Orbesen et al.: Mortality rates of Thunnus thynnus associated with pelagic longline gear in the Gulf of Mexico 
21 
Table 3 
Summary information for pop-up satellite archival tags that were deployed on bluefin tuna (Thunnus thynnus ) during 
2010-2015 in the Gulf of Mexico and failed to reach the 30-d threshold at which fish were deemed to have survived capture. 
The rate of descent (sinking rate) was calculated for all negatively buoyant tags that transmitted data. The sinking rates 
then were compared to the rate of a reference tag (this tag was dropped overboard, and its descent rate indicates the sink¬ 
ing speed of a negatively buoyant tag absent of a fish). Each tagged fish that did not survive was classified in the following 
categories: non-reporting tag, tag attachment failure (sinking ratecreference tag), or mortality (sinking rate>reference tag). 
Tag number 
Tether design 
Tag 
buoyancy 
Sinking 
rate (m/s) 
Monitoring 
days 
Release 
depth (m) 
Data type 
Category 
10A0919 
Cable tether 
Negative 
N/A 
_ 
_ 
N/A 
Non-reporting 
11A0898 
Mono single crimped 
Positive 
N/A 
- 
- 
N/A 
Non-reporting 
10A0896 
Mono single crimped 
Positive 
N/A 
10 
22 
Recovered 
Attachment failure 
10A1042 
Mono single crimped 
Positive 
N/A 
10 
288 
Transmitted 
Attachment failure 
10A1030 
Mono single crimped 
Positive 
N/A 
15 
712 
Transmitted 
Attachment failure 
10A0921 
Mono double crimped 
Negative 
0.122 
2 
1320 
Transmitted 
Attachment failure 
10A0931 
Cable tether 
Negative 
0.224 
18 
1200 
Transmitted 
Attachment failure 
10A0917 
Cable tether 
Negative 
0.227 
3 
1546 
Recovered 
Attachment failure 
Reference tag 
Cable tether 
Negative 
0.251 
1 
1808 
Recovered 
N/A 
10A0915 
Cable tether 
Negative 
0.316 
7 
1840 
Transmitted 
Mortality 
10A1041 
Cable tether 
Negative 
0.319 
7 
1850 
Recovered 
Mortality 
11A0914 
Cable tether 
Negative 
0.346 
3 
1808 
Transmitted 
Mortality 
11A0981 
Mono single crimped 
Positive 
0.408 
12 
1768 
Transmitted 
Mortality 
mal commercial practices exist. However, a study by 
Block et al. (2005) that used short experimental sets 
designed to capture bluefin tuna alive reported an at- 
vessel mortality rate of 30%, a rate that is nearly 25% 
lower than the overall nominal rate derived from the 
POP observer database (54%). Block et al. (2005) pos¬ 
tulated that mortality rates of bluefin tuna in the GOM 
PLL fishery could be a result of asphyxiation due to 
inability to ram ventilate, thermal stress from confine¬ 
ment in warm surface waters, or other capture related 
trauma that could be exacerbated by longer time on 
the line. 
Block et al. (2005) used relatively short sets de¬ 
signed to mitigate mortality. In contrast, our study op¬ 
erated under standard commercial fishing operations 
with an average soak duration of 7.5 h. Hook timer 
data indicate that fish in our study were on the lines 
for an average of 6.2 h, a period that is longer than the 
entire duration of the experimental sets in Block et al 
(2005). An additional factor related to observed mor¬ 
tality differences could be gear configuration. Although 
we did not detect significant effects of hook depth or 
SST on mortality in our analysis of POP data, the 
experimental design of their study (with a maximum 
hook depth of 200 m, compared with 97 m in our study; 
Table 2) could have allowed fish to access deeper, cooler 
waters and a fish’s ability to access such water could 
have been a mitigating factor for some of the thermal 
stress that a fish may have experienced. Furthermore, 
one static SST measurement might not accurately re¬ 
flect the range of temperatures at the locations where 
fish encountered the gear throughout sets, and the use 
of this single measurement could be the reason that 
we did not detect a significance for SST. Musyl et al. 
(2009) highlighted the importance of using fishery-spe¬ 
cific features when attempting to estimate postrelease 
survival, and our results support this notion. 
Postrelease mortality has been quantified to be rela¬ 
tively low in recreational fisheries (from 0% [95% Cl: 
0-7%] to 32% [95% Cl: 14-55%]) (Goldsmith et al., 
2017) and in commercial hand-line and rod-and-reel 
fisheries in Canada (from 3% [95% Cl: 1-13%] to 6% 
[95% Cl: 2-6%]) (Stokesbury et al., 2011). Both sets of 
authors calculated mortality in 2 ways; hence, separate 
95% CIs are given for each estimate. Although these 
fisheries and the ocean conditions where they occur are 
very different from those of the GOM, the low rate of 
postrelease mortality in those studies and in our study 
indicates that bluefin tuna, if they survive the initial 
capture process, appear to have a high probability of 
survival regardless of the gear type used or the geo¬ 
graphic region of release. 
Nonetheless, the high at-vessel mortality rate that 
we estimated (54%) for weak hooks in the GOM PLL 
fishery would diminish the effectiveness of a no-reten¬ 
tion policy in reducing fishing mortality and achieving 
stock status benchmarks (Coggins et al., 2007), in iso¬ 
lation of other measures. The relatively low postrelease 
mortality from our study, however, does provide sup¬ 
port for encouraging live release of bluefin tuna. Cur¬ 
rently, live bluefin tuna can either be retained or 
released with control over the total retention rate 
through an individual quota system for bluefin tuna, 
in which vessels are required to have quota of bluefin 
