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2013 SOUTHEASTERN NATURALIST 12(2):251–266
Observations on the Identification of Larval and Juvenile
Scaphirhynchus spp. in the Lower Mississippi River
Paul Hartfield1,*, Nathan M. Kuntz2, and Harold L. Schramm, Jr.3
Abstract - Scaphirhynchus albus (Pallid Sturgeon) and S. platorynchus (Shovelnose
Sturgeon) are sympatric and not uncommon in the lower Mississippi River from the confluence
of the Ohio River to the Gulf of Mexico, and in its distributary, the Atchafalaya
River. Reports of sturgeon larvae have been rare in the Mississippi River but have been
increasing with more effective collection methods. A suite of characters identified in
hatchery-reared larval Pallid Sturgeon and Shovelnose Sturgeon from the Yellowstone
and upper Missouri rivers has been used to distinguish larval Scaphirhynchus spp. In the
Mississippi River; however, a large proportion of wild Scaphirhynchus spp. larvae are
intermediate in these characters and have been identified by some as hybridized Pallid
Sturgeon and Shovelnose Sturgeon. We applied three diagnostic characters developed
from Missouri River sturgeon larvae to hatchery-reared progeny of Atchafalaya River
Pallid Sturgeon and found them inadequate to identify most of the known Pallid sturgeon
larvae. Additionally, fewer than 10% of a large sample of wild Scaphirhynchus spp. larvae
from the lower Mississippi River conformed to either Pallid Sturgeon or Shovelnose
Sturgeon at two or more of the characters. We also found a small mouth width relative
to head width and a concave forward barbel position may be useful for the identification
of 30% or more Scaphirhynchus spp. larvae and postlarval young-of-year as Shovelnose
Sturgeon. Established adult character indices and diagnostic measurement proportionalities
also failed to correctly identify any hatchery-reared Pallid Sturgeon juveniles
recaptured 6–7 years following their release.
Historical records of larval and small (less than ca. 300 mm fork length [FL])
Scaphirhynchus spp. from the Mississippi River are rare, as are records of juvenile
(<700 mm) Scaphirhynchus albus (Forbes and Richardson) (Pallid Sturgeon
[PLS]). This rarity was initially interpreted as recruitment failure, particularly in
PLS (USFWS 1990). Methods have been refined during the past decade to collect
larval and young-of-year (YOY) Scaphirhynchus spp. (e.g., Herzog et al. 2005),
and the numbers of larval Scaphirhynchus spp. collected in the Mississippi River
are increasing (Hrabik et al. 2007, Phelps et al. 2010).
Identification of Scaphirhynchus spp. <300 mm FL has proven difficult (Bailey
and Cross 1954, Kuhajda and Mayden 2001), and it is further complicated by
a protracted larval development period and three larval stages. Protolarva is the
1US Fish and Wildlife Service, 6578 Dogwood View Parkway, Jackson, MS 39213.
2Department of Wildlife, Fisheries, and Aquaculture, Mississippi State University, Mississippi
State, MS 39762. 3US Geological Survey, Mississippi Cooperative Fish and
Wildlife Research Unit, Mail Stop 9691, Mississippi State, MS 39762. *Corresponding
author - email@example.com.
252 Southeastern Naturalist Vol. 12, No. 2
early life stage from hatching to development of first median fin rays at 21–26
mm total length (TL), mesolarva is the morphological stage from the appearance
of first median fin rays through the appearance of last caudal fin rays at 50–60 mm
TL in Scaphirhynchus platorynchus (Rafinesque) (Shovelnose Sturgeon [SNS])
and greater than 81 mm TL in PLS, and metalarva is the stage from the appearance
of the last caudal fin rays through complete disappearance of the pre-anal
finfold at about 200 mm TL (Snyder 2002).
Snyder (2002) conducted a rigorous morphometric and meristic study of
hatchery-reared (HR) PLS and SNS larvae that were progeny of brood stock
collected from the Yellowstone River, MT, and upper Missouri River, ND, and
identified several characters diagnostic for differentiating PLS and SNS larvae at
different stages. These included: (1) pigmentation of tissues over the heart and
posterior lip lobe of protolarvae and mesolarvae (SNS with pigmentation, PLS
without); (2) inner barbel to outer barbel length ratios of protolarvae through
metalarvae (SNS < 1.4, PLS > 1.6); (3) the ventral shape of the lateral rostral
plate of mesolarvae and metalarvae (SNS semi-circular, PLS deltoid); and (4)
caudal fin ray counts of metalarvae (SNS < 65, PLS > 66).
Larval descriptions from Snyder (2002) have been used to identify sturgeon
larvae in the Mississippi River (e.g., Hrabik et al. 2007); however, a number of
Scaphirhynchus spp. larvae had characteristics of both species and were identified
as hybrid Scaphirhynchus spp. Using characters 1, 2, and 3 proposed by Snyder
(2002), above, as well as secondary characters (dorsal and anal fin ray counts),
Hrabik et al. (2007) identified 17 of 39 Scaphirhynchus spp. larvae (17.5–84 mm
TL) collected from the middle and lower Mississippi River as hybrids. These
findings appear to support reports of high proportions of hybrid Scaphirhynchus
spp. relative to total catch in this river reach (e.g., 86.4% of Scaphirhynchus spp.
reported as hybrids by Keenlyne et al. 1994).
