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Demographic, Taxonomic, and Genetic Characterization of the Snook Species Complex (Centropomus spp.) along the Leading Edge of Its Range in the Northwestern Gulf of Mexico

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  • US Fish & Wildlife Service

Abstract and Figures

A recent increase in the abundance of snook species (Centropomus sp.) in Texas has been generally associated with a broad‐scale warming trend of Texas’ inshore waters, closure of the commercial fishery in 1987, and fairly conservative restrictions on recreational catch implemented at the same time. Despite this observed increase in abundance, little is known about the snook species complex in Texas, including uncertainty about recent changes in distribution and abundance, taxonomy, and population structure. Here, abundance and distribution data from a long‐running fishery independent (FIN) data set was analyzed in synergy with mitochondrial DNA (mtDNA) and microsatellite genotypes in order to answer basic questions about the snook species complex in Texas. The main findings from this work are: (1) based on trends observed in FIN data, snook are increasing in abundance and expanding their range northward in Texas, (2) based on mtDNA sequencing, the two most common species of snook in Texas are Common Snook C. undecimalis and Largescale Fat Snook C. mexicanus, (3) a third species, the Mexican Snook C. poeyi occurs but only rarely, and (4) patterns from microsatellite genotypes suggest that the two predominant species, Common Snook and Largescale Fat Snook, probably constitute single genetic stocks in Texas, although evidence of chaotic genetic patchiness was also observed. This latter finding might be a general expectation for populations that are on the leading edge of an expanding species range, and imply that management measures in Texas should be directed towards conservation of suitable habitat corridors offering environmental and habitat refugia, as well as measures that increase the probability of survival of small localized populations, such as stock enhancement.
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ARTICLE
Demographic, Taxonomic, and Genetic Characterization of the Snook
Species Complex (Centropomus spp.) along the Leading Edge of Its Range
in the Northwestern Gulf of Mexico
Joel Anderson* and Damon Williford
Texas Parks and Wildlife Department, Perry R. Bass Marine Fisheries Research Station and Hatchery, 3864 FM 3280,
Palacios, Texas 77465, USA
Alin González
Oklahoma Cooperative Fish and Wildlife Research Unit, Oklahoma State University, 007 Agriculture Hall, Stillwater,
Oklahoma 74078, USA
Chris Chapa
U.S. Fish and Wildlife Service, Partners for Fish and Wildlife, 10711 Burnet Road, Suite 200, Austin, Texas 78758, USA
Fernando Martinez-Andrade
Texas Parks and Wildlife Department, Corpus Christi Field Ofce, Natural Resources Center Building, Suite 2500,
6300 Ocean Drive, Corpus Christi, Texas 78412, USA
R. Deborah Overath
Division of Science, Technology, Engineering, and Mathematics, Texas Southmost College, 80 Fort Brown, Brownsville,
Texas 78520, USA
Abstract
A recent increase in the abundance of snook species (Centropomus spp.) in Texas has been generally associated with a
broad-scale warming trend of Texasinshore waters, closure of the commercial shery in 1987, and fairly conservative
restrictions on recreational catch implemented at the same time. Despite this observed increase in abundance, little is known
about the snook species complex in Texas, including uncertainty about recent changes in distribution and abundance, taxon-
omy, and population structure. Here, abundance and distribution data from a long-running shery-independent (FIN) data
set were analyzed in synergy with mitochondrial DNA (mtDNA) and microsatellite genotypes to answer basic questions
about the snook species complex in Texas. The main ndings from this work are as follows: (1) based on trends observed in
FIN data, snook are increasing in abundance and expanding their range northward in Texas; (2) based on mtDNA sequenc-
ing, the two most common species of snook in Texas are the Common Snook C. undecimalis and Largescale Fat Snook C.
mexicanus; (3) a third species, the Mexican Snook C. poeyi, occurs but only rarely; and (4) patterns from microsatellite
genotypes suggest that the two predominant species, Common Snook and Largescale Fat Snook, probably constitute single
genetic stocks in Texas, although evidence of chaotic genetic patchiness was also observed. This latter nding might be a gen-
eral expectation for populations that are on the leading edge of an expanding species range and implies that management
measures in Texas should be directed toward conservation of suitable habitat corridors offering environmental and habitat
refugia as well as measures (e.g., stock enhancement) that increase the probability of survival of small, localized populations.
*Corresponding author: joel.anderson@tpwd.texas.gov
Received August 12, 2019; accepted November 19, 2019
North American Journal of Fisheries Management
©2019 American Fisheries Society
ISSN: 0275-5947 print / 1548-8675 online
DOI: 10.1002/nafm.10394
1
Centropomidae (snook) is a family of popular game
sh that are distributed along the western Atlantic Ocean,
Gulf of Mexico, and eastern Pacic Ocean, where they
inhabit warm temperate to tropical estuarine systems
(Rivas 1986). In the western Gulf of Mexico, central
Texas (near Aransas Bay) has previously been considered
the northernmost edge of the centropomidsrange (Rivas
1986). The inshore waters of Texas represent a latitudinal
environmental cline of salinity and temperature, both of
which increase from north to south (Table 1). Snook spe-
cies are vulnerable to acute temperature extremes, particu-
larly cold/freeze events (Shaand and Foote 1983; Adams
et al. 2012; Stevens et al. 2016), such that their distribution
has historically been limited to lower latitudes in Texas
where water temperatures are more tolerable (Matlock
and Osburn 1987; Pope et al. 2006). It has been suggested
that the natural range of snook species could expand fur-
ther north given the current climatic warming trend; such
a range expansion has been reported in another subtropi-
cal/tropical-associated species in Texas, the Gray Snapper
Lutjanus griseus, and has been hypothesized for other spe-
cies with similar thermal tolerance ranges (Tolan and
Fisher 2009). Recent shery-independent (FIN) monitor-
ing by the Texas Parks and Wildlife Department (TPWD,
unpublished data) suggests that the range of snook in
Texas may indeed be expanding northward, although no
formal assessment of this potential range expansion has
been made.
Snook were the target of a short-lived commercial sh-
ery in Texas. The shery crashed in the late 1920s, and
recreational catch declined within the same general time
frame (Matlock and Osburn 1987). It was hypothesized
that this dramatic decline in snook abundance was driven
by two factors: cold weather events and overshing,
although it is unknown whether other factors may have
also played a role (Matlock and Osburn 1987). In any
event, the decline in abundance seemed to persist through
the entirety of the 1900sso much so that Pope et al.
(2006) observed continued low abundance and erratic
recruitment for the most heavily targeted species, the
Common Snook Centropomus undecimalis. In 1987, the
commercial snook shery in Texas was ofcially closed
and harvest was restricted to recreational shing, with a
bag limit of 3 snook/d and a slot limit of 5171 cm. In
1995, the bag limit was reduced to 1 snook/d and the slot
size was also narrowed to 6171 cm. Subsequently, snook
abundance has increased dramatically in Texas over the
last three decades (González 2015). In particular, increases
in both abundance and presence have been observed dur-
ing FIN sampling undertaken by TPWD over that time
period (Supplemental Figures S1, S2 available in the
online version of this article).
The snook species complex in Texas is poorly charac-
terized compared to that in Florida, which is the north-
eastern range limit of the genus Centropomus (Huber et al.
2014). For instance, while it has long been assumed that
the Common Snook comprises the bulk of the snook com-
plex in Texas, other species have been notedparticularly
the Smallscale Fat Snook C. parallelus (Martin and King
1991), Largescale Fat Snook C. mexicanus, and Mexican
Snook C. poeyi (Chapa 2012). Although there are morpho-
logical characteristics that might distinguish these species,
in general these characteristics tend to overlap, making
identication based on morphology alone very difcult,
particularly for juveniles. Hybridization could further con-
tribute to the difculty in identifying snook species based
on morphology. Pfennig et al. (2016) demonstrated that
hybridization could play a key role in the persistence of
some populations at the edge of their range. Although
hybridization among species of snook has not been
observed in the wild (Tringali et al. 1999a), there is indi-
rect evidence that hybridization may occur at least occa-
sionally. Hybrids of Common Snook ×Smallscale Fat
Snook have been produced in captivity (Ferraz et al.
2012), and juvenile residency patterns of each species in
Puerto Rico imply that spawning times are likely
TABLE 1. Mean (SD in parentheses) of annual water temperature, winter water temperature (DecemberFebruary), and salinity in each of the major
bay systems of Texas, organized from north to south. Data are from monthly water samples taken by the Texas Parks and Wildlife Department dur-
ing the course of shery-independent inshore trawls and are averaged over the entire period observed in this study (19802018).
Bay Annual temperature (°C) Winter temperature (°C) Annual salinity ()
Sabine Lake 22.45 (6.86) 14.07 (3.21) 7.73 (6.77)
Galveston Bay 23.36 (6.65) 15.18 (3.3) 17.22 (9.33)
East Matagorda Bay 24.1 (6.73) 15.87 (3.59) 22.41 (8.74)
West Matagorda Bay 23.7 (6.48) 15.96 (3.7) 19.98 (9.61)
San Antonio Bay 23.71 (6.47) 15.89 (3.59) 19.28 (11.27)
Aransas Bay 24.11 (6.41) 16.32 (3.67) 20.33 (9.97)
Corpus Christi Bay 24.34 (6.27) 16.64 (3.49) 29.47 (7.26)
Upper Laguna Madre 25.17 (6.21) 17.99 (3.91) 37.21 (11.34)
Lower Laguna Madre 25.43 (5.85) 18.52 (3.87) 31.16 (8.21)
2ANDERSON ET AL.
coincident with one another (Aliaume et al. 1997). In addi-
tion to the uncertain taxonomy of snook, there is also a
lack of knowledge regarding the genetic stock structure of
snook in Texas waters. Knowledge of genetic stock struc-
ture would aid in proper management of this species
group by helping to dene management units (Waples et
al. 2008) and to prioritize conservation needs in a spatial
context (Du Toit 2010).
In this study, three key elements of snook biology in
Texas were explored. First, trends in abundance and dis-
tribution of snook in Texas over the last 38 years (1980
2018) were related to trends in environmental variables
using a long-running FIN data set collected by the
TPWD. This was done in an effort to identify environ-
mental trends that might be driving changes in snook
inshore abundance in space and time. Second, the taxo-
nomic designation and relative abundance of snook spe-
cies in Texas were directly assessed by using DNA
sequence data in order to inform unit stockdenitions
and future management measures that could presumably
target individual species. Third, intraspecic patterns of
genetic population structure were assessed in the two pre-
dominant snook species, the Largescale Fat Snook and
Common Snook, by using microsatellite DNA markers.
