MARINE AND FISHERY SCIENCES 36 (1): 91-100 (2023)

https://doi.org/10.47193/mafis.3612023010101

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.

*Correspondence:

ngardner@mail.wvu.edu

Received: 18 January 2022

Accepted: 5 August 2022

ISSN 2683-7595 (print)

ISSN 2683-7951 (online)

https://ojs.inidep.edu.ar

Journal of the Instituto Nacional de

Investigación y Desarrollo Pesquero

(INIDEP)

This work is licensed under a Creative

Commons Attribution-

NonCommercial-ShareAlike 4.0

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Marine and

Fishery Sciences

MAFIS

NOTE

Updates on putative bull shark (Carcharhinus leucas) occurrences in the

upper Mississippi River Basin of North America

RYAN SHELL1, NICHOLAS GARDNER2, * and ROBERT A. HRABIK3

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 https://orcid.org/0000-0001-6634-3127, Nicholas Gardner https://orcid.org/0000-0002-5278-7541

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

event?

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

SHELL ET AL.: UPDATES ON PUTATIVE BULL SHARK OCCURRENCES

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.

2003).

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.

2014).

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

SHELL ET AL.: UPDATES ON PUTATIVE BULL SHARK OCCURRENCES

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 (http://phylopic.org) unless noted otherwise

in acknowledgements. Sizes for species derived from common lengths reported in FishBase (http://www.fishbase.org)

or Animal Diversity Web (https://animaldiversity.org). The average length of the bull shark was derived from Ebert et

al. (2021: 535).

American alligator

Bull shark

Blue catﬁsh

Paddleﬁsh

Alligator gar

Gulf sturgeon

Memphis

stingray

Memphis

manatee

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;

Cochr