Hightower et al.: Population dynamics and relative abundance for adult Sciaenops ocellatus 163 
spawning (e.g., Lowerre-Barbieri et al., 2019), the red 
drum in the Gulf of Mexico is classified by the National 
Marine Fisheries Service as a “data-limited species” 
(SEDAR, 2016). 
The reauthorization in 2006 of the Magnuson-Stevens 
Fishery Conservation and Management Act (Magnuson- 
Stevens .. . 2021) required development and implemen- 
tation of annual catch limits for all federally managed 
stocks, a mandate that spurred significant advances in the 
development of methods for assessment of data-limited 
stocks (Newman et al., 2015). One of these methods, as 
implemented in the R package DLMtool (vers. 3.2.1; 
Carruthers and Hordyk, 2018), was recently used to assess 
a suite of data-limited species in the GOM, including the 
lane snapper (Lutjanus synagris), wenchman (Pristipo- 
moides aquilonaris), yellowmouth grouper (Mycteroperca 
interstitialis), speckled hind (Epinephelus drummond- 
hayi), snowy grouper (Hyporthodus niveatus), almaco jack 
(Seriola rivoliana), lesser amberjack (S. fasciata), and red 
drum (SEDAR, 2016). During this assessment, at least one 
method for data-limited species was identified as having 
preferable performance compared to the status quo for 
every species examined, with the notable exception of red 
drum (SEDAR, 2016). Therefore, despite new tools tailored 
to the assessment of data-limited species and a wealth of 
information about the population biology and ecology of 
red drum, the results from this assessment were not rec- 
ommended for providing management advice for red drum 
(SEDAR, 2016). 
Careful consideration of existing data sets can 
improve our ability to assess stocks like the GOM stock 
of red drum. Specific data recommendations from the 
most recent assessment of red drum include 1) expan- 
sion of efforts to collect samples, for analysis of age and 
length, at varying sizes, seasons, months, and locations, 
particularly for offshore fish; 2) identification or optimi- 
zation of fishery-independent surveys to characterize 
relative abundance in federal waters; and 3) exploration 
of ways to increase data collection from existing fishery- 
independent surveys (SEDAR, 2016). To those ends, the 
goals of this study were to combine data from fishery- 
independent surveys operating throughout the year and 
across the continental shelf to produce up-to-date over- 
all and sex-specific growth models, update estimates of 
the instantaneous natural mortality rate (M), generate 
standardized indices of relative abundance, and pro- 
vide predictions of habitat suitability for red drum in 
the north-central GOM, with the expectation that the 
results of these efforts can be used to optimize future 
fishery-independent surveys. 
Materials and methods 
Data collection 
Catch data for adult red drum were collected during 
fishery-independent bottom longline surveys conducted in 
all seasons in the north-central GOM during 2006-2018 
(Fig. 1). The locations for bottom longline surveys were 
selected by using a stratified-random sampling design 
and were sampled following standardized methods 
described in Drymon et al. (2013, 2020). Briefly, the 
mainline consisted of 1.85 km (1 nmi) of 4-mm mono- 
filament (545 kg test) that was set with 100 gangions. 
Gangions consisted of a longline snap and a 15/0 circle 
hook baited with Atlantic mackerel (Scomber scombrus). 
Each gangion was made of 3.66 m of 3-mm monofilament 
(320 kg test). All sets were soaked for 1 h, and mid-set 
measurements of surface and bottom temperatures (in 
degrees Celsius), salinity, and bottom dissolved oxygen 
(in milligrams per liter), as well as of depth at the start 
and end of each set (in meters), were recorded. During 
the retrieval of the bottom longline, all red drum encoun- 
tered were measured to the nearest millimeter (maxi- 
mum total length [TL]), weighed, and retained, and their 
sex was determined. Sagittal otoliths were extracted for 
age and growth analyses. Catch data were converted to 
catch per unit of effort (CPUE), expressed as the number 
of individuals per 100 hooks per hour. 
To augment the samples of adult red drum from the bot- 
tom longline survey, smaller red drum were collected from 
the monthly gill-net survey of the Alabama Department of 
Conservation and Natural Resources, Marine Resources 
Division, during 2006-2018; these additional samples 
were aged. This survey includes areas of coastal Alabama 
from eastern Mississippi Sound to western Perdido Bay 
and Mobile Bay (Fig. 1) (Livernois et al., 2020). The gill- 
net survey of the Marine Resources Division involves 
2 different nets: a small-mesh gill net and a large-mesh 
gill net. The small-mesh gill net consists of 5 panels that 
are 45.0 m long by 2.4 m deep, each containing stretch 
meshes in sizes of 5.1-10.2 cm. The large-mesh gill net 
consists of 4 panels that are also 45.0 m long by 2.4 m 
deep, with stretch meshes in sizes of 11.4-15.2 cm. Red 
drum caught in either gill net were measured to the near- 
est millimeter (maximum TL) and weighed, and their sex 
was determined. Sagittal otoliths were extracted for age 
and growth analyses. 
For fish of all ages combined (from both longline and 
gill-net surveys), 2-sample Kolmogorov—Smirnov tests 
were used to examine differences in length and weight 
distributions between sexes. Some red drum collected 
with longlines were not measured for TL. For red drum 
from longline surveys that had both maximum total and 
fork length (FL) measurements, linear regression was 
used to examine the relationship of maximum TL to FL, 
resulting in this equation: 
TL = 1.04(FL) + 23.53, (1) 
where TLs and FLs are expressed in millimeters (num- 
ber of samples [n]=346, coefficient of multiple determi- 
nation [R7]=0.96). This regression was used to estimate 
the TLs of red drum from the longline survey that were 
lacking a maximum TL measurement (n=238). Differ- 
ences in sex ratio were tested by using a chi-square test 
(Pearson, 1900) against an expected 1:1 male-to-female 
ratio. 
