MARINE AND FISHERY SCIENCES 36 (1): 91-100 (2023)
ABSTRACT. A previous paper in this journal by Shell and Gardner assessed various factors
around the exploration of the Mississippi River by bull sharks (Carcharhinus leucas Müller and
Henle, 1839) based on two twentieth-century occurrences. Recent evidence has suggested one of
these occurrences is a probable hoax. Here, we provide a correction to our earlier paper, as well as
additional comments on extralimital euryhaline vertebrates in the Mississippi River system, the
environmental and historical contexts for their exploration into riverine systems, and suggest steps
for any future effort to detect the usage of these river systems by bull sharks.
Key words: Freshwater elasmobranchs, extralimital, historic records, hoax.
Actualizaciones sobre las supuestas ocurrencias de tiburón toro (Carcharhinus leucas) en la
cuenca superior del Río Mississippi de América del Norte
RESUMEN. En un artículo anterior de Shell y Gardner en esta revista, se evaluaron varios fac-
tores en torno a la exploración del Río Mississippi por parte de los tiburones toro (Carcharhinus leu-
cas Müller y Henle, 1839) basados en dos sucesos del siglo XX. La evidencia reciente sugiere que
una de estas ocurrencias es un probable engaño. Brindamos aquí una corrección a nuestro artículo
anterior, así como comentarios adicionales sobre los vertebrados eurihalinos extralimitantes en el
sistema del Río Mississippi, los contextos ambientales e históricos para su exploración en los siste-
mas fluviales, y sugerimos los pasos para cualquier esfuerzo futuro en detectar el uso de estos sis-
temas fluviales por los tiburones toro.
Palabras clave: Elasmobranquios de agua dulce, extralimitantes, registros históricos, engaño.
A short summary of research into the history of bull shark (Carcharhinus
leucas Müller and Henle, 1839) occurrences in the upper Mississippi River
was published in this journal in 2021. It noted that, despite an apparent lack
of information from the fossil and archeological records and pre-20th century
historical accounts, bull sharks infrequently explore the Mississippi River
Channel in the vicinity of the city of St. Louis, Missouri (USA) (Shell and
Gardner 2021). We sought to examine this due to Mississippi River bull
sharks being an oft-cited factoid, though assessments of these statements
within the scholarly literature or attempts to understand this behavior in the
Mississippi River such as causes or frequency were virtually non-existent.
Received: 18 January 2022
Accepted: 5 August 2022
ISSN 2683-7595 (print)
ISSN 2683-7951 (online)
Journal of the Instituto Nacional de
Investigación y Desarrollo Pesquero
This work is licensed under a Creative
Commons Attribution-
NonCommercial-ShareAlike 4.0
International License
Marine and
Fishery Sciences
Updates on putative bull shark (Carcharhinus leucas) occurrences in the
upper Mississippi River Basin of North America
1Department of Vertebrate Paleontology, Cincinnati Museum Center, 1301 Western Ave, 45203 - Cincinnati, USA. 2Mary F. Shipper Library,
Potomac State College, West Virginia University, 101 Fort Avenue, 26726 - Keyser, USA. 3Missouri Department of Conservation, Oak Ridge,
USA. ORCID Ryan Shell, Nicholas Gardner
The International Shark Attack File (ISAF) notes
that bull sharks are the third most common shark
species to be implicated in an unprovoked nega-
tive encounter, accounting for 12.7% of all inci-
dents on record; with the top two species being
white sharks (Carcharodon carcharias Linnaeus,
1758) with 37.0% and tiger sharks (Galeocerdo
cuvier Péron and Lesueur, 1822) with 14.6%.
Like many shark species, bull sharks are a declin-
ing species, previously listed as ‘near threatened’
(Cascio 2017) but reassessed as vulnerable in
2020 (Rigby et al. 2021). Public perception of
bull sharks as a killing machine capable of
exploring deep into freshwater habitats have not
served the species well in popular media and are
harmful to conservation initiatives. This served as
an additional motivation to demystify bull shark
movement in the Mississippi River Basin.
The evidence for this centered on occurrences
from Alton, Illinois (USA) and the Rush Island
power station near Festus, Missouri (USA). The
former case is well-established in the ichthyolog-
ical literature (Thomerson et al. 1977) and has
never been satisfactorily falsified despite a recent
claim in a newspaper that the shark was bought at
a fish market in St. Louis (Cousins 2021).
Thomerson et al. (1977) provided sufficient argu-
ments against such a possibility and, despite
being a local oddity for the past 85 years and part
of the ichthyological literature for the past 45
years, no such first-hand evidence against its
veracity has ever been brought forward.
