360 
NOAA 
National Marine 
Fisheries Service 
Fishery Bulletin 
fb- established 1881 
Spencer F. Baird 
First U.S. Commissioner 
of Fisheries and founder 
of Fishery Bulletin 
Is the sunrey selectivity curve for Pacific cod 
iGadus macrocephalus} dome-shaped? 
Direct evidence from trawl studies 
Email address for contact author: cynthia.yeung@noaa.gov 
’ Resource Assessment and Conservation Engineering Division 
Alaska Fisheries Science Center 
National Marine Fisheries Service, NOAA 
7600 Sand Point Way NE 
Seattle, Washington 98115 
2 Resource Ecology and Fisheries Management Division 
Alaska Fisheries Science Center 
National Marine Fisheries Service, NOAA 
7600 Sand Point Way NE 
Seattle, Washington 98115 
Abstract — Survey selectivity can be 
viewed as a function of the availabil- 
ity of the stock to the sampling gear 
and the sampling efficiency of the 
gear. A dome-shaped survey selectiv- 
ity function is one in which survey 
selectivity decreases with larger and 
older fish. Such a function is esti- 
mated for eastern Bering Sea (EBS) 
Pacific cod (Gadus macrocephalus) in 
the NOAA National Marine Fisher- 
ies Service stock assessment model, 
which would be appropriate if large 
(>55 cm in fork length) Pacific cod 
avoid capture by the EBS survey 
bottom trawl. To test this assump- 
tion, a field study was conducted to 
determine whether large Pacific cod 
escape capture by either outswim- 
ming the survey trawl or by swim- 
ming above the trawl. Our results 
show that large Pacific cod do not 
outswim the trawl because catches 
did not increase when we increased 
towing speed. Additionally, large Pa- 
cific cod do not routinely swim above 
the trawl because analysis of acous- 
tic backscatter collected concurrent- 
ly with trawl hauls indicated that 
only 4% of the acoustic backscatter 
attributed to Pacific cod occurred 
at heights above the headrope. We 
found no evidence that survey-gear 
efficiency decreased with increasing 
fish length either because large fish 
outswam the trawl or because they 
tend to occur further from the bot- 
tom. Therefore the results of our 
experiment do not support the use 
of a dome-shaped survey selectivity 
function in the EBS Pacific cod as- 
sessment model. 
Manuscript submitted 16 June 2015. 
Manuscript accepted 20 May 2016. 
Fish. Bull. 114:360-369 (2016). 
Online publication date: 14 June 2016. 
doi: 10.7755/FB.114.3.8. 
The views and opinions expressed or 
implied in this article are those of the 
author (or authors) and do not necessarily 
reflect the position of the National 
Marine Fisheries Service, NOAA. 
Kenneth L. Weinberg’ 
Cynthia Yeung (contact author)’ 
David A. Somerton’ 
Grant G. Thompson^ 
Patrick H. Ressler’ 
Fisheries stock assessment surveys 
are intended to produce an index of 
relative stock abundance that varies 
over time in constant proportion to 
the true stock abundance. In stock 
assessment models, the scaler that 
relates modeled abundance to a sur- 
vey index is often considered a prod- 
uct of a constant catchability and of 
a fish age- or length-dependent sur- 
vey selectivity function (which, here- 
after, for reasons of simplicity, we re- 
fer to as length-dependent functions, 
but the same concept applies to age- 
dependent functions). Both catch- 
ability and selectivity are typically 
estimated when a stock assessment 
model is fitted to data (Maunder and 
Finer, 2015), although, in some cases, 
the catchability coefficient is fixed a 
priori (Thompson’’^’^). The selectiv- 
’ Thompson, G. G. 2013. Assessment of 
the Pacific cod stock in the eastern Ber- 
ing Sea. In Stock assessment and fish- 
ery evaluation report for the groundfish 
ity of a survey can be viewed as a 
function of the availability of the 
various biological components of the 
fish stock to the sampling gear and 
of the sampling efficiency of the gear 
(i.e., the proportion of encountered 
animals that are captured; Maunder 
et ah, 2014). However, the relative 
resources of the Bering Sea/Aleutian 
Islands regions, p. 239-380. North Pa- 
cific Fishery Management Council, An- 
chorage, AK. [Available at website.] 
2 Thompson, G. G. 2014. Assessment of 
the Pacific cod stock in the eastern Ber- 
ing Sea. In Stock assessment and fish- 
ery evaluation report for the groundfish 
resources of the Bering Sea/Aleutian 
Islands regions, p. 255-436. North Pa- 
cific Fishery Management Council, An- 
chorage, AK. [Available at website.) 
^ Thompson, G. G. 2015. Assessment of 
the Pacific cod stock in the eastern Ber- 
ing Sea. In Stock assessment and fish- 
ery evaluation report for the groundfish 
resources of the Bering Sea/Aleutian 
Islands regions, p. 251-470. North Pa- 
cific Fishery Management Council, An- 
chorage, AK. [Available at website.) 
