine Fisheries Service, National Cooperative Re- 
search Program with Industry. We are grateful for the 
advice provided by A. De Robertis on echo-integration 
techniques and for the helpful comments from our 
reviewers D. Nichol and S. Kotwicki. In addition, we 
thank our anonymous reviewers who sacrificed their 
valuable time to contribute to the advancement of fish- 
eries science. 
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