The latter case from Rush Island was based on
limited evidence which we noted in our paper. At
the time, it appeared credible, and was cited in the
literature (Burr et al. 2004, p. 249: ‘Indeed,
another bull shark was taken in the 1990s off the
screen of a power plant intake canal. This report
does not appear to be the product of a hoax, but
there is little information other than a newspaper
report’), and was believed to be true by fish and
wildlife professionals that the authors had spoken
with. Our paper was well-received in the media,
both by regional and national news outlets, was
downloaded from the journal homepage more
than 800 times, and was recently cited by another
scientist documenting freshwater occurrences of
bull sharks (Gausmann 2021). Our research
spurred an in-progress manuscript by one of us
(NG) to document hoaxes and misidentifications
of sharks in freshwater and it is not without irony
that we must now admit that we have accidentally
propagated such a case.
Around the time our manuscript was undergo-
ing review, RH independently examined the Rush
Island occurrence for his in-progress revision of
‘The Fishes of Missouri’. RH published his find-
ings in a journal with limited regional scope the
same month as our paper (Hrabik 2021). He
reported that the 96 cm long recovered specimen
was not a bull shark, but an Atlantic sharpnose
shark (Rhizoprionodon terraenovae Richardson,
1836). It is not unreasonable to conclude that the
Missouri Department of Conservation ichthyolo-
gist who first identified the shark could have con-
fused the specimen as juvenile requiem sharks
grossly resemble each other (Figure 1) and may
have made the presumption given its appearance
in freshwater that it was a bull shark considering
their known usage of riverine systems and knowl-
edge of the Alton occurrence. Confusion between
species of the Carcharhinidae, a wide ranging
family with at least 60 extant species, especially
at different life stages is not uncommon even for
ichthyologists (Branstetter 1982; Dosay-Akbulut
2008; Smart et al. 2016). This said, however,
Atlantic sharpnose sharks have never been report-
ed from riverine ecosystems even though this
species utilizes estuaries as nursery grounds like
many other requiem sharks (Ebert et al. 2021).
This led Hrabik (2021) to conclude the Rush
Island occurrence is a case of an improperly
dumped individual if not an outright hoax, rather
than a valid occurrence. RH brought our attention
to his work and we reached out to wildlife profes-
sionals in states bordering the lower part of the
Upper Mississippi River Basin to gather addition-
al information. After doing so, we felt Hrabik
92 MARINE AND FISHERY SCIENCES 36 (1): 91-100 (2023)
(2021) had established evidence as to why the
supposed bull shark was misidentified by prior
wildlife professionals and provided sufficient
arguments as to why it could not have naturally
appeared where it was found, even if there are
still many unanswerable questions such as who
perpetrated this likely hoax and their motivations.
However, the ability of the bull shark to penetrate
rivers as far inland in North America as far as
Alton, Illinois as well as its similar behavior in
other riverine systems worldwide apparently
makes it a subject of erroneous reports and
encourages false reports (compare Dukes 2016).
The discrediting of the Rush Island occurrence
means that the only known record of a bull shark
in the upper Mississippi River Basin was reported
from Alton, Illinois before the installation of the
Mississippi’s current system of locks and dams.
The fact that none can be verified from a time
after the installation of locks strongly suggests
that the bull shark’s upriver range has been limit-
ed by their installation for much of the 20th cen-
tury, so we must now reject our earlier proposal in
Shell and Gardner (2021) that unobservable pas-
sage through the most southward locks and dams
could have occurred to explain the 1995 occur-
rence. Regular reports of marine shark remains
dumped in improbable places further hampers
research into the history of this presumed habitat
loss (Gausmann 2021). We are also left with addi-
tional questions about the 1937 occurrence– was
this an isolated event or a previously more wide-
spread behavior? If the evidence is lacking for the
latter, then what factors led to this anomalous
A north-south migration following coastlines
has been observed in bull sharks. Along the east
coast of the United States it is observed that bull
sharks tend to summer in northern latitudes and
then move further south with cooler waters (Cas-
tro 2011). They commonly migrate as far north as
the Chesapeake Bay or even off the coast of New
England, which is further north latitudinally than
Alton, Illinois (38.8906° N latitude). These north-
ward migrations when in estuarine environments
can lead to riverine explorations such as the
Figure 1. Supposed bull shark recovered from the Rush Island power station near Festus, Missouri, from 1995, properly identi-
fied as an Atlantic sharpnose shark Rhizoprionodon terraenovae by Hrabik (2021). Photograph by Michael Fuhr (Mis-
souri Department of Conservation) provided to RH.
reports of sharks in the Patuxent and Potomac
Rivers from the Chesapeake Bay (Alderman
2014; Hedgpeth 2015; Chason 2018) or the more
infamous, but not definitively proven example in
Matawan Creek from the Raritan Bay in New Jer-
sey in 1916 (McCormick 1963; see also Gaus-
mann 2021). This may help contextualize the
1937 occurrence.
