149 
National Marine 
Fisheries Service 
NOAA 
Fishery Bulletin 
é established in 1881 <= 
Spencer F. Baird 
First U.S. Commissioner 
of Fisheries and founder 
of Fishery Bulletin 
Abstract—The National Marine Fish- 
eries Service conducts fishery stock 
assessments to provide the best scien- 
tific information available for the U.S. 
regional fishery management councils. 
The assessment models applied in the 
United States are often region specific, 
although the models share similar math- 
ematical and statistical attributes. How- 
ever, comprehensive comparison studies 
identifying similarities and differences 
among these assessment models remain 
scarce. We developed a multi-model 
comparison framework to evaluate the 
reliability of 4 age-structured assess- 
ment models that are commonly used 
in the United States: the Assessment 
Model for Alaska, the Age Structured 
Assessment Program, the Beaufort 
Assessment Model, and Stock Synthe- 
sis. When applied to simulated data, all 
4 models produced reliable estimates of 
assessment quantities of interest, such 
as fishing mortality, spawning biomass, 
recruitment, and biological reference 
points. Although there were differences 
among models in the calculation of the 
initial population numbers at age and 
in the bias adjustment of recruitment, 
their effects on model outputs were 
minor when estimation models were 
configured similarly. In addition, we pro- 
vide guidelines for converting unfished 
recruitment and steepness between 
2 methods of bias adjustment. We rec- 
ommend that next-generation stock 
assessment models include recruitment 
bias adjustment and that more research 
be conducted to provide guidelines for 
which methods might be preferred 
under which situations. 
Manuscript submitted 29 October 2020. 
Manuscript accepted 12 July 2021. 
Fish. Bull. 119:149-167 (2021). 
Online publication date: 30 August 2021. 
doi: 10.7755/FB.119.2-3.5 
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. 
A comparison of 4 primary age-structured stock 
assessment models used in the United States 
Bai Li (contact author)! 
Kyle W. Shertzer” 
Patrick D. Lynch? 
James N. lanelli* 
Christopher M. Legault® 
Erik H. Williams” 
Richard D. Methot Jr.° 
Elizabeth N. Brooks? 
Email address for contact author: bai.li@noaa.gov 
' National Research Council 
1315 East-West Highway, Building SSMC3 
Silver Spring, Maryland 20910 
? Southeast Fisheries Science Center 
National Marine Fisheries Service, NOAA 
101 Pivers Island Road 
Beaufort, North Carolina 28516-9722 
3 Office of Science and Technology 
National Marine Fisheries Service, NOAA 
1315 East-West Highway, Building SSMC3 
Silver Spring, Maryland 20910-3282 
4 Alaska Fisheries Science Center 
National Marine Fisheries Service, NOAA 
7600 Sand Point Way NE, Building 4 
Seattle, Washington 98115-6349 
> Northeast Fisheries Science Center 
National Marine Fisheries Service, NOAA 
166 Water Street 
Woods Hole, Massachusetts 02543 
© Northwest Fisheries Science Center 
National Marine Fisheries Service, NOAA 
2725 Montlake Boulevard East 
Seattle, Washington 98112 
Fishery stock assessment models have 
been widely used by scientific and 
management communities to evaluate 
fish population dynamics and provide 
estimates of stock abundance and fish- 
ing mortality rate (F) (Hilborn and 
Walter, 1992; Quinn and Deriso, 1999; 
Maunder and Punt, 2013; Lynch et al., 
2018). Over time, assessment models 
have become more comprehensive and 
Jonathan J. Deroba® 
Aaron M. Berger” 
Skyler R. Sagarese® 
Jon K. T. Brodziak? 
lan G. Taylor® 
Melissa A. Karp'° 
Chantel R. Wetzel® 
Matthew Supernaw"' 
7 Northwest Fisheries Science Center 
National Marine Fisheries Service, NOAA 
2032 SE OSU Drive 
Newport, Oregon 97365-5275 
8 Southeast Fisheries Science Center 
National Marine Fisheries Service, NOAA 
75 Virginia Beach Drive, Building 1 
Miami, Florida 33149-1003 
? Pacific Islands Fisheries Science Center 
National Marine Fisheries Service, NOAA 
1845 Wasp Boulevard, Building 176 
Honolulu, Hawaii 96818 
'° ECS Federal, LLC 
Office of Science and Technology 
1315 East-West Highway, Building SSMC3 
Silver Spring, Maryland 20910-3282 
"! Office of Science and Technology 
National Marine Fisheries Service, NOAA 
263 13th Avenue South 
St. Petersburg, Florida 33701 
more efficient to facilitate the inte- 
gration of data from diverse sources 
and to use increasing computational 
power. Concurrently, the complexity 
of stock assessments has increased 
considerably, creating challenges for 
both analysts and reviewers of stock 
assessments. Given the range of stocks 
requiring assessments, using generic 
stock assessment models that are not 