Identification of larger Scaphirhynchus spp. specimens (i.e., postlarval YOY,
juveniles, and adult) is based upon continuous anatomical ratios rather than
discrete meristic measurements. Diagnostic proportionalities for species identification
were established from anatomical ratios of only a few juvenile and mature
sturgeon specimens, most of which were collected in the Missouri River (Bailey
and Cross 1954), and character indices were developed from these proportionalities
(e.g., Wills et al. 2002). However, these anatomical ratios appear to change
with fish size in the lower Mississippi and Atchafalaya rivers, possibly resulting
in reports of disproportionate numbers of hybrids (USFWS 2007). Concern over
misidentification of Scaphirhynchus spp. in the Mississippi River as hybrids is
supported by Murphy et al. (2007), who compared the identification of 41 lower
Mississippi River SNS and PLS using principal component analysis (PCA) with
a character index relying on anatomical ratios (mCI) and a character index using
both anatomical ratios and a meristic character (CI). The PCA resulted in the
identification of only two morphological intermediates, i.e., putative hybrids;
whereas, the CI and mCI identified 37% and 73%, respectively, of the specimens
as hybrids (Murphy et al. 2007). Knowledge of whether Scaphirhynchus spp.
2013 P. Hartfield, N.M. Kuntz, and H.L. Schramm, Jr. 253
larvae can be distinguished in the lower 1931 km of their range, and what proportion
can be reliably identified, is essential to measuring the recruitment rates and
demographics of both PLS and SNS, as well as the extent of possible hybridization
in the Mississippi River.
Larval sturgeon characters, as well as the character indices used to identify
Scaphirhynchus spp. >300 mm FL, were developed primarily from upper
Missouri River specimens. Our initial objective was to evaluate the utility of
the Snyder (2002) characters 1, 2, and 3 (above) for identifying HR PLS larvae
and wild Scaphirhynchus spp. larvae in the extreme southern portion of their
range. We also evaluated larval mouth width relative to head width and barbel
placement, two characters commonly used to identify large (>300 mm FL)
Scaphirhynchus spp. (Bailey and Cross 1954, Forbes and Richardson 1905). Following
the recapture of age 6–7 PLS HR progeny in 2010 through 2012, we tested
the effectiveness of anatomical ratios and character indices for identification of
these larger juvenile fish.
Designation of life stages
The larval stage length ranges of Scaphirhynchus spp. used in our analysis
were modified from Snyder (2002) to accommodate our comparison of known
HR PLS to wild caught specimens potentially containing individuals of both
species. The smallest specimen we examined was 31 mm, larger than the protolarval
stage of either species (27 mm) defined by Snyder (2002). We considered
specimens up to 80 mm TL as mesolarvae, and those between 80 to 200 mm TL
For purposes of discussion and clarity, we also define post-larval lower Mississippi
River PLS. The age-0 postlarval life stage (200 to 400 mm) begins upon
the loss of the pre-anal finfold and the development of the last caudal fin rays
(Snyder 2002); the upper length limit for the first year of growth is derived from
hatchery records (Dean 1998, 2004) and a growth curve developed by Killgore
et al. (2007). The juvenile life stage (>400 to 750 mm FL) begins at age 1 and
includes individuals of both sexes showing early gonadal development; the maximum
length for this stage is based on the growth curve (Killgore et al. 2007) and
minimum age of male sexual maturity (7 years), as well as the lower sizes at
which Mississippi and Atchafalaya PLS begin to conform to diagnostic proportionalities
and character indices (P. Hartfield, N.M. Kuntz, and H.L. Schramm,
Jr., pers. observ.). The mature adult life stage (>750 mm FL) is based upon the
minimum age of female sexual maturity (10 years) and successful spawning of
PLS >800 mm FL at Natchitoches National Fish Hatchery (Dean 1998, 2004).
Age-0 hatchery-reared Pallid Sturgeon
A series of HR larval and post larval PLS (33 mm TL–260 mm FL) were obtained
from the University of Alabama Ichthyology Collection (UAIC), and 100
254 Southeastern Naturalist Vol. 12, No. 2
specimens were randomly selected for measurement and analysis (UAIC Lot #s
15246, 15247, 15249, 15254–15265, 15648, 15649, 15652). These specimens
were primarily progeny of brood stock collected in the Atchafalaya River at its
divergence from the Mississippi River in 2004 (n = 95), along with a smaller
number (n = 5) from brood stock collected in 1998. All specimens for the analysis
were spawned and reared at Natchitoches National Fish Hatchery during the
years of brood stock collection.
Brood stock for the 2004 HR PLS included three females and four males
visually identified as PLS, which were confirmed by character index (Kuhajda
and Mayden 2001) and genetic analysis (Dean 2004). The water temperature
was maintained at 19 °C throughout incubation, and eggs began hatching on
11 May 2004. Approximately 14,000 fingerlings were produced in 2004 (Dean
2004). Larvae were randomly subsampled at 10 to 71 days post hatch, fixed, and
preserved in 10% buffered formalin, and deposited in the University of Alabama
Ichthyology Collection. Surviving HR age-0 fish were stocked into the Atchafalaya
(n = 965) and lower Mississippi rivers (n = 2600) in October 2004.
