Practical Field Methods of Sexing Largemouth Bass

Abstract

Several rapid and simple methods of sexing live mature largemouth bass (Micropterus salmoides) were assessed: the presence or absence of a swollen reddish genital papilla, the shape of the scaleless area surrounding the urogenital opening, and the depth and angle of probe penetration into the urogenital opening. The genital papilla method was 89% successful during the spring, but only 48% successful during the fall. The shape of the urogenital opening was a poor indicator of sex (53% of the fish were sexed correctly). Probing the urogenital opening was the best single method; it provided success rates of 90% (probe depth) and 94% (probe angle). The highest rate of sexing success (98%) was realized during the spring spawning season from a combination of the papilla and probing methods. The probing methods that were used successfully in this study on largemouth bass might be used with similar success in sexing other fish species.

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Practical Field Methods of Sexing
Largemouth Bass
G. W. Benz a & R. P. Jacobs a
a Bureau of Fisheries, Connecticut Department of
Environmental Protection State Office Building, Hartford,
Connecticut, 06115, USA
Version of record first published: 09 Jan 2011.
To cite this article: G. W. Benz & R. P. Jacobs (1986): Practical Field Methods of Sexing
Largemouth Bass, The Progressive Fish-Culturist, 48:3, 221-225
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COMMUNICATIONS 221
hp) are provided for both supply and discharge
lines, making the unit independent of local water
pressures. The column is 12 inches in diameter
and is constructed of schedule-80 polyvinyl chlo-
ride pipe. The supply and discharge lines are 2
inches in diameter.
An important feature of this degasser is the con-
trol system (Figure 7). The unit can be operated
manually by using the sight glass in association
with motorized water and vacuum valves, or the
operator may choose an automatic mode. At the
flip of a switch, a preprogrammed computer mon-
itors water levels, controls pumps and valves, and
monitors discharge gas concentrations. The au-
tomatic system also provides alarms for high water
levels in the column and high dissolved gas in the
discharge water.
Acknowledgments
I thank the engineers of the U.S. Fish and Wild-
life Service for their work in developing the design
criteria for vacuum degassers. In particular, Bill
Miller and Jim Kelley of the Region 3 Fort Snelling
Office, and Terry McLaughlin of the Denver En-
gineering Center have been very generous in shar-
ing their experiences.
The Progressive Fish-Culturist 48:221-225, 1986
Practical Field Methods of Sexing Largemouth Bass
G. W. BE•z • AtqD R. P. JACO}•S
Bureau of Fisheries
Connecticut Department of
Environmental Protection
State Office Building
Hartford, Connecticut 06115, USA
Abstract. -- Several rapid and simple methods of sexing
live mature largemouth bass (Micropterus salmoides) were
assessed: the presence or absence of a swollen reddish
genital papilla, the shape of the scaleless area surround-
ing the urogenital opening, and the depth and angle of
probe penetration into the urogenital opening. The gen-
ital papilla method was 89% successful during the spring,
but only 48% successful during the fall. The shape of the
urogenital opening was a poor indicator of sex (53% of
the fish were sexed correctly). Probing the urogenital
opening was the best single method; it provided success
rates of 90% (probe depth) and 94% (probe angle). The
highest rate of sexing success (98%) was realized during
the spring spawning season from a combination of the
papilla and probing methods. The probing methods that
were used successfully in this study on largemouth bass
might be used with similar success in sexing other fish
species.
It is often desirable for biologists and fish cul-
turists to be able to sex fishes without sacrificing
them. Sexing any individual fish is a binomial con-
i Present address: Department of Zoology, 6270 Uni-
versity Boulevard, The University of British Columbia,
Vancouver V6T 2A9, Canada.
cept and, hence, without a graded scale of success.
In sexing numbers of fishes, however, success may
be graded. In population studies, it is often only
necessary for a sexing method to approach some
level of success to be considered applicable. In
precise breeding programs that require no error in
determining sex, however, procedures used to
identify each sex must be applicable to the strin-
gency of the binomial concept.
Different methods of sexing live largemouth bass
(Micropterus salmoides) have produced varying
levels of success. Using hematocrit values to sep-
arate the sexes, Steucke and Atherton (1965) found
that 52% of the largemouth bass in their sample
had to be discarded to ensure the method's overall
accuracy. Snow (1963) noted that by a combina-
tion of stripping eggs or milt, estimating fullness
of the abdomen, and noting the presence or ab-
sence of a urogenital papilla (i.e., swollen vent),
one could separate the sexes at spawning time.