The ndings associated with these data represent a base-
line upon which future assessments of distribution, abun-
dance, taxonomy, and stock structure in Texas snook
populations can be anchored.
METHODS
Demographic analysis.Trends in abundance of Cen-
tropomus spp. were evaluated using a long-running FIN
monitoring data set collected by TPWD. Gill nets have
been deployed by TPWD since the 1970s to measure
trends in abundance of all bay-associated nsh species in
each of the state's major bays. Gill nets were deployed in
10 major inshore bays (Figure 1) for 10 weeks in the spring
(AprilJune) and 10 weeks in the fall (SeptemberNovem-
ber) each year throughout the period 19802018. A total
of 45 nets were deployed in each system across each 10-
week season, with the exception of East Matagorda Bay
(n=20 nets) and Cedar Lakes (n=10 nets). Each bay sys-
tem was subsectioned into 1-min
2
grids aligned with the
geographic coordinate system, and nets were deployed
overnight along shorelines in grids chosen using a strati-
ed random sampling design (stratied by bay). Each net
extended 182.9 m from shore and consisted of equally
sized panels with four different mesh sizes (76, 102, 127,
and 152 mm). Upon retrieval of each net, specimens were
enumerated and the TL of each specimen was measured
to the nearest millimeter. Additionally, two water
quality variablestemperature (°C) and salinity ()
were assessed concurrently with net deployment. These
sampling protocols remained unchanged during the entire
duration of the sampling period; as a result, changes in
catch can be reasonably assumed to be related to changes
in the abundance and distribution of snook species over
the sample period. Due to difculty in properly identifying
species, including overlap in key characteristics for almost
all specimens observed during a previous study in Texas
(Chapa 2012), snook species were combined into a single
data set for the demographic analysis.
Logistic regression was used to evaluate the presence/
absence of snook in any single net, with year and latitude
used as predictors of temporal and spatial change. Each
gill net set during the period 19802018 represented a sin-
gle observation, with a sample size of 29,276 total data
points for the regression. Temperature and salinity were
originally included in the logistic model described above;
however, both variables were collinear with latitude, and
neither variable was as important as latitude in the nal
model. The coastal waters of Texas represent a cline of
cooler, less saline waters in the north and warmer, higher
salinity waters in the south. Temperature is driven primar-
ily by a natural climatic cline, whereas increasing salinity
from north to south is driven by decreasing rainfall and
freshwater inows into southern bays (Tolan 2007). As
such, we would expect latitude to be correlated signi-
cantly with both temperature and salinity. We used
ANOVA to evaluate differences in mean temperature and
salinity between nets with zero catch and nets with non-
zero catch. This analysis was broken up into seasons
(spring versus fall) to account for different water condi-
tions during these time periods.
Latitude was evaluated further by plotting each individ-
ual observation onto a map of the Texas coastline using
ArcMap version 10.1 (Environmental Systems Research
Institute, Redlands, California). It was noted that catches
in higher latitudes were observed more commonly during
later years in the study; therefore, the map plot was bro-
ken down by two eras, 19802009 and 20102018, to eval-
uate changes in snook distribution during the most recent
10-year period. The mean latitude of all catches was calcu-
lated for each year, and the relationship between year and
mean latitude was evaluated with simple linear regression.
Sample collection and DNA extraction.Fin tissue
samples were collected from Centropomus spp. captured in
a variety of sampling areas between Aransas Pass, Texas,
and the Rio Grande along the TexasMexico border
between 2009 and 2013 (Table 2). Adults were collected
via hook and line, and juveniles were collected by using
seines; all individuals were sampled separately from the
FIN sampling program described above. Additional sam-
ples from Campeche, Mexico (n=24), and Jacksonville,
Florida (n=7), were collected for inclusion in the mito-
chondrial DNA (mtDNA) phylogenetic analysis. Genomic
DNA was extracted from tissues using the DNeasy Blood
CHARACTERIZATION OF THE SNOOK SPECIES COMPLEX 3
and Tissue Kit (Qiagen, Germantown, Maryland) and the
Puregene DNA Isolation Kit (Gentra Systems, Min-
neapolis, Minnesota) following the manufacturersproto-
cols. The nal rehydration volume varied between 75 and
200 μL depending on DNA pellet size.
It should be noted that genetic sample selection was
not identical between mtDNA and microsatellite data sets.
This was the result of combining two data sets generated
in two different laboratories during the course of indepen-
dent projects (Chapa 2012; González 2015). Although
there was overlap between studies in a large number of
samples (n=443 individuals that were sequenced for 16S
mtDNA were also genotyped, including 18 individuals
from Mexico), a subset of individuals (n=138, including
samples from Florida [n=7] and Mexico [n=6]) was
assigned mtDNA haplotypes but not genotyped and a sec-
ond subset (n=112, all from Texas) was genotyped but
not assigned mtDNA haplotypes.
16S mitochondrial DNA sequencing and phylogenetics.
Mitochondrial DNA haplotypes were obtained from 581
unique individual snook (Texas: n=550; Mexico: n=24;
Florida: n=7). We used DNA primers described by
Palumbi (1996) to amplify a fragment (440 bp) of the mito-
chondrial 16S ribosomal RNA gene using the PCR protocol
of Tringali et al. (1999b). The PCRs were carried out using
Ready-To-Go PCR beads (GE Healthcare, Piscataway,
New Jersey) and reaction mixes of the following: 1 μLof
template DNA, one Ready-To-Go bead, 12 μL of 0.4-μM
FIGURE 1. Distribution of snook catch in Texas shery-independent gill-net sets, 19802018. Ten major inshore areas (labeled) were sampled using a
stratied random design. Three of those areas (Aransas Bay, Upper Laguna Madre, and Lower Laguna Madre) were subsampled using hook-and-line
sampling as well as bag seines targeting snook. Specic targeted sampling locations are described in Table 2. Eras were differentiated in order to
qualitatively assess the distribution of the catch before and after 2010.
4ANDERSON ET AL.
forward primer, and 12 μL of 0.4-μM reverse primer for a
total volume of 25 μL. After amplication, PCR products
were puried using ExoSAP-IT (USB, Cleveland, Ohio) via
the manufacturer's recommended protocol. Sequencing
reactions were then carried out in 10-μL volumes using
Quick Start Master Mix DTCS (Beckman Coulter, Fuller-
ton, California). Primers for sequencing were the same as
those used in PCR. Sequencing reactions were precipitated
by adding 1/20 volume of a cocktail containing 2 μLof
sodium acetate (3 M), 2 μL of EDTA (100 mM), and 1 μL
of glycogen, followed by 2 volumes of 95% ethanol. Precipi-
tated sequence extracts were then centrifuged at 3,700 revo-
lutions/min for 30 min to form pellets. The resulting pellets
were then rinsed twice with 70% ethanol, dried, and rehy-
drated by using a formamide sample loading solution
(Beckman Coulter). Finally, the sequences were separated
and analyzed on a Beckman CEQ8000 capillary sequencer
(Beckman Coulter) using default sequencing module
parameters. Forward and reverse sequence traces were
aligned with Sequencher version 5.4 (Gene Codes, Ann
Arbor, Michigan).
The program Clustal X version 2.0.3 (Larkin et al.
2007) was used to align 16S mtDNA sequences. The num-
ber of haplotypes in the entire data set was computed
using DnaSP version 6.12.01 (Rozas et al. 2017). The spe-
cies identity of each haplotype was determined rst by
using the Basic Local Alignment Search Tool (BLAST)
algorithm and the MegaBLAST optimization (Altschul et
al. 1990; Zhang et al. 2000; Morgulis et al. 2008) in the
GenBank database (National Center for Biotechnology
Information; https://blast.ncbi.nlm.nih.gov/Blast.cgi). To
further assess the species identity and evolutionary rela-
tionships of the haplotypes, a maximum likelihood phylo-
genetic analysis was performed in MEGA7 (Kumar et al.
2016) by using the haplotypes and 16S mtDNA sequences
of Centropomus species and outgroup taxa from previous
studies available in GenBank (Table 3). The phylogenetic
analysis employed Kimura's (1980) two-parameter gamma
distribution model, which was selected as the best-tting
model of DNA sequence evolution based on the Bayesian
information criterion (Schwarz 1978) implemented in the
MEGA7 model selection tool. The reliability of the
inferred relationships was assessed using 1,000 bootstrap
replicates (Felsenstein 1985).
Microsatellite genotyping and population genetic analysis.
Genotypes were generated using microsatellite markers for
555 unique snook individuals (Texas: n=537; Mexico: n=
18). Eight microsatellite loci developed by Seyoum et al.
(2005) were amplied and uorescent-labeled using the
M13-tail labeling procedure described by Schuelke (2000).
Volumes for PCR were as follows: 5 μL of GoTaq Green
Master Mix (Promega Corp., Madison, Wisconsin), 0.25 μL
of 10-μM forward primer, 0.50 μL of 10-μM reverse primer,
0.25 μL of 10-μM FAM-M13 labeled primer, 3 μLof
TABLE 2. Collection locations and sample sizes (n)ofCentropomus species that were used for genetic analyses.
