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Fishery Bulletin 1 12(4) 
-3 - 
-4 - 
T 1 1 1 1 1 — 
0 20 40 60 80 100 
0 20 40 60 80 100 
Depth (m) 
Figure 3 
Predicted relationship between depth and release mortality of red snap- 
per ( Lutjanus campechanus ) in the Gulf of Mexico derived from the meta- 
analytical model in which coefficients were equally weighted to produce 
the predicted values (average model). (A) Relationship plotted with ef- 
fect-size data weighted by the inverse of its variance; dot size, then, is 
proportional to the inverse of the corresponding sampling variance, and 
increasing dot size indicates better fit to the model. (B) Same relationship 
plotted with effect size data converted back to original proportions to bet- 
ter visualize predictions of release mortality. 
reduced during those seasons (Fig. 5; Table 4). Inclu- 
sion of the commercial data in the model indicated that 
release mortality rates were significantly higher in the 
commercial fishing sector than in the recreational fish- 
ing sector (Fig. 6; Table 4). 
Discussion 
The mixed-effects modeling approach in which a ran- 
dom effect is estimated for each individual study, while 
estimating discard mortality as a function of key fac- 
tors of interest, provides a robust method for dealing 
with specific differences due to either experimental 
protocols or other factors. The functional relationships 
developed in this model had been used in the most re- 
cent red snapper stock assessment in 2012, and that 
use was a change from previous assessments based on 
region-specific point estimates and that did not pro- 
vide an estimate a depth-related function (Campbell et 
al. 3 ). The various differences in experi- 
mental protocols represented nuisance 
factors for the estimation of a range 
of discard mortality rates by depth. 
Nonetheless, significant residual het- 
erogeneity was observed in the model, 
indicating that there likely were other 
unquantified variables that influence 
release mortality. Results from the 
various studies have to be evaluated 
within the context of the experimental 
methods used, but overall our meta- 
analytical model resulted in consistent 
results that isolated important factors 
for release mortality in the red snap- 
per fishery. 
There was a consistent, positive 
correlation between depth and release 
mortality estimates regardless of vent- 
ing treatment, season, or fishing sec- 
tor (Tables 1-4; Figs. 3-6). Presence of 
a positive correlation between depth 
and mortality is frequently reported 
in the literature, and the relationship 
is thought to be associated primarily 
with injuries sustained during decom- 
pression, such as overexpansion and 
rupture of the gas bladder, esophageal 
eversion, cloacal prolapse, exophthal- 
mia, and gas infusion into vital organs 
(Davis, 2002; Rummer and Bennett, 
2005; Hannah et ah, 2008). The de- 
velopment of a predictive relationship 
with depth is important because pre- 
vious stock assessments of red snap- 
per, completed before this model, were 
based on single estimates that were 
fixed by region (i.e., east and west 
GOM) rather than on a depth relation- 
ship. Still, although depth was a consistent factor for 
the explanation of release mortality, the results from 
the studies examined were complicated by study-spe- 
cific experimental methods. 
Estimated rates of release mortality were signifi- 
cantly higher for the commercial sector than for the 
recreational sector, but, unfortunately, they were de- 
rived from a single surface-release study that was 
conducted in a single region (Nieland et ah, 2007). 
Further complicating the commercial data was the 
fact that no fish were vented before release in that 
study. Comparable surface-release studies of the rec- 
reational sector revealed that at least some amount 
of venting occurred, and all of those studies resulted 
in lower estimates of release mortality (Patterson et 
ah, 2001; Dorf, 2003; Campbell et ah, 2010a, Patter- 
son 4 ). Because commercial fishing operations were 
observed directly in the Nieland study, the estimates 
are at least reflective of common venting practices for 
that fishing sector at that time and region. Impor- 