Toots (1972) also recommended using the pres-
ence or absence ofa urogenital papilla to sex large-
mouth bass at spawning time and, additionally,
noted that body coloration was sometimes helpful
in sexing spawning males. Shape of the scaleless
area surrounding the urogenital opening was rec-
ommended by Parker (1971), and later was criti-
cized by Manns and Whiteside (1980), as a means
of sexing largemouth bass. Glass et al. (1962) used
a urogenital-probing technique to accurately sex
(100% correc0 11 largemouth bass. Driscoll (1969)
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222 COMMUNICATIONS
used an otoscope to successfully sex 97% of a large
sample (N = 154) of pre- and postspawning bass.
The otoscope served as both a probe and viewing
aid and, although the latter role was not discussed,
probing the urogenital opening was described as
being important.
During stock assessment of largemouth bass
populations in selected lakes throughout Con-
necticut, we found it necessary to determine the
sex of fish. Because killing many of these valuable
bass was undesirable, we needed a rapid and re-
liable method of sexing live fish in the field. There-
fore, we compared several practical sexing meth-
ods. Because the primary goal of this effort was to
acquire a quick, easy, and relatively accurate
(•90%) method of sexing live fish for our assess-
ment purposes, an exhaustive evaluation of all po-
tential methods was not undertaken.
Methods
Four solitary methods of sexing largemouth bass
were assessed: (1) noting the presence (on females)
or absence (from males) of a genital papilla, a red-
dish protuberance surrounding the urogenital
opening; (2) examining the shape of the scaleless
area surrounding the urogenital opening (round in
males, oval in females) when the fish is held upside
down (Parker 1971); and noting (3) the depth, and
(4) the angle of penetration of a 1-mm-diameter
plastic broom straw inserted into the genital duct
(Glass et al. 1962). In females, the probe passes
through the oviduct and deep into the ovary at an
angle oblique to the main body axis; in males the
probe generally does not enter the genital duct, but
rather passes a short distance perpendicular to the
main body axis into the urinary bladder (Figure
1).
We also evaluated these methods in combina-
tions of, first, the two probing techniques (3 and
4), and, second, the gential papilla and the two
probing techniques (1, 3, and 4). In order for sex
identification to be made by the combined tech-
niques, each technique had to predict the same
outcome. If predictions were incongruent, one ob-
servation had to be considered inconclusive and
the sex was predicted by the remaining explicit
observation.
Largemouth bass were collected by electrofish-
ing between the spring of 1981 and the fall of 1982
in three Connecticut lakes: Candlewood Lake, Lake
Lillinonah, and Pachaug Pond. After sex was pre-
dicted by the above methods, it was verified by
dissection and examination of the gonads; fish
deemed immature (based on shape and size of
gonads) were omitted from the results (the smallest
male and female fish included were 272 mm and
300 mm, respectively).
Results and Discussion
The success of the genital papilla method as a
means of identifying sex varied seasonally. During
the spring spawning period, most mature females
developed a highly noticeable eurythemic papilla
and the method proved almost 90% effective
(Table 1). During the fall, however, the papilla
apparently atrophies and, correspondingly, the
method greatly overestimated the number of males.
In contrast, McComish (1968) was able to sex ma-
ture bluegills (Lepomis macrochirus) throughout
the year with absolute success even though sea-
sonal variation in papilla size was noted.
The shape of the scaleless area surrounding the
urogenital opening was a poor indicator of sex,
and was not much better than chance (53% correct)
in predicting sex correctly (Table 1). These results
corroborate those of Manns and Whiteside (1980)
who judged this method to be of limited value in
sexing Guadalupe bass (Micropterus treculi) and
largemouth bass (67% accurate for fish 355 mm
or longer). Both the present study (Table 1) and
that of Manns and Whiteside (1980) indicate that
this method grossly overrepresents males. AI-
TABLE 1 .--Percent of largemouth bass correctly sexed
by six different methods. Sample sizes are in parentheses.