Location Major bay system State or country Latitude Longitude n
Jacksonville Jacksonville Florida 7
East Yucatan Campeche Mexico 24
Brazos River Galveston Bay Texas 28°52.771N95°22.898W1
Bridge Harbor surfside Galveston Bay Texas 28°57.852N95°17.539W1
Oyster Creek Galveston Bay Texas 29°01.079N95°22.641W1
Bastrop Bayou Galveston Bay Texas 29°05.875N95°12.190W4
Hitchcock diversion canal Galveston Bay Texas 29°20.129N95°01.389W1
Colorado River West Matagorda Bay Texas 28°40.797N95°58.603W2
Carancahua Bay West Matagorda Bay Texas 28°44.265N96°24.107W5
Aransas Pass ditch Aransas Bay Texas 27°53.566N97°09.188W73
Packery Channel Upper Laguna Madre Texas 27°36.829N97°11.907W55
Rio Grande/San Martin Lower Laguna Madre Texas 26°00.113N97°17.911W87
Brownsville Ship Channel Lower Laguna Madre Texas 26°00.125N97°17.913W29
South Bay Lower Laguna Madre Texas 26°01.529N97°10.272W82
Mexiquita ats Lower Laguna Madre Texas 26°04.045N97°11.826W1
Brazos Santiago Pass Lower Laguna Madre Texas 26°04.356N97°09.994W21
White Sands boat ramp Lower Laguna Madre Texas 26°04.462N97°12.873W3
Laguna Vista ditch Lower Laguna Madre Texas 26°05.705N97°17.024W 169
Arroyo Colorado Lower Laguna Madre Texas 26°20.998N97°23.483W 122
Port Manseld jetties Lower Laguna Madre Texas 28°33.799N97°16.483W2
Exact location unknown Texas 3
Total 693
CHARACTERIZATION OF THE SNOOK SPECIES COMPLEX 5
deionized water, and 1.5 μL of genomic DNA. Fragment
length analysis was conducted by the Genomics Core Labo-
ratory at Texas A&M UniversityCorpus Christi with an
ABI 3730 DNA Analyzer (Applied Biosystems, Foster City,
California). Allele lengths were scored using the program
GeneMapper version 5.0 (Applied Biosystems) and binned
using Tandem (Matschiner and Salzburger 2009) with the
default settings.
For microsatellite analyses, individuals were initially
assigned to species based on their mtDNA haplotype. If
no haplotype information was available, individuals were
grouped as unknownand were assigned species based
on genotype cluster analysis (see below). Since only four
Mexican Snook were observed and since microsatellite
amplication of these individuals resulted in poorly
resolved genotypes, this species was excluded from
microsatellite analyses. We used Structure version 2.3.4
(Pritchard et al. 2000) to test for hybrids between Com-
mon Snook and Largescale Fat Snook and to assign indi-
viduals for which there were no mtDNA haplotype data
(n=112) to species. To determine the number of signi-
cant genetic clusters, Structure was run iteratively while
varying Kfrom 1 to 5 clusters under the admixture model.
For each level of K, 10 independent iterations were run,
with each iteration consisting of 50,000 burn-insteps,
followed by 950,000 run steps. All runs (total n=50 runs)
were then used to calculate the ΔKstatistic of Evanno et
al. (2005), which more consistently resolves true Kthan
the log-likelihood probability values generated by Struc-
ture (Evanno et al. 2005). The ΔKstatistic for each level
of Kwas determined using Structure Harvester (Earl and
vonHoldt 2012). Individuals that had Structure Q-scores
TABLE 3. Species identity, accession numbers, and references of 16S mitochondrial DNA sequences downloaded from GenBank that were used in
the phylogenetic analysis.
Species Accession number Reference
Longspine Snook Centropomus armatus HQ731414 Li et al. (2011)
HQ731415 Li et al. (2011)
U85010 Tringali et al. (1999b)
Swordspine Snook C. ensiferus HQ731408 Li et al. (2011)
HQ731418 Li et al. (2011)
U85008 Tringali et al. (1999b)
Blackn Snook C. medius EF120864 Smith and Craig (2007)
HQ731409 Li et al. (2011)
HQ731413 Li et al. 2011
JQ939047 Betancur-R et al. (2013)
U85019 Tringali et al. (1999b)
Largescale Fat Snook C. mexicanus KU745737 S. Seyoum, M. D. Tringali,
J. Dutka-Gianelli, and R. G. Taylor,
unpublished
Black Snook C. nigrescens U85015 Tringali et al. (1999b)
Smallscale Fat Snook C. parallelus U85016 Tringali et al. (1999b)
Tarpon Snook C. pectinatus U85018 Tringali et al. (1999b)
Mexican Snook C. poeyi U85014 Tringali et al. (1999b)
Yellown Snook C. robalito DQ307688 Peregrino-Uriarte et al. (2007)
U85011 Tringali et al. (1999b)
Common Snook C. undecimalis AF247436 Orrell and Carpenter (2004)
HQ731428 Li et al. (2011)
U85012 Tringali et al. (1999b)
Humpback Snook C. unionensis U85009 Tringali et al. (1999b);
White Snook C. viridis DQ307689 Peregrino-Uriarte et al. (2007)
DQ532849 Peregrino-Uriarte et al. (2007)
JQ939047 Betancur-R et al. (2013)
U85013 Tringali et al. (1999b)
Barramundi Lates calcarifer (outgroup) DQ010541 Lin et al. (2006)
Nile Perch L. niloticus (outgroup) KY213963 Gann et al. (2017)
Waigieu Seaperch Psammoperca
waigiensis (outgroup)
HQ731401 Li et al. (2011)
6ANDERSON ET AL.
less than 0.7 for all structure clusters were assumed to rep-
resent potential hybrids. Additionally, individuals having
a microsatellite genotype that clustered with one species
and mtDNA representative of the alternate species were
assumed to be late-generation hybrids (back-crossed).
An exploratory biplot of the microsatellite genotype
data was generated using a principal components analysis
(PCA) as implemented in the R package Adegenet (Jom-
bart 2008). Individuals with the representative mtDNA
haplotype of one species but a microsatellite genotype that
appeared (qualitatively) to cluster with the alternate spe-
cies in a plot of the rst two axes from the PCA were
again considered to be potential hybrids. The PCA was
used to compare and contrast the results of a parametric
analysis (Structure) with those of a nonparametric multi-
variate approach (PCA).
Once species assignments were resolved via Structure,
microsatellites were checked within each species for devia-
tion from HardyWeinberg equilibrium (HWE) expecta-
tions as well as linkage disequilibrium using Genepop
(Raymond and Rousset 1995; Rousset 2008). Signicant
deviation from HWE was tested using simulations as
implemented in Genepop, with 1,000 dememorizations,
100 batches, and 1,000 iterations/batch. Loci that had
inbreeding coefcients (F
IS
) values greater than 0 were con-
sidered to have deviated from HWE. Signicant linkage
disequilibrium was also detected via simulation with the
same simulation parameters, although in this case a Holm
Bonferroni adjustment (Holm 1979) was used to adjust the
targeted alpha (initial α=0.05) downward to account for
results from 28 simultaneous tests between pairs of loci.
Species-specic population structure was tested using
additional Structure runs. Individuals that were identied
as putative hybrids were removed, the remaining individu-
als were pooled by species, and Structure was run inde-
pendently for each species. For each level of K(K=15
genetic clusters), 10 iterations were run with 50,000 burn-
in steps and 950,000 run steps. Cursory Structure runs
suggested that results were generally similar among vari-
ous admixture models; therefore, the results from the
model assuming admixture among clusters and no prior
probabilities are reported here. The appropriate value of
Kwas again chosen by using the ΔKstatistic of Evanno
et al. (2005). In each species run, individuals were orga-
nized by bay to test for differences in cluster proportions
between different bays; F-statistics were generated using
the R package hierfstat (Goudet 2004), and statistical sig-
nicance of divergence among bays was determined using
1,000 data simulations. For Largescale Fat Snook, sample
locations were (from north to south) Aransas Bay, Upper
Laguna Madre, and Lower Laguna Madre. For Common
Snook, only individuals from Aransas Bay and Lower
Laguna Madre were compared (this species was not cap-
tured in the sample from Upper Laguna Madre).
A discriminant analysis of principal components
(DAPC) was used to validate additional population struc-
ture that was observed in the species-specic Structure
runs. First, a PCA was used to reduce redundancy in the
data set; a discriminant analysis of the rst 20 principal
components was then performed among clusters identied
by Structure. The DAPC was carried out with the number
of groups constrained to the same value of Kthat was
observed in Structure runs. Individual scores on the rst
two discriminant axes were plotted by Structure assign-
ment, and a chi-square (χ
2
) test with a null assumption of
random assignment among groups was used to determine
whether there was signicant correlation between assign-
ment results from DAPC and Structure.
RESULTS
Snook Abundance and Distribution
Fishery-independent gill-net sets resulted in the observa-
tion of 1,129 snook during the period 19802018 (Table
4). The size range of snook encountered in nets was 270
872 mm TL, and the length frequency plot suggested a
wide distribution in length, with most individuals occur-
ring between 350 and 650 mm TL (Figure 2).
Logistic regression results suggested that both year and
latitude signicantly impacted snook presence/absence,
and the overall model was signicant (r
2
=0.26, P<
0.0001; Table 5). Latitude was negatively correlated (i.e.,
presence was higher in lower latitudes) and was the most
important variable in the model (χ
2
=1,029.7, P<0.0001);
year was positively correlated (i.e., later years had higher
presence; χ
2
=215.2, P<0.0001). The three southernmost
bays (Corpus Christi Bay, Upper Laguna Madre, and
Lower Laguna Madre) yielded 1,017 of all snook landed
in FIN sampling (~90%; Table 4). However, the coastwide
relative distribution of snook changed signicantly in later
years of the study based on two observations: (1)
TABLE 4. Total catch of all species of snook, by bay and overall, in
shery-independent (FIN) sampling by the Texas Parks and Wildlife
Department, 19802018. Bays are organized from north to south.
Major bay Catch
Sabine Lake 1
Galveston Bay 16
East Matagorda Bay 33
West Matagorda Bay 19
San Antonio Bay 23
Aransas Bay 20
Corpus Christi Bay 75
Upper Laguna Madre 67
Lower Laguna Madre 875
Total catch, FIN sampling 1,129
CHARACTERIZATION OF THE SNOOK SPECIES COMPLEX 7
individuals were caught in the northern bays between East
Matagorda Bay and Sabine Lake more commonly in the
latest 9-year period (20102018) than in all other years of
the study combined (19802009; Figure 1); and (2) there
was a signicant and positive relationship between year
and mean latitude (Figure 3), as the mean latitude of catch
increased by 0.017 decimal degrees/year from 1980 to
2018 (r
2
=0.40, P<0.0001). Latitude is generally corre-
lated negatively with both temperature and salinity in Tex-
asinshore waters. As a result, catch of snook was
correlated with relatively high temperature and salinity in
both spring and fall (Table 6).