The 1998 HR PLS were progeny from a single pair of visually identified (i.e.,
morphological) PLS. Water temperature was held at 17.2 °C throughout incubation,
and eggs hatched 23–25 March 1998. On 26 March 1998, 8030 fry were
transferred to rearing troughs with flow-through water from a local stream. Rearing
water temperatures were not reported. On 14–15 October 1998, 35 surviving
age-0 fish were PIT tagged and released into the Atchafalaya River in Concordia
Parish, LA (Dean 1998).
Wild-caught Age-0 Scaphirhynchus spp.
As no hatchery has produced SNS from the lower portion of the range, HR
SNS were not available for this analysis. Wild Scaphirhynchus spp. larvae and
age-0 postlarvae (<300 mm TL) were collected during 2008–2010 from various
locations in the lower Mississippi River between Rosedale (rkm 935) and
Vicksburg (rkm 698), MS, and deposited at the Mississippi Museum of Natural
Science (MMNS Registration # 9868). A 3.05-m otter trawl with 3.18-mm bar
mesh (Innovative Net Systems, Lafayette, LA) was used during May and June,
and a 3.66-m otter trawl with 31.75-mm bar mesh and a 3.18-mm bar mesh cod
liner (Memphis Net and Twine Company, Memphis, TN) was used during other
months. Specimens were fixed in 10% buffered formalin, then rinsed and transferred
to 70% denatured ethanol after approximately 2 weeks of fixation. We
randomly selected 94 meso-, meta-, and post-larval YOY wild sturgeon (31 mm
TL–301 mm FL) for examination and measurement. An additional 6 specimens
cataloged in the MMNS Fish Collection (MMNS Cat # 43530–43533, 43535,
53694) were also included in the analysis.
Age-0 specimen analysis
All specimens were measured (mm) for TL (larvae) or FL (postlarvae) using a
30-cm ruler and assigned to a developmental stage as defined above. We recorded
2013 P. Hartfield, N.M. Kuntz, and H.L. Schramm, Jr. 255
(1) the presence or absence of pigmentation of the integument ventral to the heart
and on the posterior lip lobes, (2) the lengths of inner and outer barbels, (3) the
head width, (4) the mouth width, (5) the shape of the lateral rostral plate, and (6)
the relative position of the inner and outer barbels. All measurements other than
TL and FL were measured to the nearest 0.01 mm using digital calipers (Traceable
Control Co., Friendswood, TX) and a 1.25x illuminated magnifier. We did
not attempt to use fin ray counts (defined as secondary characters by Hrabik et
al. 2007) because of discrepancies in counts between the original description
(Forbes and Richardson 1905) and the re-description (Bailey and Cross 1954),
the variability of counts due to developmental water temperature (Snyder 2002),
and the difficulty of accurate fin ray counts even in larger specimens (Bailey and
Snyder (2002) considered an outer barbel-inner barbel ratio (OB/IB) <1.4
diagnostic for SNS, and >1.6 diagnostic for PLS for mesolarvae and larger specimens;
however, ratios of 1.5 from meso- and metalarvae could be indicative of
either species. For the purposes of our study, we selected a measurement ratio
of <1.45 as characteristic of SNS and a ratio >1.55 as characteristic of PLS. The
most intact pair of left or right barbels was selected for meas urement.
The head width/mouth width ratio (HW/MW) was recognized by Forbes
and Richardson (1905) as a diagnostic character for distinguishing adult PLS
(1.4–1.6) and SNS (1.6–1.9), but, to our knowledge, this ratio has not been
evaluated for larval or postlarval Scaphirhynchus spp. Mouth width is typically
measured as the distance between the outside of the lips; however, lips may be
distorted by fixation and preservation. To avoid this problem, we defined mouth
width as the inside width of the cartilaginous border at the posterior of the buccal
cavity; head width was the widest measurement of head. In our analysis, we
compared mouth width relative to head width (MW/HW), the reciprocal of the
ratio used by Forbes and Richardson, to reflect a higher value as mouth width
increases relative to head width. Using this ratio, the Forbes and Richardson
(1905) diagnostic HW/MW ratios for adult specimens would correspond to MW/
HW ratios ≥0.63 for PLS, and <0.63 for SNS.
Snyder (2002) identified the shape of the lateral rostral plate in ventral aspect
as diagnostic for meso- and metalarvae of PLS and SNS. He described the more
angular lateral rostral plate of PLS larvae as “deltoid” and widest near the barbel;
whereas, in SNS, the semicircular plates are widest near the middle. To compare
quantitatively the shape of the lateral rostral plate, we measured total length
of the plate and distance from the anterior margin to its widest point. Shape of
the rostral plate was quantified as length at the widest point divided by its total
length. Measurement ratios <0.30 were considered deltoid, and ratios >0.40 were
Bailey and Cross (1954) noted that the origin of the four barbels typically lie
in a straight line for SNS, occasionally with the bases of the outer pair originating
slightly forward of the inner pair (i.e., straight or concave forward); whereas,
in PLS the outer barbels originate posterior to the inner barbels (i.e., convex
256 Southeastern Naturalist Vol. 12, No. 2
forward). Snyder (2002) described barbel position only for prot olarvae and only
relative to other structures. We determined barbel position by placing a straightedge
along the anterior base of the barbels. Barbel position was recorded as
convex forward, straight, or concave forward.
