15 
National Marine 
Fisheries Service 
NOAA 
Fishery Bulletin 
«>• established in 1881 ■<?< 
Spencer F. Baird 
First U S Commissioner 
of Fisheries and founder 
of Fishery Bulletin 
At-vessel and postrelease mortality rates of bluefin 
tuna (Thunnus thynnus) associated with pelagic 
longline gear in the northern Gulf of Mexico 
Email address for contact author: eric.orbesen@noaa.gov 
Abstract —Bluefin tuna (Thunnus 
thynnus ) of the western Atlantic 
Ocean are often incidentally caught 
in the pelagic longline fishery that 
targets swordfish (Xiphias gladius) 
and yellowfin tuna (Thunnus alba- 
cares) in the Gulf of Mexico. Data on 
at-vessel and postrelease mortality 
are lacking. Using the database of 
the NOAA Southeast Fisheries Sci¬ 
ence Center’s Pelagic Observer Pro¬ 
gram, we estimated the mortality 
rate occurring at-vessel to be 54% 
(95% confidence interval [Cl]: 46- 
62%) when the currently mandated 
weak circle hook (with a reduced di¬ 
ameter <3.65 mm) was used. To esti¬ 
mate rates of postrelease mortality, 
we deployed 41 pop-up satellite ar¬ 
chival tags (PSATs) on bluefin tuna 
captured in the pelagic longline fish¬ 
ery operating in the northern Gulf of 
Mexico from May 2010 through April 
2015. Data from the PSATs indicate 
that 29 fish survived for at least 30 
d and that 4 fish died within 12 d 
of tagging. Six PSATs detached prior 
to the programed release date, and 
2 PSATs did not report. We estimate 
a postrelease mortality rate be¬ 
tween 12% and 29%. Combining the 
postrelease mortality estimate with 
the at-vessel mortality rate, we es¬ 
timate a total mortality rate of 59% 
(95% Cl: 47-71%) associated with 
capture and subsequent release of 
bluefin tuna in this fishery accord¬ 
ing to its current fishing practices. 
Manuscript submitted 8 May 2018. 
Manuscript accepted 14 November 2018. 
Fish. Bull. 117:15-23 (2019). 
Online publication date: 12 December 2018. 
doi: 10.7755/FB. 117.1.3 
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. 
Eric S. Orbesen (contact author) 
Craig A. Brown 
Derke Snodgrass 
Joseph E. Serafy 
John F. Walter III 
Southeast Fisheries Science Center 
National Marine Fisheries Service, NOAA 
75 Virginia Beach Drive 
Miami, Florida 33149-1003 
The bluefin tuna (Thunnus thynnus) 
is the largest member of the family 
Scombridae and has a specialized 
cardiovascular physiology that allows 
it to exploit subarctic to subtropical 
pelagic waters (Carey and Lawson, 
1973; Block et al., 2005). Bluefin 
tuna of the western Atlantic Ocean 
forage in the North Atlantic Ocean, 
and many of them migrate to the Gulf 
of Mexico (GOM) (Mather et al., 1995; 
Block et al., 2005; Knapp et al., 2014) 
to spawn, although spawning is not 
exclusive to the GOM (Mather et al., 
1995; Richardson et al., 2016). Blue¬ 
fin tuna can be found in the GOM 
from December through July, but the 
timing can vary with oceanographic 
conditions (Block et al., 2005; Teo et 
al., 2007; Galuardi et al.; 2010). Dur¬ 
ing these months, bluefin tuna are in¬ 
cidentally caught by the U.S. pelagic 
longline (PLL) fleet, which targets 
swordfish (Xiphias gladius) and yel¬ 
lowfin tuna (Thunnus albacares) in 
the northern GOM (Springer, 1957). 
Currently, the bluefin tuna in the 
GOM is managed as an incidental 
bycatch species, and no active target¬ 
ing of it is allowed. As a result, vari¬ 
ous management actions have either 
required or resulted in substantial 
numbers of fish released alive or dis¬ 
carded dead. Beginning in 1981, the 
National Marine Fisheries Service 
(NMFS) prohibited directed fisher¬ 
ies for bluefin tuna on the GOM 
spawning grounds and established 
quotas for the fishery (Federal Reg¬ 
ister, 1981). To further reduce the 
incentive to target bluefin tuna, the 
NMFS enacted target species catch 
requirements for bluefin tuna re¬ 
tention (Federal Register, 1992). In 
2011, the NMFS mandated that all 
U.S. PLL vessels fishing in the GOM 
use a weak hook with a reduced wire 
diameter (i.e., <3.65 mm) (Federal 
Register, 2011). A weak hook is a cir¬ 
cle hook designed to allow a bluefin 
tuna and other similarly large ani¬ 
mals to straighten the hook, thereby 
reducing their catch by more than 
50% while not significantly reducing 
the catch of target species (Foster 
and Bergmann 1 ). Additional mea¬ 
sures were taken in 2006 with the 
1 Foster, D., and C. Bergmann. 2010. 2010 
interim report: update on Gulf of Mexico 
pelagic longline bluefin tuna mitigation 
research, 11 p. [Available from Har¬ 
vesting Eng. Branch, Southeast Fish. 
Sci. Cent., Natl. Mar. Fish. Serv., 3209 
Frederic St., Pascagoula, MS 39567.] 