Mitochondrial DNA Phylogenetics
Overall, 428 bp of the 16S mtDNA gene were observed
from 581 Centropomus specimens, and these were col-
lapsed into 21 haplotypes (Table 7). Haplotype sequences
were submitted to GenBank as accession numbers
MN068225MN068245. Haplotype 1 was the most abun-
dant haplotype and occurred in 56% of samples. Haplo-
types 2 and 3 were less common, occurring in 23% and
15% of the samples, respectively. The BLAST searches
revealed that 13 of the haplotypes most closely matched
sequences of Common Snook (accession number
HQ731428 or U85012), whereas 6 haplotypes most closely
matched a sequence of Largescale Fat Snook
(KU745737). Haplotypes 7 and 19 most closely matched a
sequence of Mexican Snook (U85014). The results of the
phylogenetic analysis supported the BLAST search results
(Figure 4). Most haplotypes clustered with sequences from
specimens identied as Common Snook (AF247436,
HQ731428, and U85012). This clade was closely related
to another clade composed of haplotypes 7 and 19 and
the GenBank sequence of Mexican Snook (U85014).
Haplotypes 1, 11, 12, 13, 17, and 18 clustered with the
Largescale Fat Snook sequence (KU745737).
Of the total 581 specimens that were assigned a haplo-
type in this study, 336 were identied via BLAST and phy-
logenetic analysis as Largescale Fat Snook (n=332 from
Texas; n=4 from Mexico). An additional 241 specimens
were identied as Common Snook (n=215 from Texas; n=
19 from Mexico; n=7 from Florida). Finally, four individu-
als were identied as Mexican Snook (n=3 from Texas; n
=1 from Mexico). The top results from the BLAST search
were concordant with the phylogenetic analysis in all cases.
Interestingly, there were qualitative differences in haplo-
type distribution for specimens from outside of Texas that
were identied as Common Snook. For instance, 6 of 7
specimens (86%) sampled from Florida possessed haplo-
type 3, whereas only 6 of 18 individuals (33%) from Mex-
ico possessed this haplotype. In contrast, 1 of 7 individuals
(14%) from Florida possessed haplotype 2, compared to 10
of 18 individuals (56%) from Mexico. Qualitatively, Texas
had high numbers of both haplotypes 2 and 3 (n=126 and
74, respectively), suggesting that it may receive migrants
from both areas. Due to small sample sizes in areas outside
of Texas, a more quantitative statistical assessment of stock
contribution was not possible.
Genetic Variation and Population Structure
When individual Common Snook, Largescale Fat
Snook, and unknowns were included in the initial Struc-
ture run, the ΔKstatistic suggested that there were two
genetic clusters (K=2). An examination of each group
under the K=2 scenario yielded the following ndings: (1)
257 of 259 individuals that were identied as Largescale
Fat Snook with mtDNA generally fell into a single cluster
(Figure 5); (2) 182 of 184 individuals that were identied
as Common Snook with mtDNA fell into the alternate
cluster; and (3) the estimated genetic divergence between
clusters was high and indicative of what might be expected
between congeneric species (F
ST
=0.183). The four
remaining individuals with an unknown species assign-
ment showed evidence of hybrid background. One of these
individuals (SN550) had the mtDNA haplotype of Larges-
cale Fat Snook but a genotype that indicated Common
Snook. Three additional individuals (SN31, SN522, and
SN573) had genotypes that indicated admixture (Q<0.7
for any genetic cluster) despite being assigned a diagnostic
mtDNA haplotype (Common Snook: n=2; Largescale
Fat Snook: n=1). Of the 112 individuals with no mtDNA
FIGURE 2. Length frequency (mm TL) of snook (all species combined)
captured during shery-independent sampling in Texas, 19802018.
TABLE 5. Results of logistic regression evaluating the impact of year
and latitude on presence/absence of snook in shery-independent gill nets
deployed during 19802018. Both variables contributed signicantly to
presence/absence in the model.
Term Estimate SE χ
2
P(>χ
2
)
Intercept 79.68 8.262 93.0 <0.0001
Year 0.06 0.004 215.2 <0.0001
Latitude 1.65 0.051 1,029.7 <0.0001
8ANDERSON ET AL.
data (i.e., no haplotype) included in the Structure analysis,
73 had genotypes that were consistent with cluster 1 (Lar-
gescale Fat Snook), while an additional 39 had genotypes
that were consistent with cluster 2 (Common Snook).
Results from the PCA supported those from the Struc-
ture analysis. Most of the variance observed in the
microsatellite data set was aligned with the rst principal
component (Figure 6). Clustering of individuals along this
axis generally coincided with taxonomic expectations based
on mtDNA haplotypes, with three notable exceptions. One
individual (SN550) had a genotype that clustered with
Common Snook but had a Largescale Fat Snook mtDNA
haplotype. An additional two individuals (SN522 and
SN573), both with mtDNA representative of Common
Snook, had genotypes that did not cluster tightly with either
group. All three individuals coincided with those identied
as potential hybrids in Structure. The fourth individual
identied as a potential hybrid by Structure (SN31) clus-
tered tightly with other Largescale Fat Snook in the PCA,
in contrast to the results from Structure.
Most microsatellite loci deviated from HWE, and this
was observed in both species, including deviation from
HWE at 7 of 8 loci in Largescale Fat Snook and at 6 of 8
loci in Common Snook (Table 8). As a result, both species
deviated signicantly from HWE across all loci combined
(Largescale Fat Snook: F
IS
=0.134, P<0.0001; Common
Snook: F
IS
=0.187, P<0.0001). Four pairs of loci also
deviated signicantly from linkage expectations after
adjustment for multiple tests (CUN14CUN19,CUN14
CUN16,CUN4ACUN22, and CUN4ACUN16). An
additional nine pairs of loci showed evidence of linkage
disequilibrium under a static alpha of 0.05 (with no Bon-
ferroni adjustment).
Examination of the ΔKstatistic from the species-speci-
c Structure analysis of Common Snook suggested that
there were four genetic clusters (K=4). The plot of Q-
scores from this model suggested a high degree of admix-
ture within most individuals and complex population
structure within and among sample areas (Figure 7). The
divergence among sample areas (Aransas Bay versus
Lower Laguna Madre) was small but signicant (F
ST
=
0.010, P=0.003). Both the multivariate PCA and the dis-
criminant analysis based on the rst 20 principal compo-
nents demonstrated clustering that validated observable
FIGURE 3. Mean latitude of the snook catch (all species combined) in shery-independent gill-net samples in Texas, 19802018. Gray lled circles
represent point means of each year. The dashed line is a trend line (linear regression) t to the data (regression parameters and t are reported in the
upper left corner).
TABLE 6. Mean values of temperature and salinity for samples that had zero catch ( =0) or positive catch (>0) of snook in spring and fall sampling
efforts. Analysis of variance was used to determine whether mean differences in catch were signicantly greater than zero.
Variable Snook catch =0 Snook catch >0FP
Mean spring temperature (°C) 26.8 27.9 19.6 <0.0001
Mean fall temperature (°C) 25.7 26.3 8.4 0.004
Mean spring salinity () 22.2 31.6 120.8 <0.0001
Mean fall salinity () 23.9 29.2 82.9 <0.0001
CHARACTERIZATION OF THE SNOOK SPECIES COMPLEX 9
differences between individuals assigned to Structure clus-
ters 14 (Figure 8). The chi-square test comparing Struc-
ture clusters to DAPC assignment was highly signicant
(χ
2
=182.4, df =9, P<0.0001).
The species-specic Structure analysis of Largescale Fat
Snook also suggested a complex population structure. The
ΔKstatistic implied that Kwas equal to 3, and as in Com-
mon Snook the plot of Q-scores from this model suggested a
high degree of admixture within most individuals and com-
plex population structure within and among populations
(Figure 9). The global F
ST
(among sample areas: Aransas
Bay, Upper Laguna Madre, and Lower Laguna Madre) was
small but signicant (F
ST
=0.006, P=0.005). The PCA and
DAPC both seemed to validate observable differences among
individuals assigned to Structure clusters 13 (Figure 10),
and the comparison of assignments from Structure and
DAPC was highly signicant (χ
2
=290.0, df =4, P<0.0001).
DISCUSSION
Snook Range Expansion and Abundance
Two different trends observed here suggest that the dis-
tribution and abundance of snook species in Texas have
changed dramatically in the 38-year span of this data set.
First, snook have increased in general abundance; there
was a signicant positive correlation with year in the logis-
tic regression, indicating a higher frequency of occurrence
in later years. In fact, the last 2 years of the observed per-
iod (2017 and 2018) had the two highest coastwide fre-
quencies of occurrence compared to all other years of the
study. Second, the distribution of snook has expanded
northward over the observed period at a rate of approxi-
mately 0.017 decimal degrees/year (between 1 and 2 km).
Latitude was in fact the most important variable in the
logistic regression of catch, and most catches occurred in
southern bays. However, examination of the relationship
between year and the mean latitude of catch demonstrated
a clear range expansion of snook into northern latitudes
areas where these species were only rarely observed prior
to 2010. This range expansion has coincided with a warm-
ing climate and the development of tropical conditions in
temperate and subtropical waters (Staten et al. 2018),
resulting in the range expansion of tropical marine sh
species into temperate zones worldwide (e.g., Figueira and
Booth 2010; Nakamura et al. 2013; Verges et al. 2014;
Heck et al. 2015). In this context, perhaps the most parsi-
monious conclusion is that the northward expansion of
snook species in Texas is driven by warming trends that
are also likely to sustain this new distribution. With that
TABLE 7. Basic Local Alignment Search Tool (BLAST) results for each of the 21 haplotypes observed in 581 samples of Centropomus spp. collected
from the Texas coast, including the number of individuals (N) that carried each haplotype, the top matching species in GenBank, the total BLAST
score and percent identical of that match, and the GenBank accession number of the top matching sequence.
Haplotype NTop match Total score % Identical Accession number
1 328 Largescale Fat Snook
C. mexicanus
773 99.07 KU745737
2 134 Common Snook C. undecimalis 739 97.49 HQ731428
3 86 Common Snook 745 97.72 U85012
4 5 Common Snook 739 97.49 U85012
5 1 Common Snook 739 97.49 U85012
6 1 Common Snook 734 97.26 HQ731428
7 1 Mexican Snook C. poeyi 747 98.15 U85014
8 6 Common Snook 734 97.26 HQ731428
9 2 Common Snook 739 97.49 U85012
10 1 Common Snook 739 97.49 U85012
11 1 Largescale Fat Snook 767 98.84 KU745737
12 1 Largescale Fat Snook 767 98.84 KU745737
13 2 Largescale Fat Snook 767 98.84 KU745737
14 1 Common Snook 739 97.49 U85012
15 1 Common Snook 734 97.26 HQ731428
16 1 Common Snook 734 97.26 HQ731428
17 1 Largescale Fat Snook 767 98.84 KU745737
18 3 Largescale Fat Snook 767 98.84 KU745737
19 3 Mexican Snook 752 98.38 U85014
20 1 Common Snook 734 97.26 HQ731428
21 1 Common Snook 739 97.49 U85012
10 ANDERSON ET AL.
FIGURE 4. Maximum likelihood condensed tree of Centropomus taxa (see Table 3) inferred from 428 bp of 16S mitochondrial DNA sequence data.