A chi-square analysis was used to test whether morphological characteristics
changed with larval stage. Exact tests were used to accommodate the low cell
size for some comparisons, and HR age-0 postlarvae were excluded from analysis
because of the low sample size.
Juvenile and adult Pallid Sturgeon
During 2010 through 2011, 8 HR PLS (500–674 mm FL) were captured on
trotlines within the combined outflow channel of the Old River Control Complex
(Atchafalaya River) or just downstream (Red River Miles 7–10). Coded wire
tags (CWT) extracted from two of the individuals confirmed that they were from
the same family lots as our larval specimens, released as age-0 postlarvae into
the lower Mississippi and Atchafalaya rivers in 2004. Over the past 2 years, we
have also captured, measured, and released 7 HR PLS (625–741 mm FL) with
CWT and/or elastomer marks in their rostrums from the Mississippi River near
Baton Rouge, LA (Mississippi River Mile [MRM] 234. 246), Vicksburg, MS
(MRM 431, 454, 459), and Rosedale, MS (MRM 580). We applied diagnostic
proportionalities (Bailey and Cross 1954) and two character indices currently in
use for identifying PLS and SNS to these specimens (Table 1). The mCI (Wills
et al. 2002) was developed to distinguish PLS, SNS, and presumptive hybrids in
the middle Mississippi and lower Missouri rivers; the BKI (Kuhajda and Mayden
2001) was developed to conservatively select brood stock for pr opagation in the
lower Mississippi and Atchafalaya rivers and identifies morphological intermediates
as “intermediate”, rather than as “hybrid”. Measurements used in the
proportionalities and character indices include: mouth width (MW), the distance
from the center of the mouth to the front edge of the right inner barbel insertion
Table 1. Diagnostic proportionalities from Bailey and Cross (1954) and character index values
(mCI from Wills et al. 2002, BKI from Kuhajda and Mayden 2001) used to identify Scaphirhynchus
spp. throughout their range. Measurements used in the proportionalities and indices are defined in
Pallid Sturgeon Intermediate or hybrid Shovelnose Sturgeon
MW:MIB >1.63 1.42–1.63 <1.42
IRL:MIB >2.29 2.19–2.29 <2.19
HL:MIB >5.54 5.04–5.54 <5.04
OBL:IBL >1.72 1.48–1.72 <1.48
IRL:IBL >2.63 2.50–2.63 <2.50
HL:IBL >6.35 5.76–6.35 <5.76
mCI <-0.70 -0.70–0.41 >0.41
BKI >4.70 3.60–4.70 <3.60
2013 P. Hartfield, N.M. Kuntz, and H.L. Schramm, Jr. 257
(MIB), the distance from the tip of the snout to the front edge of the right outer
barbel insertion (IRL), head length as measured from the tip of the snout to the
rear margin of the right opercle (HL), outer barbel length (OBL), and inner barbel
length (IBL) (USFWS 1990).
Characteristics of Age-0 hatchery-reared Pallid Sturgeon
Preserved HR PLS ranged from 33 mm TL–260 mm FL and included 36 mesolarvae
(<80 mm TL), 62 metalarvae (80–200 mm TL), and 2 age-0 postlarvae
(206 and 260 mm FL). Only 24 HR PLS specimens conformed to Snyder’s (2002)
three characters as PLS. Barbel length ratios, shape of lateral rostral plate, or both
either conformed to Shovelnose Sturgeon or were intermediate in value in the
other 76 specimens.
No pigmentation was evident in the middle of the posterior lip lobes of any of
the HR PLS larvae. A single mesolarva (49 mm TL) exhibited a few small, scattered
specks of pigment over the heart. Few to moderate numbers of small specks
of pigmentation were observed on the outer mar gin of the lips near the posterior
corners of the mouth of 39 of the specimens (Table 2). These included 18 mesolarvae
and 21 metalarvae (up to 155 mm TL). Pigmentation was not observed
on the lips of any HR metalarvae >155 mm TL or the 2 age-0 postlarvae. The
general lack of pigmentation in HR PLS larvae conformed to Snyder’s (2002)
description; however, almost 40% of specimens exhibited light to moderate pigmentation
on the outer margin of the lip lobes near the posterior corners of the
mouth, a character Snyder reported as an occasional occurrence in either species.
Hatchery-reared PLS OB/IB ratios ranged from 1.16–1.96. Barbel ratios for
36 of the HR specimens fell within the range for SNS (<1.45), 29 had intermediate
values (1.45–1.54), and 35 had ratios >1.55 that are considered diagnostic
of PLS (Table 2). Therefore, 65 of the known HR PLS did not conform to the
expected barbel ratio value.
Relative shape of the lateral rostral plate in HR PLS varied from 0.16 (deltoid,
PLS condition) to 0.49 (semicircular, SNS condition). Most (n = 72) were deltoid
(<0.30), but 1 mesolarva and 4 metalarvae exhibited a strongly semicircular
shape (>0.40) (Table 2). Eight specimens were marginal to our defining ratio
(0.30, 0.31), and 5 of these conformed to PLS in barbel ratios.
