National Marine 
Fisheries Service 
NOAA 
Fishery Bulletin 
é established in 1881 <= 
First U.S. Commissioner 
of Fisheries and founder 
Spencer F. Baird ( 
of Fishery Bulletin al 
Abstract—Understanding fishery- 
independent survey selectivity is funda- 
mental to relating relative abundance 
indices to total population size. The 
selectivity of a survey in an assess- 
ment model represents a combination 
of gear selectivity and availability of 
fish to the gear. Concerns have been 
raised about possible bias in sampling 
of the bottom trawl survey (BTS) of 
the NOAA Northeast Fisheries Science 
Center (NEFSC) caused by the chal- 
lenges associated with towing in rough- 
bottom habitat. These difficulties may 
affect the availability of some fish, 
such as large (2100 cm in total length 
[TL]) Atlantic cod (Gadus morhua). 
To evaluate the potential presence of 
BTS habitat-related bias, we compared 
catches of Atlantic cod and white hake 
(Urophycis tenuis) from BTS sampling 
in the Gulf of Maine with catches from 
the NEFSC bottom longline survey, 
which focuses on rough-bottom habitats 
in the same region. Differences between 
survey catches were apparent for large 
white hake (£90 cm TL), supporting 
the premise of availability differences 
between surveys for white hake and 
the assumption of dome-shaped selec- 
tivity for the BTS. In contrast, results 
for Atlantic cod did not support the 
hypothesis of habitat-related bias in 
sampling of the BTS, supporting con- 
tinued use of asymptotic selectivity for 
Atlantic cod in the BTS. 
Manuscript submitted 16 November 2020. 
Manuscript accepted 22 September 2021. 
Fish. Bull. 119:231-242 (2021). 
Online publication date: 3 November 2021. 
doi: 10.7755/FB.119.4.3 
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. 
Comparison of a bottom longline survey and a 
bottom trawl survey for 2 groundfish species 
in the Gulf of Maine to evaluate habitat-related 
availability of large fish 
W. David McElroy (contact author)' 
Jessica Blaylock? 
Gary R. Shepherd (retired)? 
Christopher M. Legault? 
Paul C. Nitschke* 
Katherine A. Sosebee? 
Email address for contact author: dave.mcelroy@noaa.gov 
" Cooperative Research Branch 
Northeast Fisheries Science Center 
National Marine Fisheries Service, NOAA 
166 Water Street 
Woods Hole, Massachusetts 02543 
? Fisheries Monitoring Operations Branch 
Northeast Fisheries Science Center 
National Marine Fisheries Service, NOAA 
166 Water Street 
Woods Hole, Massachusetts 02543 
Fishery-independent scientific surveys 
can be used to produce relative abun- 
dance indices for fish stocks, as well as 
demographic and biological data, that 
are free of biases associated with com- 
mercial catches, and as such they are 
an integral part of many stock assess- 
ments (Hilborn and Walters, 1992). In 
the northeastern United States, the 
NOAA Northeast Fisheries Science 
Center (NEFSC) uses data from its 
bottom trawl survey (BTS) to inform 
assessments of finfish and marine 
invertebrate stocks. The BTS was 
designed to sample the northwestern 
Atlantic Ocean between Cape Hatteras, 
North Carolina, and Nova Scotia, 
Canada (Grosslein!; Azarovitz, 1981) 
and follows a stratified random design 
with stratification determined by depth 
and geographic region (Stauffer, 2004). 
Stock assessments by the NEFSC 
' Grosslein, M. D. 1969. Groundfish survey 
methods. Bur. Commer. Fish., Woods Hole 
Lab. Ref. Doc. 69-02, 34 p. [Available from 
website. ] 
3 Population Dynamics Branch 
Northeast Fisheries Science Center 
National Marine Fisheries Service, NOAA 
166 Water Street 
Woods Hole, Massachusetts 02543 
assume that the BTS data can be used 
to produce representative estimates of 
the characteristics of each stock, includ- 
ing trends in abundance, distribution 
across habitats, sex structure, and size 
and age compositions. 
The catch of a given species from a 
fishing event depends on a combination 
of 2 main factors: fish availability and 
gear selectivity (Crone et al.”; Maunder 
et al., 2014). Availability refers to the 
probability that the fish are in prox- 
imity to the gear and are vulnerable to 
possible capture. As such, availability 
is primarily determined by the spatial 
distribution of different sizes of fish 
relative to the distribution of survey 
sampling. Gear selectivity (hereafter 
referred to as selectivity) refers to the 
probability that a fish of a given length 
2? Crone, P., M. Maunder, J. Valero, 
J. McDaniel, and B. Semmens (eds.). 
2013. Selectivity: theory, estimation, and 
application in fishery stock assessment 
models. Workshop Ser. Rep. 1, 9 p. Cent. 
Adv. Popul. Assess. Methodol., La Jolla, 
CA.[Available from website.] 