Values on branches are bootstrap values. Nodes with bootstrap values less than 50% were collapsed into polytomies. Individual haplotypes observed
in this study are designated with Haplotype #and correspond to the haplotypes listed in Table 7.
CHARACTERIZATION OF THE SNOOK SPECIES COMPLEX 11
said, the current data are inadequate to distinguish the
effects of long-term climate change versus the positive
impact of restrictive shery regulations that were
implemented in the 1980s, including a ban on commercial
entanglement gears in Texas, a daily bag limit of 1 snook/
d, and a conservative slot limit of 610711 mm (2428 in).
FIGURE 5. Cluster results from Structure analysis of all snook individuals genotyped in this study, sorted by mitochondrial DNA haplotype results
from the Basic Local Alignment Search Tool (generally by species). Individuals with no haplotype available were sorted by Q-scores from Structure.
The four labeled individuals showed evidence of admixture: SN550 had a Largescale Fat Snook Centropomus mexicanus haplotype and a Common
Snook C. undecimalis genotype (Q>0.7); SN31 had a Largescale Fat Snook haplotype and a hybrid genotype (Q<0.7); and SN522 and SN573 had a
Common Snook haplotype and a hybrid genotype (Q<0.7).
FIGURE 6. Principal components analysis of snook microsatellite genotype data. Each color represents individuals whose mitochondrial DNA
haplotype implied a taxonomic designation of Largescale Fat Snook Centropomus mexicanus (black) or Common Snook C. undecimalis (red). Three
individuals (labeled) that we identied as putative hybrids with Structure also clustered with the alternate species group (SN550) or did not cluster
tightly with either group (SN522 and SN573).
12 ANDERSON ET AL.
Previous literature suggests that the historical distribution
of Common Snook included areas as far north as Galve-
ston Bay (Rivas 1986; Pope et al. 2006); thus, the combi-
nation of favorable climate, conservative shing
regulations, and perhaps other factors might be driving
the range expansion of Centropomus. However, the
observed presence of snook in Sabine Lake in this study
implies that they are expanding beyond even their histori-
cal distribution, which emphasizes the importance of cli-
mate in the observed expansion.
In addition to year and latitude, salinity and tempera-
ture appear to inuence the probability of encountering a
snook in Texas. Snook are associated with warmer, saltier
water. From a qualitative standpoint, temperature may be
the most important factor driving the distribution of
snook in Texas, as these species are susceptible to cold dis-
turbances that can signicantly increase mortality (Adams
et al. 2012; Stevens et al. 2016). The likelihood of snook to
be found more commonly in southern latitudes in Texas
underscores that these relatively warmer bays historically
have proven to be more suitable habitat for Centropomus
than cooler northern bays.
Snook Species Composition and Hybridization
Based on mitochondrial haplotypes and microsatellite
genotypes, the Largescale Fat Snook is the most com-
monly encountered snook in Texas (63%), followed clo-
sely by Common Snook (36%) and occasional
observations of Mexican Snook (0.4%). To our knowl-
edge, this is the rst study documenting Mexican Snook
as far north as Texas. A systematic review by Rivas
(1986) suggested that this species is generally found from
Tampico, Tamaulipas, Mexico, southward to Belize;
hence, this species has been generally thought to have the
narrowest range of any snook species (Kubicek et al.
2018). We observed two specimens from Lower Laguna
Madre and a third specimen in Aransas Bay, suggesting
occasional incursions of this species into Texas. Given the
distance between Tampico and Aransas Bay (~630 km),
these data indicate that the known natural range of Mexi-
can Snook may have expanded into more northerly areas
than previously noted.
These data also suggest that Common Snook and Lar-
gescale Fat Snook may occasionally hybridize in Texas.
To our knowledge, evidence for wild hybrids has not been
demonstrated (Tringali et al. 1999a), but hybrid snook
(Common Snook ×Smallscale Fat Snook) have been pro-
duced in the laboratory (Ferraz et al. 2012), indicating the
potential for occasional hybridization events in the wild.
The species diversication in Centropomus was a relatively
recent event (Tringali et al. 1999b); therefore, it is reason-
able to assume that viable hybrids could occasionally be
produced, particularly in relatively sparsely populated
TABLE 8. Expected heterozygosity (H
e
), F
IS
, and the P-value of F
IS
at
each microsatellite locus assayed in this study for Largescale Fat Snook
and Common Snook. The P-values in bold italics indicate that loci were
signicantly different from the null expectation of F
IS
=0.
Locus
Largescale Fat Snook Common Snook
H
e
F
IS
PH
e
F
IS
P
CUN4A 0.808 0.058 <0.0001 0.74 0.0292 0.9641
CUN9 0.816 0.003 0.9268 0.93 0.4147 <0.0001
CUN14 0.799 0.096 <0.0001 0.86 0.114 <0.0001
CUN16 0.577 0.208 0.0017 0.85 0.0555 <0.0001
CUN17 0.669 0.151 <0.0001 0.77 0.1997 <0.0001
CUN19 0.661 0.362 <0.0001 0.69 0.0914 0.2134
CUN20 0.789 0.085 <0.0001 0.78 0.0876 0.0065
CUN22 0.889 0.163 <0.0001 0.95 0.4185 <0.0001
FIGURE 7. Results from Structure cluster analysis of Common Snook genotypes. Individuals are aligned on the x-axis and grouped by sample
location (Aransas Bay and Lower Laguna Madre, Texas). The four colors represent the proportions of contribution to each individual from the K=4
genetic clusters observed in the Structure analysis.
CHARACTERIZATION OF THE SNOOK SPECIES COMPLEX 13
areas at the edge of the speciesrange (Pfennig et al.
2016). Nevertheless, the nding of hybrids in this data set
could also be the result of the poor statistical resolution
expected from a small genetic data set. This question
could be answered with a larger genetic or genomic data
set, as the number of loci employed here (n=8) is proba-
bly not sufcient to conclusively resolve hybrids. It should
be noted also that one of the putative hybrids identied
by Structure clustered with its expected species in the PCA
model, which underscores the weakness of these data in
truly identifying hybrids with any reliability.
Anal note regarding the taxonomy of Texas snook
is the unexpected lack of Smallscale Fat Snook.This
contrasts with the results of Martin and King (1991),
FIGURE 8. Results from multivariate clustering of individual Common Snook Centropomus undecimalis (PCA =principal components analysis;
DAPC =discriminant analysis of principal components; DA =discriminant analysis). The gure on the left presents a PCA of microsatellite
genotypes. Individuals are colored based on their assigned cluster from Structure analysis (clusters 14). The x- and y-axes represent the rst and
second axes of ordination in the PCA. The gure on the right is a DA of the rst 20 principal components, with individuals again colored based on
their assigned genetic cluster from Structure.
FIGURE 9. Results from Structure cluster analysis of Largescale Fat Snook genotypes. Individuals are aligned on the x-axis and grouped by sample
location (Aransas Bay, Upper Laguna Madre, and Lower Laguna Madre, Texas). The three colors represent the proportions of contribution to each
individual from the K=3 genetic clusters observed in the Structure analysis.
14 ANDERSON ET AL.
who observed both juveniles and advanced-stage female
Smallscale Fat Snook near the mouth of the Rio
Grande. There are two potential explanations for this
disparity. First, the small group observed by Martin
and King (1991) could have been transient. To our
knowledge, prior to the ndings of Martin and King
(1991) there were no records of Smallscale Fat Snook in
the literature from Texas. These historical catches
occurred during 19861989, a time period that was
bookended by two historic freezes (in February and
December 1989), which killed an estimated 17 million
sh combined (TPWD, unpublished data). It is possible
that a small but established population of Smallscale
Fat Snook near the Rio Grande was extirpated as a
result of these freeze events. A second, more plausible
explanation is that the taxonomic designation of Smalls-
cale Fat Snook in the earlier study was incorrect and
that the specimens described by Martin and King (1991)
were actually Largescale Fat Snook. Morphologically,
these two species are very similar and can only be sepa-
rated using differences in lateral line scale counts and
size (Rivas 1986). These species are also very closely
related biochemically, with approximately 0.5% diver-
gence at the mtDNA 16S locus (Tringali et al. 1999b).
Anecdotally, the taxonomic designation of the C.
parallelus/mexicanus group in Texas has historically been
equivocal; as such, it is likely that the specimens
described by Martin and King (1991) and the specimens
described in the current work are in actuality members
of the same species.
Population Genetics of Snook in Texas
For the two more common snook species in Texas,
analysis of genetic population structure suggested equivo-
cal results. In both cases (Largescale Fat Snook and Com-
mon Snook), the Structure analysis and multivariate
statistical analyses suggested the presence of multiple
genetic clusters within species, but there was little actual
geographic structure that could be associated with these
clusters. The complex admixture pattern observed in pop-
ulation analyses was coupled with very high rates of
genetic disequilibrium. The weak and irregular differentia-
tion among individual cluster assignments, coupled with
deviation from equilibrium expectations, has been identi-
ed before in marine shes and has previously been attrib-
uted to chaotic genetic patchiness caused by small
effective population size and localized kin aggregations
(Selkoe et al. 2006; Selwyn et al. 2016). Simulation studies
have suggested that chaotic genetic patchiness is driven by
(1) genetic drift created by small local effective population
FIGURE 10. Results from multivariate clustering of individual Largescale Fat Snook Centropomus mexicanus (PCA =principal components analysis;
DAPC =discriminant analysis of principal components; DA =discriminant analysis). The gure on the left presents a PCA of microsatellite
genotypes. Individuals are colored based on their assigned cluster from Structure analysis (clusters 13). The x- and y-axes represent the rst and
second axes of ordination in the PCA. The gure on the right is a DA of the rst 20 principal components, with individuals again colored based on
their assigned genetic cluster from Structure.
