Hightower et al.: Population dynamics and relative abundance for adult Sciaenops ocellatus 171 
Marginal effect 1 
-0.2 -0.1 0 0.1 
Surface northward velocity (m/s) 
Marginal effect 2 
Surface temperature (°C) 
Marginal effect 3 
10 20 30 40 50 
l 
10 20 30 40 50 
10 20 30 40 50 
Depth (m) 
Figure 6 
Marginal effect plots for the 3 variables identified through analysis with boosted regression trees (BRTs) as 
the most influential in predicting relative abundance of red drum (Sciaenops ocellatus) in (A-C) spring, (D-F) 
summer, and (G—I) autumn in the north-central Gulf of Mexico: northward velocity of the surface current, 
surface temperature, and depth. Data used in the BRT analysis were from surveys conducted in 2006-2018. 
the assessment (0.16—0.18 year). Unfortunately, the cur- 
rent approach for stock assessments (DLMtool; Carruthers 
and Hordyk, 2018) does not allow for age-dependent esti- 
mates of M. The inability to model age-dependent mortality 
is potentially problematic for red drum because fishing 
pressure is higher for juveniles, which likely experience dif- 
ferent M than that experienced by older individuals. As the 
red drum becomes less data-limited, development of the 
ability to account for age-based differences in M should be 
prioritized. 
The development of a Gulf-wide index of relative 
abundance generated from fishery-independent bottom 
longline surveys is critical for future assessments of 
red drum. During the last stock assessment, 6 potential 
methods were considered for generating catch advice. The 
only method to meet the performance criteria was Islope, 
which is solely based on an index of relative abundance 
(Carruthers and Hordyk, 2018). For GOM red drum, the 
index of relative abundance deemed most representative 
of the adult spawning stock was the index based on data 
from our bottom longline survey. Therefore, the index of 
relative abundance generated in this study is an import- 
ant step toward producing catch advice for this data- 
limited species. This index indicates that the relative 
abundance of red drum has varied little over the past 
13 years. However, given the long life span of red drum, 