Only 39 of the 100 HR PLS larvae and postlarvae had the convex forward
barbel position indicative of PLS; all others were straight, and none were concave
forward (Table 2). Convex-forward specimens ranged from 33 mm TL–260
mm FL, included fish in all stages of development, and 12 conformed to PLS in
all measurements. Specimens with straight barbel position ranged from 42 mm
TL–206 mm FL and included 61 fish in all stages of development. Twenty of
these conformed to PLS in all other measurements.
The MW/HW ratios for HR PLS ranged from 0.49 to 0.65, and only a single
HR PLS had a MW/HW ratio < 0.5 (0.49). However, only 8 mesolarvae and 2
258 Southeastern Naturalist Vol. 12, No. 2
Table 2. Ventral head measurements, barbel position, and pigmentation of hatchery-reared (HR) Pallid Sturgeon and wild Scaphirhynchus spp. larvae and
postlarvae from the lower Mississippi River.
Pigmentation Barbel ratio Barbel position Rostral plate Mouth width:head width
None Heart Lips <1.45 1.45–1.55 >1.55 Concave Straight Convex <0.3 0.3–0.4 >0.4 <0.49 0.49–0.51 >0.51
# HR 61 1 39 36 29 35 0 61 39 72 23 5 0 4 95
-Meso 18 1 18 10 12 14 0 21 15 28 7 1 0 1 34
-Meta 41 0 21 26 17 19 0 39 23 42 16 4 0 3 59
-Post 2 0 0 0 0 2 0 1 1 2 0 0 0 0 2
Chi-square 4.14 1.972 0.200 1.316 0.222
df 2 2 1 2 1
Pexact 0.089 0.373 0.670 0.562 1.000
# Wild 84 10 0 62 22 14 25 62 13 42 50 6 29 33 37
-Meso 16 10 0 10 12 10 1 22 9 18 12 2 3 8 21
-Meta 38 0 0 26 9 3 14 23 1 15 20 2 15 14 9
-Post 30 0 0 26 1 1 9 17 3 9 18 2 11 11 7
Chi-square 14.930 26.702 18.060 4.299 17.507
df 2 4 4 4 4
Pexact <0.001 <0.001 <0.001 0.375 <0.001
2013 P. Hartfield, N.M. Kuntz, and H.L. Schramm, Jr. 259
metalarvae had a ratio > 0.63 considered diagnostic of large PLS by Forbes and
Developmental stage did not appear to be a factor in identification of HR
PLS as there were no differences between meso- and metalarval stage in any
characters measured (P > 0.089; Table 2). Thus, although SNS and intermediate
characters were found in some HR PLS, the frequency of occurrence remained
the same between larval stages.
Characteristics of wild-caught Age-0 Scaphirhynchus spp.
Wild-caught Scaphirhynchus spp. larvae and postlarvae ranged from 31 mm
TL to 301 mm FL and included 32 mesolarvae, 39 metalarvae, and 29 postlarvae.
Excluding pigmentation, 7 specimens (1 mesolarvae, 3 metalarvae, 3 postlarvae)
conformed to SNS in barbel ratio and lateral rostral plate, while 9 conformed to
PLS (7 mesolarvae, 1 metalarva, 1 postlarva).
Ventral pigmentation over the heart was evident in only 10 of the wild
Scaphirhynchus spp. specimens, and only in mesolarvae 38–70 mm TL. All
of these were intermediate in barbel ratio and/or lateral rostral plate. Pigmentation
in most of these specimens consisted of a few small specks, but 1
specimen had a distinct patch of pigmentation over the heart conforming to
Snyder’s (2002) description of SNS. No pigmentation was observed on the lips
of any of the wild specimens.
Barbel ratios ranged from 1.2–1.7, with 14 greater than 1.55 and 22 with intermediate
values (Table 2). High, low, and intermediate barbel ratios were found
for all developmental stages.
Shape of the lateral rostral plate ranged from 0.19 to 0.45, similar to the HR
specimens (Table 2). Almost half of the wild specimens exhibited a deltoid lateral
rostral plate, and only 6 exhibited a strongly semicircular sha pe (>0.40).
Thirteen specimens had convex forward barbel placement, 62 were straight,
and 25 were concave forward (Table 2). Most of the specimens with concave
forward barbels were metalarvae and postlarvae; only 1 mesolarva had concave
forward barbel placement. Three of the 7 specimens identified as SNS,
and 1 of the 9 identified as PLS by barbel ratio and lateral rostral plate exhibited
concave forward barbel placement, while 1 specimen identified as SNS
and 4 identified as PLS exhibited convex forward barbel placement.
The MW/HW ratios ranged from 0.43 to 0.64, and 29 individuals had MW/
HW ratios of <0.50 (Table 2). Of those 29, almost half had concave barbels and
none exhibited convex barbels. Specimens of all developmental stages were represented
in the 70 wild sturgeon with MW/HW ratio > 0.49. Only 2 mesolarvae
had MW/HW > 0.63.