CHARACTERIZATION OF THE SNOOK SPECIES COMPLEX 15
sizes and (2) collective dispersal at the larval phase (Bro-
quet et al. 2013). Selwyn et al. (2016) noted that patchiness
in the marine realm is generally characterized by very low
but signicant genetic divergence among areas as well as
deviation from equilibrium expectations, both of which
were observed for Largescale Fat Snook and Common
Snook in the present study. The patchiness pattern
observed in microsatellite data could be further reinforced
by multiple factors associated with the distribution of
snook species at the edge of their population ranges. First,
Texas represents the leading northern edge of the snook
speciesrange in the western Gulf of Mexico, and previous
studies on population leading edges have also observed
patterns of disorganized but signicant genetic divergence
over small spatial scales (Eschbach et al. 2014; Shirk et al.
2014; Hagen et al. 2015; Tollefsrud et al. 2016). Second,
the demographic data reported here suggest persistent
expansion and contraction of snook in Texas, and these
pulses of abundance at the edge of the speciesrange can
be expected to be coupled with complementary pulses of
gene ow. Persistent gene ow into sparsely occupied
areas at the edge of the range could drive genetic patterns
similar to that expected from repeated founder events,
resulting in genetic disequilibrium. In the case of Common
Snook in particular, a previous study noted spawning
ground delity in this species (Adams et al. 2009), which
would also be expected to reinforce a pattern of genetic
patchiness (Selwyn et al. 2016). Finally, it should be noted
that the unique reproductive strategy of snook species
may also play a role in the population genetic patterns
observed here. Snook are protandric hermaphrodites, with
all individuals beginning life as males but switching to
females upon reaching maximum size (Vidal-López et al.
2019 and references therein). Sequential hermaphroditism
has the potential to impact effective population size by
limiting the number of individuals that successfully spawn
as one sex or the other, but it is poorly understood how
this strategy may impact genetic drift. A previous study of
sequential hermaphrodites suggested that species exhibit-
ing this strategy are not more genetically structured than
other species (Chopelet et al. 2009), and there is no evi-
dence from previous snook population genetics work that
would indicate an impact of this unique reproductive
strategy (Tringali and Bert 1996).
Taken together, although these patterns suggest a lack
of temporally stable population structure over a broad
geographic scale in Texas, neither Largescale Fat Snook
nor Common Snook can be reasonably characterized as
entirely panmictic. It is more likely that while some degree
of genetic divergence might be expected among local pop-
ulations in larvae or younger individuals, over broader
spatial scales both of the predominant snook species in
Texas probably represent single genetic stocks. As these
populations continue to expand and as more individuals
become involved in localized spawning events (i.e.,
increased effective population size), this pattern may lose
some effect over time since genetic disequilibrium would
be expected to break down.
Management Implications
The low but signicant values of F
ST
between areas
and the lack of geographical resolution in the Structure
models imply that both Largescale Fat Snook and Com-
mon Snook in Texas exist as single genetic stocks. As pre-
viously discussed, the statistical signicance of measured
divergence among areas is more likely driven by a pattern
of chaotic genetic patchiness reinforced by low effective
population size at the edge of each speciesrange than by
traditional population structure (i.e., isolated populations).
Thus, we recommend that each of the two predominant
snook species in Texas be managed as a single stock unit.
One caveat to this recommendation is that the genetic
data set used to generate this interpretation was very small
(n=8 loci). New genomic-based methods (e.g., Peterson
et al. 2012) allow for simultaneous locus discovery and
genotyping of potentially thousands of genetic markers for
a reduced per-sample cost. These genomicmethods have
frequently demonstrated a pattern whereby small numbers
of loci (presumably under directional selection) show ele-
vated divergence relative to the genomic mean in marine
shes (Portnoy et al. 2015; Hollenbeck et al. 2018; Ander-
son et al. 2019), a nding that underscores the risk of
overinterpreting smaller genetic data sets. Nevertheless, in
the absence of such genomic data, the present data suggest
a tentative single-stock management unit for each of the
predominant snook species in Texas.
Large-scale changes in climate present challenges for the
management of wild species (Hulme 2005; Thomas et al.
2010), and range expansion can be included among these
challenges as tropical species can be generally expected to
advance poleward (Lawson et al. 2012). It has been sug-
gested that management of local populations should shift
toward facilitation of their inevitable range expansion
(Galatowitsch et al. 2009; Lawson et al. 2012). Although
Texas historically supported a commercial snook shery,
the shery crashed after the 1920s; it has been hypothesized
that this crash was driven by an interplay of freezes and
overshing (Matlock and Osburn 1987), followed by low
abundance and erratic recruitment (Pope et al. 2006).
Warming of inshore water temperatures over the last several
decades, coupled with no commercial shing pressure and
limited recreational pressure, has resulted in the increasing
abundance and range of snook species in Texas. In this con-
text, management efforts should be focused on further facil-
itating this expansion by (1) increasing connectivity between
suitable habitat patches and (2) enhancing population sur-
vival (Lawson et al. 2012). Regarding the latter, Stevens et
al. (2016) noted that snook population resilience in Florida
16 ANDERSON ET AL.
after cold events was variable across estuaries and that this
variability in resilience was likely tied to estuary geomor-
phology and the accessibility of thermal refugia. Using the
Stevens et al. (2016) study as a model for the expected resili-
ence of snook in Texas after cold events, one might expect
that bay-specic variability in available thermal refugia and
localized extinction during extreme cold are the most
important challenges to snook expansion. Thus, efforts to
enhance population survival may have the most promise for
facilitating snook expansion in Texas. In 2005, the TPWD
enacted regulations to close easily accessible, deepwater
refugia to shing during extreme freeze events. The spatial
and temporal extent of these closures could be expanded to
include known snook habitat areas that are not currently
protected. Additionally, snook species in Texas represent
excellent candidates for future stock enhancement efforts,
as mortality caused by persistent acute cold events could
potentially be offset by the release of captive-raised juve-
niles. In any event, the increasing abundance and range of
snook species in Texas represent increased opportunity for
anglers, and measures to facilitate this expansion should
balance conservation and recreational access. This work
represents a baseline upon which future assessments of
snook can be anchored, and management efforts going for-
ward should take into account the diversity and distribution
of snook species in coastal waters of Texas.
ACKNOWLEDGMENTS
The following individuals assisted with preparation and/or
internal review of this work: M. Fisher of TPWD, A. Landry
formerly of Texas A&M Galveston, C. Bird of Texas A&M
Corpus Christi, and M. Iacchei of Hawai'iPacicUniversity.
We thank the various teams within TPWD, TAMU-Corpus
AndTAMU-Galvestonthatassistedwithcollectionand
identication of eld specimens and lab work, especially C.
Barnes. This work was supported by a grant from the Sport
Fish Restoration Program (U.S. Fish and Wildlife Service).
A. González was supported by a fellowship from the Hispa-
nic Leaders in Agriculture and the Environment. C. Chapa
was supported by the Southeast Texas Sportshing Associa-
tion and McDaniel Charitable Foundation. There is no con-
ict of interest declared in this article.
ORCID
Alin González https://orcid.org/0000-0003-4041-0496
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SUPPORTING INFORMATION
Additional supplemental material may be found online
in the Supporting Information section at the end of the
article.
CHARACTERIZATION OF THE SNOOK SPECIES COMPLEX 19
... The feeding ecology of snooks has been well documented in several parts of their range highlighting the breadth of their diets which include fish, shrimp and crabs (Blewett et al. 2006, Boucek and Rehage 2013, Contente et al. 2009, Lira et al. 2017, Getz et al. 2021). In the northwestern Gulf of Mexico (GOM), 4 species of snook have been previously observed (Common Snook Centropomus undecimalis, Largescale Fat Snook C. mexicanus, Smallscale Fat Snook C. parallelus and Mexican Snook C. poeyi), although recent genetic work has categorized the Fat Snook in the northwestern GOM as C. mexicanus and questioned the validity of previous C. parallelus observations in the area (Anderson et al. 2020, Seyoum et al. 2022. ...
... Since the collapse of the fishery, snook have been heavily regulated in Texas allowing for a small population to persist into the early 2000s (Pope et al. 2006). Recently, snook abundance has increased dramatically and range shifts have been documented northward along the Texas coast (Chapa 2012, Anderson et al. 2020. Poleward expansion has also been documented in other parts of their range including the northeastern GOM (Purtlebaugh et al. 2020) and the western Atlantic Ocean . ...
... Given the context of increased snook abundance and dispersion, effective management efforts will need to draw on the small but growing body of work on snook biology and ecology in Texas (Chapa 2012, Huber et al. 2014, Anderson et al. 2020, Getz et al. 2021. For example, previous work has focused on characterizing populations and nursery habitat of snook in the Rio Grande River (Rio Grande; Chapa 2012, Huber et al. 2014. ...
Article
Snook (Centropomus spp.) support important fisheries in the Gulf of Mexico and populations are increasing in Texas. However, a primary snook nursery, the Rio Grande River, remains one of the most imperiled rivers in North America. In 2001—2002, low flows led to the formation of a sandbar and the complete blockage of the Rio Grande, eliminating exchange with coastal waters. Here, data collected by the Texas Parks and Wildlife Department were used to determine how the Rio Grande closure affected juvenile and subadult snook and the estuarine community. Seine and trawl samples were collected prior to (1992—1997) and during (2001—2002) the closure. Snook size, seasonality and distribution were assessed. Additionally, multivariate analyses were used to describe community composition before and during the Rio Grande closure. Juvenile and subadult abundance was high and snook were observed year—round, suggesting that the Rio Grande functions as an important nursery. River mile (i.e., distance in miles from the mouth of the river) had a significant effect on snook abundance, with snook preferring upstream portions of the river with low salinity. Fish/invertebrate community results highlighted shifts in assemblage with decreased species richness and increased community dominance during the river closure, as well as decreases in abundance of species like white shrimp. Although snook persisted in the Rio Grande during the closure, community—wide impacts were observed and potentially impacted snook indirectly. As south Texas becomes more arid, managers may need to consider the likelihood of similar events affecting important sport fish like snook in the future.
... The feeding ecology of snooks has been well documented in several parts of their range highlighting the breadth of their diets which include fish, shrimp and crabs (Blewett et al. 2006, Boucek and Rehage 2013, Contente et al. 2009, Lira et al. 2017, Getz et al. 2021). In the northwestern Gulf of Mexico (GOM), 4 species of snook have been previously observed (Common Snook Centropomus undecimalis, Largescale Fat Snook C. mexicanus, Smallscale Fat Snook C. parallelus and Mexican Snook C. poeyi), although recent genetic work has categorized the Fat Snook in the northwestern GOM as C. mexicanus and questioned the validity of previous C. parallelus observations in the area (Anderson et al. 2020, Seyoum et al. 2022. ...