Method Sexes
(season) Male Female combined
A: Presence or absence
of red papilla
(1981-1982) a
May-Jun 97 (98) 79 (79) 89 (177)
Sep-Nov 95 (65) 48 (67) 71 (132)
B: Shape of ufogenital
region (spring and
fall 1981) b
Fish 300-355 mm 51 (37) 46 (24) 49 (61)
Fish >355 mm 81 (37) 39 (52) 56 (89)
C: Depth of straw pene-
tration (spring and
fall 1982) c 90 (84) 89 (71) 90 (155)
D: Angle of straw pene-
tration (spring and
fall 1982) c 94 (31) 95 (38) 94 (69)
A plus C
(spring 1982) 100 (35) 95 (19) 98 (54)
C plus D
(fall 1982) 97 (32) 91 (32) 94 (64)
a Two additional bass could not be sexed by this method.
b Eleven additional bass could not be sexed by this method.
c Two additional bass could not be Sexed by this method.
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COMMUNICATIONS 223
/\
a uo
-__
FIGURE i.--(A) Illustration of a female largemouth bass with sexing probes inserted into the oviduct (o) and
urinary bladder (u). (B) Illustration of male largemouth bass with probes inserted into the sperm duct (s) and urinary
bladder (u). Note that the angle of probe penetration into the genital duct of both male and female fish is at an
oblique angle to the main body axis whereas penetration into the bladder is generally perpendicular. Note also that
the sexing probe penetrates much deeper into the oviduct than it does into the sperm duct (the lumen of the oviduct
being wider), whereas it penetrates to a similar depth into the urinary bladder of both sexes. In practice, because
of the differential ease of penetrating the various ducts, our i-ram-diameter probe would typically pass deeply into
the oviduct of females, but would usually only enter the urinary bladder of males. (C) Schematic drawing of the
urogenital region of largemouth bass: anus (a); gonad (g); urinary bladder (u); genital duct (gd: solid lines = male
duct, dashed lines = female extension); urogenital opening (uo).
though the method may be successful for experi-
enced people who continually examine large-
mouth bass (Parker 1971), we feel that the criterion
"round or oval scaleless area around the ufogenital
opening" is too subjective and variable. The shape
of the scaleless area may be influenced by seasonal
gonad enlargement or stomach fullness, or spatial
attitude during examination. Manns and White-
side (1980) found the accuracy of sexing their small
sample oflargemouth bass increased to 84% when
they excluded fish exhibiting distended or concave
abdomens. We did not attempt to duplicate this
effort as we felt the original technique was already
overly subjective.
Probing the urogenital opening to measure depth
or angle of penetration proved an effective way to
sex largemouth bass; about the same levels of ac-
curacy were achieved for males and females (Table
1). Depth of penetration ranged from 0 to 40 mm
for males (83% of probes were within the range of
0-20 ram) and from 0 to 60 mm for females (82%
of probes were within the range of 30-60 ram;
Figure 2). Therefore, we adopted 25 mm as the
cutoff depth (Figure 2) for sexing largemouth bass
with our 1-mm probe (•25 mm = male; -•25
mm = female). Subsequently no attempt to insert
the probe to the furthest depth was needed to sex
most females accurately.
Sexing bass with a 0.8-mm probe, Glass et al.
(1962) noted penetration depths of 8-13 mm for
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224 COMMUNICATIONS
Permtratio. D•h (ram)
FIGURE 2.--Frequency distribution of probe penetra-
tion through the urogenital openings of 155 largemouth
bass subsequently sexed by gonad inspection; open cir-
cles represent males, solid circles represent females. In
determining the sex with depth of probe penetration as
the criterion, 25 mm was established as the cutoff depth
separating the sexes (<25 mm = male, >25 mm = fe-
male).
males and 30-58 mm for females. Additionally,
Glass et al. (1962) reported an oblique angle of
probe penetration into the genital ducts of both
sexes, and a perpendicular angle of penetration if
the probe was directed into the urinary duct. In
the present study, the angle of probe penetration
into the genital duct of female largemouth bass
was typically oblique to the ventral body surface,
whereas in males (if the probe could be inserted
at all) it penetrated perpendicularly, presumably
into the bladder via the urinary duct (Figure 1).
We believe the differences noted here between these
two studies can be explained by the fact that the
probe used by Glass et al. (1962) was 0.2 mm
smaller in diameter than that used in the present
study and thus allowed entry into the sperm duct
of some male fish.