Other euryhaline marine vertebrates are capa-
ble of widespread dispersals into freshwater sys-
tems, and the environmental influences on this
behavior are complex (currents, depth, salinity,
temperature, food availability, etc.). Understand-
ing what may represent rare or previously unob-
served behavior versus actions of wayward indi-
viduals will depend on confirming the frequency
of valid observations. Other brackish or marine
vertebrates for which osmoregulatory competen-
cy is not a factor in freshwater survivability have
demonstrated surprising long-distance dispersals
into riverine systems.
One notable instance of a wayward euryhaline
marine vertebrate in the lower Mississippi River
Basin was that of a 3 m long West Indian manatee
(Trichechus manatus Linneaus, 1758) (Mam-
malia: Sirenia) which wandered more than
1,200 km upriver to near Memphis, Tennessee
(USA) (Manatee... 2006; Romero 2006). First
spotted on October 23, 2006 only to ultimately be
found dead 49 days later, the individual evaded
detection by teams of federal and state wildlife
and law enforcement professionals as well as an
independent private team from SeaWorld Orlan-
do who utilized aerial and boat-based spotting,
side-scan sonar, and FLIR imaging (Officials...
2006). Though no cause of death was reported,
hypothermia seems most probable and starvation
less likely given healthy sirenians may last up to
seven months without depleting their fat reserves
(Best 1983) and variation in water salinity is not
a stress inducer for manatees when invading
riverine systems (Ortiz et al. 1998; Callejas-
Jiménez et al. 2021), unlike other marine mam-
mals (Pemberton 2011). This large-scale move-
ment upriver may have taken 36-44 days to com-
plete based on modest estimations of migratory
behavior in T. manatus (27.3-33.5 km d-1 typical-
ly over 10-15 days according to Deutsch et al.
Consequently, if such a large and otherwise
conspicuous species such a manatee, for which
salinity is not an ecological barrier, can navigate
this far upriver without detection and evade con-
centrated human effort to locate it, it raises ques-
tions as to the potential to evade human detection
by smaller species such as bull sharks. These
sharks also engage in difficult to observe behav-
iors in freshwater and brackish systems, such as
swimming low in the water column when moving
upriver during the daytime (Heupel et al. 2010), a
behavior retained during migrations (Daly et al.
Similar accounts of long-range dispersals
upriver in the lower Mississippi River Basin have
been made by other typically brackish or offshore
elasmobranchs, such as dasyatids (McCormick
1963; Brown 2010), and common freshwater
species which frequently grow larger than bull
sharks (alligator gars, blue catfishes, Gulf stur-
geon, and paddlefishes) often move up and down
the Mississippi River without being detected until
they are caught on a line or in a net (Broom 2015)
(Figure 2).
Despite the near lack of information in the
upper Mississippi River Basin, archeological
reports indicate an indigenous interest in the
recovery of non-fossil shark material in central
Louisiana (USA) from approximately 1,300
years before present (though extra regional trade
cannot be ruled out to explain its presence)
(Springer 1980). Furthermore, a supposed white
shark was captured in Natchez, Mississippi
(USA) in 1829 more than 500 km up-river from
the Gulf (Another… 1829), though the size of
the animal and its freshwater location are more
suggestive of a bull shark. Its identification as C.