... Since the collapse of the fishery, snook have been heavily regulated in Texas allowing for a small population to persist into the early 2000s (Pope et al. 2006). Recently, snook abundance has increased dramatically and range shifts have been documented northward along the Texas coast (Chapa 2012, Anderson et al. 2020. Poleward expansion has also been documented in other parts of their range including the northeastern GOM (Purtlebaugh et al. 2020) and the western Atlantic Ocean . ...
... Given the context of increased snook abundance and dispersion, effective management efforts will need to draw on the small but growing body of work on snook biology and ecology in Texas (Chapa 2012, Huber et al. 2014, Anderson et al. 2020, Getz et al. 2021. For example, previous work has focused on characterizing populations and nursery habitat of snook in the Rio Grande River (Rio Grande; Chapa 2012, Huber et al. 2014. ...
Article
Full-text available
Snook (Centropomus spp.) support important fisheries in the Gulf of Mexico and populations are increasing in Texas. However, a primary snook nursery, the Rio Grande River, remains one of the most imperiled rivers in North America. In 2001—2002, low flows led to the formation of a sandbar and the complete blockage of the Rio Grande, eliminating exchange with coastal waters. Here, data collected by the Texas Parks and Wildlife Department were used to determine how the Rio Grande closure affected juvenile and subadult snook and the estuarine community. Seine and trawl samples were collected prior to (1992—1997) and during (2001—2002) the closure. Snook size, seasonality and distribution were assessed. Additionally, multivariate analyses were used to describe community composition before and during the Rio Grande closure. Juvenile and subadult abundance was high and snook were observed year—round, suggesting that the Rio Grande functions as an important nursery. River mile (i.e., distance in miles from the mouth of the river) had a significant effect on snook abundance, with snook preferring upstream portions of the river with low salinity. Fish/invertebrate community results highlighted shifts in assemblage with decreased species richness and increased community dominance during the river closure, as well as decreases in abundance of species like white shrimp. Although snook persisted in the Rio Grande during the closure, community—wide impacts were observed and potentially impacted snook indirectly. As south Texas becomes more arid, managers may need to consider the likelihood of similar events affecting important sport fish like snook in the future.
... The occurrence of warmer, mild winters is contributing to elevated water temperatures, making previously cooler regions suitable for tropical and subtropical fishes (Osland et al., 2021;Stevens et al., 2021). Species limited to warmer climates may find the new conditions favorable, allowing them to expand their range poleward (Anderson et al., 2019;Stevens et al., 2021;Vergés et al., 2014). Warm thermal refuges may also serve as a mechanism for range expansion, enabling subtropical and tropical species to persist within an expanded range despite stochastic winter extremes (Blewett & Stevens, 2014;Boucek et al., 2017;Stevens et al., 2021). ...
... A tropical species that has recently expanded its range poleward is the common snook Centropomus undecimalis, referred to as snook hereafter (Figs. 1 and 2; Anderson et al., 2019;Purtlebaugh et al., 2020). Historically, their range along Florida's Gulf Coast extended northward to Tarpon Springs (Taylor et al., 1998;Trotter et al., 2021). ...
Article
As climate change leads to rising temperatures, tropical fishes such as common snook Centropomus undecimalis (hereafter snook) are expanding poleward, necessitating an understanding of their ability to tolerate cold temperatures and rapid temperature drops. To investigate this ability, we conducted chronic lethal minimum (CLmin) and critical thermal minimum (CTmin) trials in the laboratory using fish collected from a latitudinal gradient along the Gulf of Mexico coast of Florida. Individual lower lethal temperatures ranged from 7.9 to 10.5 °C. On average, the northernmost snook population exhibited the most cold hardiness, ceasing feeding at 14.4 °C and dying at 8.6 °C. These thermal endpoints were lower than for populations collected farther south and are significant in the context of passing cold fronts. In the CTmin trial that reduced water temperatures more quickly, and is sub-lethal, snook lost equilibrium at temperatures almost 2 °C warmer than those in our chronic trial, underscoring the necessity of simulating realistic cold events to fully understand species’ cold tolerance. These findings help managers predict the effects of variation in timing and extent of severe cold events on snook across different estuaries, allowing for targeted management approaches should conditions warrant actions to facilitate population recovery. Metrics associated with a species’ cold hardiness can inform climate modeling, fisheries management, and freshwater inflow regulations affecting thermal refugia, aiding in the management and conservation of tropical fish populations in the face of global climate change.
... The expansion of its distribution range may be related to the intensification of the warm Brazil Current [5]. In the Northern Hemisphere, the abundance of snooks (Centropomus spp.) in previously unoccupied areas has been observed, and this has been associated with a trend of warming coastal waters [60,61]. A warming trend in Florida water temperatures has been observed in the last decade. ...
... Later, the limit was reduced to three snooks per day with a size limit of 51-71 cm TL. In 1995, the catch limit was reduced to one snook per day, with a size restriction of 61-71 cm TL [60,72]. In Florida, commercial fishing was banned in 1957 [114]. ...
Article
Common Snook (Centropomus undecimalis) is widely distributed across the tropical Atlantic Coasts and has a significant economic impact. This review aims to assess the knowledge status of common snook, contributing significantly to the development of sustainable aquacultural practices. The review was conducted using Web of Science and Google Scholar to identify scientific articles analysing the biology and ecology, the status of snook fisheries and developments in aquaculture production. Researchers in the USA, Mexico and Brazil have conducted 37.95%, 25.52% and 26.21% of published research, respectively, focusing mainly on reproduction in natural environments, status of fisheries and aquaculture production. From an environmental perspective, it is essential to understand the factors impacting C. undecimalis populations. Climate change effects and human alterations to river courses pose significant threats. In addition to fishing regulations, there is a promising potential for further fisheries research and to deepen the understanding of the life cycle to support the implementation of sound aquacultural practices to replenish exploited stocks and to develop commercial aquaculture. Currently, C. undecimalis populations in Mexico and the United States are overexploited, focusing on México, which now has a management plan. Advances in controlled reproduction and larval development have been made. However, many challenges remain unclear, such as larval conditions that continue to represent a bottleneck to block continuous and large-scale aquaculture production, larval nutrition, disease management and culture systems. However, aquaculture holds the potential to enhance the sustainability of this species by reducing fishing pressure and aiding population recovery.
... They are commonly known as snooks or robalos in various regions of Central and South America where they have great commercial importance (Vergara-Chen 2014). Snooks have a high tolerance to low salinity but are vulnerable to temperature extremes (Tringali et al. 1999;Anderson et al. 2020). The morphology of centropomid species is conservative, with strong similarities among species. ...
... The phylogenetic relationships between these families are still a subject of debate. Centropomidae has been proposed to be sister to Latidae (Tringali et al. 1999;Li et al. 2011;Betancur-R et al. 2013Betancur-R et al. , 2017Carvalho-Filho et al. 2019;Anderson et al. 2020;Figueiredo-Filho et al. 2021), or Sphyraenidae (Near et al. 2013;Mirande 2017;Rabosky et al. 2018;Girard, Davis, and Smith 2020). Latidae, with three extant genera, is distributed in Indo-West Pacific and African freshwater basins, often exhibiting endemism to specific lakes (Otero 2004). ...
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Aim Amphiamerican New World fishes provide a unique opportunity to explore the impact of geological processes and the formation of geographic barriers on biological diversification across both spatial and temporal dimensions. We employed phylogenetic and biogeographic methods to assess the impact of the emergence of the Isthmus of Panama on the evolutionary history of snooks. Location Eastern tropical Pacific and western Atlantic Oceans. Methods Bayesian methods were used for phylogenetic inference and divergence time estimation, incorporating the fossil record of Carangidae, Centropomidae, Istiophoriformes, Latidae and Sphyraenidae to establish a timeline using the methods of stratigraphic intervals for node calibration density specification. Biogeographic models were fitted to test the hypothesis that transisthmian vicariant events are coeval with the Isthmus closure. To estimate ancestral range probability and perform stochastic mapping, we utilised BioGeoBears and the parameters from the best‐fitting model. This allowed us to estimate the quantity and kind of biogeographical events. Results Our results suggest a sister relationship between Centropomidae and Sphyraenidae with a common ancestor that originated in the Upper Cretaceous (~78.51 Ma). The biogeographic model BAYAreaLIKE + j indicated speciation in sympatry and dispersal (founder effect) as the primary modes of speciation in the genus Centropomus . The dispersion in the family Centropomidae was estimated from the Tropical Eastern Pacific to the tropical western Atlantic since the Oligocene. Main Conclusions The alignment of divergence times with ancestral species distributions suggests a possible synchrony between the current distribution in Centropomus species and the processes of the formation of the Isthmus of Panama during the Miocene. However, the evidence of only two transisthmic pair suggests that this event was not determinant in allopatric interbasin speciation. Furthermore, recent diversification events within each basin imply an influence of post‐closure environmental conditions on the evolution of this group of fishes.
... A warming climate and a trend towards milder winters is probably the other major factor contributing to increased abundance of spot along the Texas coast. Several other species of warm-temperate and subtropic fishes have also increased in abundance along the Texas coast in the last 40 years, including ladyfishes (Elops spp., Williford et al. 2022), gafftopsail catfish (Bagre marinus (Mitchill, 1815)), Cates et al. 2023), common snook [Centropomus undecimalis (Bloch, 1792] (Anderson et al. 2019), and gray snapper [Lutjanus griseus (Linnaeus, 1758)] (Tolan and Fisher 2009;Anderson et al. 2022). Warmer climate may benefit juvenile fishes by lengthening the growing season and increasing food resources (Barrow et al. 2018;Huss et al. 2019;Kotowych et al. 2023). ...