Concurrent use of several sexing methods pro-
duced the highest levels of accuracy (Table 1). For
instance, the highest rate of sexing success (98%)
occurred during the spring, when fish were sexed
by inspection for a urogenital papilla plus probing
of the urogenital opening. During the fall, a com-
bination of probing depth and angle of penetration
resulted in an overall accuracy of 94%. Addition-
ally, some females began to exhibit a reddish uro-
genital papilla by late fall.
Problems that hampered probe insertion into
largemouth bass were (1) insufficient polishing of
the probe tip, which caused it to snag on the genital
duct wall, (2) distension of the stomach with food
so that the genital duct was distorted, (3) occa-
sional occurrence of underdeveloped ovaries, and
(4) presence of a thin layer of skin, which closes
off the oviduct during late summer (Braungart
1951; Glass et al. 1962) and which had to be punc-
tured before insertion was possible. Despite oc-
casional problems with probing, we feel it was
generally a very practical method of sexing large-
mouth bass. After some experience, most fish could
be sexed in less than 20 s. Several authors have
reported using similar techniques to successfully
sex various other fish species (Moen 1959; Glass
et al. 1962; Norton et al. 1976; Mohr 1982). We
feel that this rapid method of sexing fish may be
applicable to many more species.
Other Considerations
When a probing method is used to sex large-
mouth bass, several trials should be made on each
fish to ensure that the probe has not accidentally
missed the urogenital opening and that it passes
deep enough to permit confidence in predicting
sex (Figures 1, 2). Gently rotating the probe as it
is inserted facilitates maximal penetration. Fur-
thermore, because the genital duct is anterior to
the urinary duct, the probe should be inserted clos-
er to the anterior wall of the urogenital opening
when the separate openings cannot be distin-
guished (Glass et al. 1962). Finally, it is necessary
to use a very light touch when probing, so as not
to puncture the duct walls. We noted that a light-
probing technique allowed us to feel when the probe
was confronting the somewhat elastic gonadal or
urinary bladder walls long before puncturing them.
While we did not assess changes in mortality and
reproductive success associated with probe punc-
tures, we did recapture numerous largemouth bass
that we had previously sexed. Furthermore, Ross
(1981) noted no mortality or effects on growth and
behavior after catheterization of the genital tract
and gonads of female wrasses (Thalassoma du-
perrey). Shehadeh et al. (1973) noted no abnormal
oocyte development associated with weekly cath-
eterization.
Catheterization is commonly used to extract ga-
metes and gonadal tissue in order to monitor re-
productive status and to identify the sex of sex-
ually monomorphic fishes (Stevens 1966; Chen et
at. 1969; Shehadeh et al. 1973; Ross 1981, 1984).
Although advanced oocytes and mature ova can
often be identified macroscopically, microscopic
examination of extracted tissue is required during
certain reproductive phases. Additionally, the
catheter (usually a glass hematocrit tube or poly-
ethylene tubule) must be disposable or cleaned
between uses. Catheterization, therefore, can be
more time consuming than the probing technique
in identifying the sex of fishes. Catheterization is
a very effective technique to extract oocytes and
ova, but difficulties in introducing the catheter into
male fish and in obtaining sperm samples have
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COMMUNICATIONS 225
been noted (Ross 1984). When no gametes or other
reproductive tissues are extracted, the catheter-
ization technique relies on the same criteria as the
probing method (Ross 1984). It appears that at
certain times of the year a combination of the
catheter and probing methods would allow many
fishes to be accurately sexed. However, the probing
method alone might provide high enough levels
of accuracy for many population studies.
A hierarchy of techniques exists for sexing live
largemouth bass. The simultaneous use of several
methods can increase the percentage of correct sex
determinations. When predictions made by sev-
eral methods are inconsistent, input from various
sexing methods must be weighed in light of known
seasonal variations (e.g., stripping gameres and
presence ofa urogenital papilla). It seems probable
that the highest rates of sexing success can be at-
rained in the spring when sexual dimorphism is
most pronounced and the greatest combination of
sexing methods can be used simultaneously.
Acknowledgments
We thank E. Beckwith, C. Phillips, and R. Or-
ciari for assistance with field operations, and P.
Kaminski for drawing the figures. E. Beckwith, R.
Jones, J. Moulton, and C. Schaefer commented on
the manuscript. Our reviewers' comments were
also helpful. This study was partially funded by
Federal Aid in Fish Restoration, Project F57R.
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