carcharias is probably more reflective of the
limited ichthyological knowledge of that time
94 MARINE AND FISHERY SCIENCES 36 (1): 91-100 (2023)
(Carcharhinus leucas was not yet described until
1839). This is much further than other reported
cases of freshwater penetrations by bull sharks in
the Mississippi River system by Gunter (1938) at
300 km up-river for Simmesport, Louisiana and
440 km up-river for Jonesport, Louisiana,
although Matich et al. (2020) gave information
on a bull shark record from 950 km up-river in
the Red River, Louisiana, Arkansas, Oklahoma,
and Texas. Thomerson et al. (1977) cited Gunter
(1938) and observed ‘[Bull sharks] undoubtedly
occur much more commonly in the lower Missis-
sippi than the scientific record discloses, and the
wonder is that they have not been reported much
farther up the river than they have’ (p. 168). It
would therefore seem that the exploration, even
if not long term habitation, of North American
rivers, as a behavior, has a deep Holocene history
in bull sharks, and the presence of their DNA in
the Alabama-Tensaw River of Alabama (USA) as
recently as 2018 (Drymon et al. 2021) indicates
that this pattern is ongoing in the lower reaches
of the Mississippi River Basin, exploration in
other rivers along the Atlantic and Gulf coastal
plains is documented though not fully under-
stood. The same applies to sustained migrations
of the species in coastal and oceanic environ-
ments (Carlson et al. 2011; Daly et al. 2014; Lea
et al. 2015). For now, however, evidence of bull
shark exploration into the upper Mississippi
River Basin is given only by a singular occur-
rence, and it is possible river impediments pre-
vent bull sharks (and other anadromous fish) to
move into the upper stretches of the Mississippi
River. If this occurrence represents anomaly
rather than evidence of a more common, yet dif-
ficult to detect, behavior, reasons for that conclu-
sion have not been explained to date, and the
post-1937 manmade impediments on the river
may have impeded subsequent occurrences as
Figure 2. Relative sizes of wayward individuals of marine species (inside yellow box) compared with large regular freshwater
or brackish species of the Lower Mississippi River Basin and the average size of male and female bull sharks (Car-
charhinus leucas Müller and Henle, 1939). Silhouettes courtesy of Phylopic ( unless noted otherwise
in acknowledgements. Sizes for species derived from common lengths reported in FishBase (
or Animal Diversity Web ( The average length of the bull shark was derived from Ebert et
al. (2021: 535).
American alligator
Bull shark
Blue catfish
Alligator gar
Gulf sturgeon
Alton bull shark
Rush Island shark
2 m
suggested by Thomerson et al. (1977). However,
historic reports of bull shark records from further
large streams of the world such as the Tigris
River, Iraq (Günther 1874) and the Amazon
River in Brazil, Colombia, and Peru (Thorson
1972) suggest that long-distance movements of
bull sharks up to thousands of kilometers are
exceptional and rare events, and the majority of
immature bull sharks presumably utilize the
lower reaches of rivers.
Increased surveillance for large predatory fish
in the lower Mississippi River Basin may help to
better understand exploration of riverine systems
by bull sharks and other marine fishes. This
increased surveillance could take the form of
environmental DNA monitoring (such as that of
Drymon et al. 2021), through the tagging of indi-
vidual bull sharks in nearshore ecosystems which
could reveal riverine exploration, or a wider pop-
ulation survey of fishes in near shore and
upstream localities. The authors encourage an
extensive investigation of the Mississippi River
Basin as a potential nursery area for bull sharks,
as the few records from this river systems indi-
cate this function, and moreover, the influence of
anthropogenic impact such as water pollution on
the fish fauna that inhabit this system. Currently,
the Mississippi River must be valued as a data-
poor area regarding the occurrences of sharks,
and the importance of this river system for elas-
mobranchs is unclear.
While the behavior of bull sharks in estuarine
and riverine environments has been the subject of
previous studies (Heupel and Simpfendorfer
2008; Ortega et al. 2009; Heupel et al. 2010; Cur-
tis et al. 2011; Curtis et al. 2013), it is important
to continue to monitor for behavioral changes
associated with climate change for which temper-
ature increases and sea level rises are predicted to
alter or increase the frequency in which bull
sharks may utilize these near-coastal brackish to
freshwater environments (Heithaus et al. 2009;
Matich et al. 2020). Recent studies have already
demonstrated that given their reliance on these
ecosystems as nurseries, bull sharks are suscepti-
ble to climate change-driven alterations of estuar-
ies (Niella et al. 2022). There is already evidence
for these behavioral shifts in the global fossil
record of bull sharks dating back at least as far as
the Pleistocene (Lopes et al. 2020) and oppor-
tunistic exploration by bull sharks into man-made
waterways are well documented worldwide
(Alive... 1896; Montoya and Thorson 1982;
Werry et al. 2012; Curtis et al. 2013). There are
increased reports of bull sharks in these environ-
ments globally (Gausmann 2021; Hasan et al.
2021; Gausmann and Hasan 2022), though it
could be due to increased effort to gather these
records rather than a behavioral shift.
Additionally, the widespread digitization of
historical records such as newspapers or archival
image and manuscript collections have provided
a powerful new tool in understanding the past
geographic distributions and relative abundance
of fishes (Cochran 2007; McClenachan 2009;