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Spot (Leiostomus xanthurus) is a species in the family Sciaenidae that occurs in the western Atlantic and Gulf of Mexico. Abundance and life history of spot in the Gulf of Mexico have been poorly documented in comparison with the Atlantic component of its distribution. Therefore, we used 38 years (1986–2023) of fishery-independent data collected with bag seines, gill nets, and bay and offshore trawls to assess the long-term trends in abundance and length of spot and assess juvenile growth on the Texas coast. The abundance of spot increased along the Texas coast as indicated by positive trends observed in annual trawl and gill net catch-per-unit effort (CPUE). Mean annual total length (TL in mm) of spot also increased over the 38-year time series in each sampling gear. Spot in Texas begin to recruit into fishery-independent bag seines and trawls in January at approximately 22 mm TL and grow at a rate of 10.6 mm TL/month (0.35 mm TL/day) to approximately 138 mm TL by December. Analysis of monthly growth of juvenile spot through the time series suggests that growth rates have increased in more recent years, and year was a significant predictor that accounted for 34% of the variation in annual growth. Increasing catch, size, and growth rates could all be driven by the decline of commercial shrimp trawling in Texas coupled with a warmer climate in the Gulf of Mexico.
... The Common Snook Centropomus undecimalis is a subtropical-tropical species that supports popular fisheries throughout its range, including South America (e.g., Ferreira et al. 2019;Gonzalez et al. 2019), Central America (e.g., Perera-García et al. 2011;Hernández-Vidal et al. 2014), the Caribbean Sea (e.g., Aliaume et al. 2000;Adams and Murchie 2015), and the southern United States (e.g., Young et al. 2020;Trotter et al. 2021). The geographic range of Common Snook is expanding farther north in Texas and Florida, where it supports new fisheries (Anderson et al. 2020;Purtlebaugh et al. 2020). As early juveniles, Common Snook are obligate users of mangrove creeks, coastal ponds, and other transitional habitats, and they are considered a flagship umbrella species for the conservation of coastal wetlands in Florida (Wilson et al. 2023). ...
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Objective We investigated the validity of daily age estimates for juvenile Common Snook Centropomus undecimalis by using sectioned and sanded sagittal otoliths. Methods Common Snook have a protracted spawning season, which is problematic for validation of daily ages because a hatch date—needed to calculate age—cannot be reasonably assigned like it can for species with a short spawning period (<30 days). To help overcome this, two readers independently counted presumed daily increments in otoliths collected from hatchery‐reared Common Snook of known age (100–240 days; n = 91). Result Differences between known ages and those estimated from otoliths were small (mean absolute difference = 3.4 days) for individuals aged 100 days, but these differences increased after 100 days, mainly due to the crowding of increments along the otolith margin. Underestimation of ages was 8% at 120 days, 29% at 180 days, and 36% at 240 days. Conclusion Ideally, analyses based on counts of daily increments in Common Snook otoliths should be limited to fish with an age of 100 days or younger.
... One conclusion from this finding may be that although no significant temporal trends were observed, unusually high catch rates during the most recent years in the time series might indicate the beginning of an expansion of Atlantic Tripletail abundance in Texas estuaries. Notably, increases in other warmwater fish species have been observed in Texas during recent years (e.g., Gray Snapper: Tolan andFisher 2009, Anderson et al. 2022; snook Centropomus spp.: Anderson et al. 2020, Getz et al. 2021, suggesting a consistent trend of tropicalization of estuarine community assemblages occurring throughout Texas. Increasing catch rates of tropical predatory species, in general, may represent increasing opportunities for anglers in Texas and the northern GOM, which should be coupled with increasing effort by fishery biologists to monitor and manage these species sustainably through fishing regulations. ...
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Objective The Atlantic Tripletail Lobotes surinamensis is a globally distributed subtropical and tropical fish species that inhabits estuaries throughout the northern Gulf of Mexico (GOM), particularly during warm months. Little is known about distribution and residency patterns within estuaries, as the species is rarely caught in the recreational fishery, and virtually no commercial fishery exists for the species in the GOM. Methods We used data from a long‐term fishery‐independent gill‐net survey to model estuarine distribution throughout Texas and to relate environmental variables to the Atlantic Tripletail catch. Result Although there were no observable temporal trends in catch over the time series (1990–2022), the most recent 6 years included record catch in six of the 10 major Texas estuaries, possibly indicating a recent pulse in abundance. Catch throughout the time series was spatially aggregated in a small number of “hot spots” observed coastwide. Latitude was the best predictor of catch, although wind fetch and wind aspect (wind direction in relation to shoreline direction) were important predictors, and catch was highest near GOM inlets. The Texas Parks and Wildlife Department gill‐net sampling program caught a range of Atlantic Tripletail between 171 and 880 mm total length, indicating a potential gear bias against juveniles. Conclusion Despite this gear bias, these data shed light on the factors that drive Atlantic Tripletail estuarine distribution and abundance in the northwestern GOM. Wind‐driven passive movements in the estuary, combined with active selection of polyhaline habitats near GOM inlets, might be primary drivers of Atlantic Tripletail catch, thus supporting findings from previous studies.
... Similar taxonomic issues are observed in the family Centropomidae because of their remarkable conservative morphology, as presently reported in samples morphologically identified as C. pectinatus that actually corresponded to C. undecimalis [59] (Fig 5). In Table 2, we highlight the main morphological characteristics that contributed to the misidentification of the analyzed specimens. ...
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The efficiency of the DNA barcoding relies on sequencing fragment of the Cytochrome C Subunit I (COI) gene, which has been claimed as a tool to biodiversity identification from distinct groups. Accordingly, the goal of this study was to identify juvenile fish species along an estuary of Caeté River in the Brazilian Blue Amazon based on. For this purpose, we applied the DNA barcoding and discuss this approach as a tool for discrimination of species in early ontogenetic stages. A 500-bp fragment was obtained from 74 individuals, belonging to 23 species, 20 genera, 13 families and seven orders. About 70% of the 46 haplotypes revealed congruence between morphological and molecular species identification, while 8% of them failed in identification of taxa and 22% demonstrated morphological misidentification. These results proved that COI fragments were effective to diagnose fish species at early life stages, allowing identifying all samples to a species-specific status, except for some taxa whose COI sequences remain unavailable in public databases. Therefore, we recommend the incorporation of DNA barcoding to provide additional support to traditional identification, especially in morphologically controversial groups. In addition, periodic updates and comparative analyses in public COI datasets are encouraged.
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The Amazonian snook (Centropomus irae) is the most recently described fish species of the genus Centropomus, which is endemic to the marine biogeographic region known as the Amazon River Plume, on the northern coast of Brazil. In the present study, we evaluated the population genetics, phylogeography, and evolutionary history of this species based on sequences of the Control Region of the mitochondrial genome. We analyzed a total of 87 specimens, including 39 C. irae and 48 specimens of the sister species Centropomus undecimalis, from the coast of the western South Atlantic. The results of the study revealed high levels of genetic diversity in both species, with no evidence of any genetic differentiation among the localities sampled for either species. This may be related to the typical migratory and reproductive behavior of the snooks. The analyses of the demographic history of the species identified a discrete process of population expansion in C. undecimalis at around 20,000 years ago, whereas C. irae passed through a marked population expansion approximately 30,000 years ago, which was probably linked to the formation of the Amazon plume and the fluctuations in sea levels that occurred during the late Pleistocene.
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We present the latest version of the Molecular Evolutionary Genetics Analysis (MEGA) software, which contains many sophisticated methods and tools for phylogenomics and phylomedicine. In this major upgrade, MEGA has been optimized for use on 64-bit computing systems for analyzing bigger datasets. Researchers can now explore and analyze tens of thousands of sequences in MEGA. The new version also provides an advanced wizard for building timetrees and includes a new functionality to automatically predict gene duplication events in gene family trees. The 64-bit MEGA is made available in two interfaces: graphical and command line. The graphical user interface (GUI) is a native Microsoft Windows application that can also be used on Mac OSX. The command line MEGA is available as native applications for Windows, Linux, and Mac OSX. They are intended for use in high-throughput and scripted analysis. Both versions are available from www.megasoftware.net free of charge.
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Restriction site‐associated DNA (RAD) sequencing was used to characterize neutral and adaptive genetic variation among geographic samples of red drum, Sciaenops ocellatus, an estuarine‐dependent fish found in coastal waters along the southeastern coast of the United States (Atlantic) and the northern Gulf of Mexico (Gulf). Analyses of neutral and outlier loci revealed three genetically distinct regional clusters: one in the Atlantic and two in the northern Gulf. Divergence in neutral loci indicated gradual genetic change and followed a linear pattern of isolation by distance. Divergence in outlier loci was at least an order of magnitude greater than divergence in neutral loci, and divergence between the regions in the Gulf was twice that of divergence between other regions. Discordance in patterns of genetic divergence between outlier and neutral loci is consistent with the hypothesis that the former reflects adaptive responses to environmental factors that vary on regional scales, while the latter largely reflects drift processes. Differences in basic habitat, initiated by glacial retreat and perpetuated by contemporary oceanic and atmospheric forces interacting with the geomorphology of the northern Gulf, followed by selection, appear to have led to reduced gene flow among red drum across the northern Gulf, reinforcing differences accrued during isolation and resulting in continued divergence across the genome. This same dynamic also may pertain to other coastal or nearshore fishes (18 species in 14 families) where genetically or morphologically defined sister taxa occur in the three regions.
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The geographic structure of marine fish populations is an important element used in defining stock units, and genetic data have historically been used for this purpose. Here, genetic data were used to elicit population genomic patterns for Atlantic Croakers Micropogonias undulatus collected at five locations in the Gulf of Mexico (hereafter, "Gulf") and a single location in the southern U.S. Atlantic. Mitochondrial DNA (mtDNA) haplotypes were used as a baseline for historical lineage delineation and in a comparison with a previous Atlantic Croaker study that was centered on the Atlantic coast. A genomic data set consisting of 3,682 single-nucleotide polymorphism (SNP)-containing loci was used to assess contemporary gene flow throughout the sampled area. Both the mtDNA and SNP data sets showed significant between-basin estimates of genetic divergence (ɸ st = 0.049 and F ST = 0.002, respectively), while pairwise F ST implied a low magnitude of divergence (F ST ≤ 0.002) at all geographic scales. Comparison of patterns obtained from putatively "neutral" versus "outlier" SNP loci suggested contrasting genetic patterns at the extremes of the sampling distribution in the Gulf. Putatively neutral SNPs implied a single stock in the Gulf, whereas a handful of outliers suggested distinct populations in the eastern and western Gulf. The pattern observed at outlier loci could imply either the presence of natural selection impacting a small number of loci or otherwise could be explained as a remnant pattern reflective of historical geographic isolation. In either case, the weak population structure observed at a small number of SNP loci may be indicative of a more significant demographic structure; for this reason, caution is urged when treating Atlantic Croakers as a single stock in the Gulf.
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