For wild Scaphirhynchus spp., barbel ratio, barbel position, MW/HW ratio,
and pigmentation all differed between larval stages (P < 0.001; Table 2). All 4
characters indicated a reduced frequency in PLS characters and an increase in
SNS characters as fish advanced in development stage.
260 Southeastern Naturalist Vol. 12, No. 2
Characteristics of juvenile and adult Pallid Sturgeon
All 8 HR PLS (500–674 mm FL) captured in the Atchafalaya River 7 years
after release were visually identified by field crews as PLS and confirmed to be
progeny from the 2004 Atchafalaya hatchery release by the presence of coded
Table 4. Diagnostic proportionalities from Bailey and Cross (1954) and character indices applied
to 7 Pallid Sturgeon from the Atchafalaya River used as brood stock for propagation. Identification
of all measurements and their specific diagnostic value can be found in Table 1. Pallid Sturgeon
characters are in bold, Shovelnose Sturgeon characters are in italics, all others are considered intermediate.
date length Sex IRL:MIB HL:MIB OBL:IBL IRL:IBL HL:IBL mCI BKI
3/19/04 855 M 2.88 6.25 2.22 3.19 6.94 -1.22 5.10
4/29/04 880 M 2.75 6.13 2.21 2.82 6.28 -1.10 4.96
4/29/04 890 M 2.62 5.95 1.89 2.44 5.56 -0.72 4.51
4/29/04 903 M 2.53 6.13 3.06 3.26 7.90 -1.37 5.59
4/29/04 935 F 2.59 6.22 2.29 2.59 6.22 -0.82 4.88
4/29/04 970 F 2.88 6.80 2.25 2.88 6.80 -1.02 5.13
4/29/04 1015 F 2.50 5.83 2.76 3.11 7.24 -1.18 5.26
Table 3. Diagnostic proportionalities from Bailey and Cross (1954) and character indices applied
to 8 recaptured hatchery-reared Pallid Sturgeon from the Atchafalaya River. Diagnostic values for
these metrics are presented in Table 1. Pallid Sturgeon characters are in bold, shovelnose stur geon
characters are in italics, all others are considered intermedia te.
date length MW:MIB IRL:MIB HL:MIB OBL:IBL IRL:IBL HL:IBL mCI BKI
12/16/2010 500 1.38 2.23 5.14 1.68 2.16 4.98 -0.22 3.91
12/16/2010 513 1.35 2.16 5.18 1.61 2.18 5.24 0.01 3.77
3/14/2011 564 1.81 2.46 5.46 1.80 2.12 4.70 -0.65 4.26
3/14/2011 591 1.71 2.23 5.13 1.89 2.40 5.52 -0.37 4.12
3/14/2011 591 1.72 2.41 5.86 1.94 2.52 6.12 -0.38 4.35
3/14/2011 619 1.96 2.43 5.43 2.04 2.83 6.32 -0.65 4.47
3/13/2011 654 1.35 1.79 4.33 1.93 2.57 6.20 0.02 3.72
3/14/2011 674 1.53 2.15 5.01 2.24 2.91 6.79 -0.51 4.39
Table 5. Diagnostic proportionalities from Bailey and Cross (1954) and character indices applied
to 7 recaptured hatchery-reared Pallid Sturgeon from the Mississippi River. Diagnostic values for
these metrics are presented in Table 1. Pallid Sturgeon characters are in bold, Shovelnose Sturgeon
characters are in italics, all others are considered intermedia te.
date length MW:MIB IRL:MIB HL:MIB OBL:IBL IRL:IBL HL:IBL mCI BKI
3/12/2012 625 1.65 2.32 5.77 1.74 2.12 5.26 -0.09 4.06
12/16/2011 630 1.60 2.34 5.24 1.40 1.96 4.40 -0.20 3.74
11/17/2011 645 1.70 1.50 6.12 1.86 1.43 5.84 1.64 3.36
3/12/2012 680 1.57 2.31 5.43 1.79 2.08 4.87 -0.34 4.11
12/14/2011 688 1.73 2.10 5.08 1.88 2.24 5.41 -0.16 3.98
2/16/2012 691 1.81 2.37 5.63 2.00 2.50 5.94 -0.47 4.37
6/8/2010 791 1.90 2.48 6.13 2.71 2.83 7.00 -1.01 5.19
2013 P. Hartfield, N.M. Kuntz, and H.L. Schramm, Jr. 261
wire tags. However, head measurements applied to the proportionalities defined
by Bailey and Cross (1954) failed to identify any of the recaptured fish as PLS,
and 6 of the 8 specimens exhibited 1 or more proportions within the SNS range
(Table 3). Additionally, none of the measurement ratios of the 2 smallest HR fish
were within the ranges identified for PLS. There was some indication of the effect
of allometric growth in that proportionalities associated with inner barbel length
(IBL) increased with size. Both character indices (mCI, BKI) failed to correctly
identify any of the recaptured fish as PLS (T able 3).
Established measurement proportionalities and character indices do not appear
diagnostic for lower Mississippi and Atchafalaya river age-0 and <700 mm
FL PLS. Alternatively, it is possible that the HR PLS could be hybrids. However,
the brood stock reportedly conformed both morphologically and genetically to
PLS identifications. To test the morphological identification of the brood stock,
we obtained and compared the morphometric measurements of the PLS brood
stock (J. Dean, US Fish and Wildlife Service, Natchitoches National Fish Hatchery,
Natchitoches, LA, unpubl. data) to 5 of the 6 diagnostic proportionalities of
Bailey and Cross (1954) (mouth width measurements were unavailable) and the
two character indices (Table 4). Three of the 4 males and 2 of the 3 females were
identified as morphological PLS by all proportionalities. A single male scored
as SNS on IRL:IBL and HL:IBL ratios, and 1 female scored intermediate at the
same ratios. All brood stock were identified as morphological PLS by the mCI.
The BKI identified 1 male as an intermediate and all others as PLS. It appears
that the intermediate proportionalities in 2 specimens and the BKI identification
of 1 of these as an intermediate are because of a disproportionately longer IBL in
We also applied the 6 diagnostic proportionalities and the character indices to
6 juvenile and 1 small adult HR fish captured in the Mississippi River (Table 5).
Only the adult HR fish scored as PLS in all proportionalities and indices. One
juvenile (645 mm FL) was identified as SNS by both indices. While at least
some of these 7 fish may be from the 2004 lower Mississippi release, we cannot
be certain because >45,000 HR YOY were released in the riverine reach of the
Missouri River between 2003 and 2006, many marked with CWT and elastomer
marks in the rostrum. Genetic samples of these specimens have been provided to
the US Fish and Wildlife Service for brood stock parental screening but have not
yet been analyzed.
Our analysis indicates that morphological characters considered diagnostic
for Missouri River larval Scaphirhynchus spp. failed to correctly identify all but
a small proportion of HR PLS larvae reared from the Atchafalaya River brood
stock. Atchafalaya River HR PLS meso- and metalarvae deviated from Snyder’s
(2002) characters by 1) a higher incidence of pigmentation on the outer margins
of the posterior lip lobes, 2) a high degree of overlap and variation in barbel
262 Southeastern Naturalist Vol. 12, No. 2
length ratios, and 3) the occurrence of semicircular lateral rostral plates in some
HR PLS specimens. Therefore, characteristics found useful for identifying larval
PLS in the upper Missouri River do not appear reliable for identifying meso- and
metalarval PLS from the lower Mississippi River Basin.
Application of the Missouri River characters to wild larval and postlarval
Scaphirhynchus spp. from the lower Mississippi River indicates limited value
of these characters for identification of either PLS or SNS in this portion of
the range. Based on the low ratio of PLS:SNS adult captures in this reach of the
Mississippi River (1:16; Killgore et al. 2007), we would expect that the majority
of wild larval Scaphirhynchus spp. would consist of SNS and conform to that
larval description, but this was not the case. Snyder (2002) reported that ventral
pigmentation on the lower lip lobes was diagnostic for SNS mesolarvae, and
pigmentation over the heart was characteristic for SNS meso- and metalarvae.
All three characters (pigmentation, semi-circular lateral rostral plate, and low
OB/IB ratios) were not observed in any wild specimen. Excluding pigmentation,
less than 50% of the wild larvae conformed to SNS in either barbel length ratio
or shape of lateral rostral plate, and only 7 conformed to SNS in both characters.
Therefore, the suite of characteristics diagnostic for SNS larvae in the Missouri
River appear to be ineffective in the lower Mississippi River.
We also observed 2 characters in the wild larvae that were not present in
the HR PLS meso- and metalarvae: MW/HW ratio < 0.49, and concave barbel
position. Because these characters were not present in any of the HR PLS,
they may have some utility for identification of SNS larvae in the Mississippi.
However, while a low MW/HW ratio may be unique to some SNS specimens,
we do not believe that a wide mouth (>0.49) is diagnostic of PLS because,
as noted above, it is unlikely that the large percentage (70%) of wild Scaphirhynchus
spp. specimens with MW/HW ratios > 0.49 in our study are all PLS.
Similarly, barbel position is only partially informative. While no HR PLS exhibited
a concave barbel position, the majority (>60%) of both HR PLS and
wild larvae exhibited a straight barbel alignment.
During our analysis, we noted differences in barbel papillation of metalarvae
and age-0 postlarvae between the HR PLS and the wild Scaphirhynchus spp.
samples. Papillae were not evident on the barbels of any HR PLS of any stage,
or on any wild mesolarvae. However, prominent papillae or fine serrations indicative
of papillae development were evident on both the anterior and posterior
margins of all four barbels on the majority of wild metalarvae above 100 mm TL.
Only 2 of the larger (>100 mm TL) wild metalarvae exhibited smooth barbels
similar to the HR specimens of similar size. Both of these specimens (181 mm,
206 mm TL) exhibited other PLS characters, including convex barbel positions,
deltoid lateral rostral plates (0.23), and wide mouths (>0.5 M/H). Barbel ratio in
the smaller specimen was also within PLS range (1.68), while barbel ratio in the
larger specimen was within SNS range (1.42). We suggest that the degree and
location of barbel papillae on larval and postlarval Scaphirhynchus spp. warrants
2013 P. Hartfield, N.M. Kuntz, and H.L. Schramm, Jr. 263
There are several possible explanations for the deviation of Atchafalaya River
HR PLS from the 3 characters described by Snyder (2002) and our ability to
identify only a few of the wild larvae that we examined with these characters.
1) Hatchery effects: Snyder’s characters (barbel length, pigmentation, lateral
rostral plate) may be affected by the hatchery environment; however, these characters
have been successfully applied to wild larvae collected in the Missouri
River. While hatchery effects might explain differences between HR PLS larvae
reared in south Louisiana vs. South Dakota, they do not explain the inability to
positively identify any lower Mississippi River SNS larvae with all 3 characters.
2) Adaptation to regional environmental differences: The habitat and environment
for brood stock that produced the progeny used in Snyder’s analysis (Missouri
River, North Dakota) differs from that in the Atchafalaya River, LA, relative to
hydrograph, discharge, temperature regimes, and other factors. Differences have
been noted in size and longevity between adults from the upper Missouri River
and the lower Mississippi River populations (USFWS 2007). Additionally, genetic
studies have shown genetic differences between PLS populations increase
with spatial separation (e.g., Campton et al. 2000, Schrey and Heist 2007, Tranah
et al. 2001). Therefore, local adaptation to regional environmental differences
cannot be discounted as an influence on larval characters. In other words, while
the adults from both the middle Missouri River and the Atchafalaya River all
appear to be PLS, local adaptations in the adults may be manifest in morphological
differences in their progeny. 3) Introgression: The high proportion of
intermediate morphological characters in Mississippi River Scaphirhynchus
spp. has been attributed to hybridization (e.g., Carlson et al. 1985, Keenlyne et
al. 1994, Hrabick et al. 2007, Schrey and Heist 2007). Introgression might also
explain the deviation of the age 6–7 recaptured HR PLS from the Bailey and
Cross (1954) morphometric proportionalities and character indices (Kuhajda and
Mayden 2001, Wills et al. 2002). This hypothesis is supported to some degree
by Allendorf et al. (2001) and Schrey et al. (2011), who concluded the amount
of genetic introgression observed in Scaphirhynchus spp. would require multiple
generations. If such is the case, however, both species have continued to maintain
their adult morphologies. Such prolonged hybridization between sympatric
species is not unexpected (e.g., Wallace 1912), has been demonstrated in other
species (e.g., Saether et al. 2007), is not uncommon in sturgeon species, and
has had a primary role in sturgeon speciation (Birstein et al. 1997, Robles et al.
2005, Vasil’ev 1999). The occurrence, extent, and significance of hybridization
in Scaphirhynchus spp. populations have yet to be fully resolved (USFWS 2007).
Our analysis of the 8 recaptured Atchafalaya River HR PLS and the 7 Mississippi
River HR PLS indicates that lower Mississippi River juvenile PLS
are likely to be identified as morphological intermediates, or “hybrids”, by
measurement proportionalities and character indices currently in use. These
results support the findings of Murphy et al. (2007) that morph ometric indices
should not be exclusively used to classify juvenile or adult sturgeon specimens
264 Southeastern Naturalist Vol. 12, No. 2
The paucity of larval and juvenile PLS records in the Mississippi River portion
of the range has been previously attributed to recruitment failure (e.g.,
USFWS 1990). We contend, however, that this rarity reflects insufficient historical
collection efforts, a poor understanding of phenotypic and genotypic
variation in lower Mississippi River Scaphirhynchus spp., and the inability
to accurately identify larval, postlarval, and juvenile PLS with identification
methods developed in other portions of its range. Large Scaphirhynchus spp.
(>700 mm FL) classified as PLS have been collected from numerous locations
throughout the Mississippi River (e.g., Killgore et al. 2007, and our unpublished
collections). Our results demonstrate that larval and juvenile fish reared
from morphological and genetic PLS collected from the Atchafalaya River are
identified as “hybrids” or SNS by larval identification methods and character
indices. Therefore, it follows that wild larval and juvenile PLS cohorts are also
recruiting, but have likely been identified and discounted as “hybrids” or even
as SNS. While multi-generation genetic introgression may play a role in the
morphological differences between lower Mississippi and upper and middle
Missouri river PLS populations, further refinement of both genetic and morphological
methods of identification of PLS and SNS are needed to understand
Larval collections of Scaphirhynchus spp. were conducted under US Fish and Wildlife
Subpermit TE198846-1, Louisiana Department of Wildlife and Fisheries Freshwater
Scientific Collecting Permit 1235, and Mississippi Department of Wildlife, Fisheries, and
Parks Administrative Scientific Collection Permit 1206102. Funding and support for our
Scaphirhynchus collection efforts were provided by the US Fish and Wildlife Service,
US Geological Survey, and Arkansas Game and Fish Commission. Additional support
was provided by Mississippi State University and Catfish Point Hunt Club, Benoit, MS.
We express our appreciation to Jan Dean, Natchitoches National Fish Hatchery, and
Bobby Reed, Louisiana Department of Wildlife and Fisheries, for their information on
PLS broodstock from the Atchafalaya River, and to Bernie Kuhajda and the University of
Alabama Ichthyology Collection for the loan of the HR PLS series. Wild specimens collected
for this study have been deposited at the Mississippi Museum of Natural Science.
The findings and conclusions in this article are those of the authors and do not necessarily
represent the views of the US Fish and Wildlife Service.
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