MARINE AND FISHERY SCIENCES 34 (2): 263-267 (2021)

https://doi.org/10.47193/mafis.3422021010607

ABSTRACT. Rare appearances of bull sharks (Carcharhinus leucas) in river systems and other

freshwater ecosystems have been reported from five continents. The wide geographic range of this

phenomenon, the physiological adaptations of this species to allow for movement into rivers, and a

fossil record in the Miocene Epoch, all imply that this behavior has a long history. In the Mississippi

River basin, however, only two specimens were captured in the river’s upper portion during the

entire 20th century. Further historical, archeological, and paleontological records of these animals

appearing farther than 500 km upstream from the Mississippi delta is almost nonexistent. Bull shark

movements in the upper portions of this river are likely sufficiently rare or cryptic to avoid detec-

tion using historical, archaeological, paleontological, and modern fisheries conservation tech-

niques. Deeper research into these sharks, and their historical biogeography is needed to better

understand the relationship these large predators have to the freshwater ecosystems of the American

mid-continent.

Key words: Bull shark, Carcharhinus leucas, biogeography, Illinois, Missouri, Mississippi River,

cryptic species, zooarchaeology.

Movimiento del tiburón toro (Carcharhinus leucas) en la cuenca superior del Río Mississippi,

América del Norte

RESUMEN. Se han reportado raras apariciones de tiburones toro (Carcharhinus leucas) en sis-

temas fluviales y otros ecosistemas de agua dulce en cinco continentes. El amplio rango geográfico

de este fenómeno, las adaptaciones fisiológicas de esta especie para permitir el movimiento hacia

los ríos y un registro fósil en la época del Mioceno, implican que este comportamiento tiene una

larga historia. En la cuenca del Río Mississippi, sin embargo, solo dos especímenes fueron captura-

dos en la parte superior del río durante todo el siglo XX. Los registros históricos, arqueológicos y

paleontológicos a más de 500 km río arriba del delta del Mississippi son casi inexistentes. Es pro-

bable que los movimientos del tiburón toro en las porciones superiores de este río sean lo suficien-

temente raros o crípticos como para evitar su detección utilizando técnicas históricas, arqueológicas,

paleontológicas y modernas de conservación de la pesca. Se necesita una investigación más profun-

da sobre estos tiburones y su biogeografía histórica para comprender mejor la relación que estos

grandes depredadores tienen con los ecosistemas de agua dulce del continente americano medio.

Palabras clave: Tiburón toro, Carcharhinus leucas, biogeografía, Illinois, Missouri, Río Mississip-

pi, especies crípticas, zooarqueología.

263

*Correspondence:

ryanshell501@gmail.com

Received: 5 April 2021

Accepted: 27 April 2021

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

International License

Marine and

Fishery Sciences

MAFIS

NOTE

Movement of the bull shark (Carcharhinus leucas) in the upper Mississippi

River Basin, North America

RYAN SHELL1, * and NICHOLAS GARDNER2

1Department of Vertebrate Paleontology, Cincinnati Museum Center, 1301 Western Ave, 45203 - Cincinnati, USA. 2Potomac State College,

West Virginia University, 101 Fort Avenue, 26726 - Keyser, USA. ORCID Ryan Shell https://orcid.org/0000-0001-6634-3127,

Nicholas Gardner https://orcid.org/0000-0002-5278-7541

The bull shark, Carcharhinus leucas Müller

and Henle, 1839, is an extant, large-bodied

requiem shark that is well-known for its ability

to move between freshwater and saltwater

ecosystems (Ebert et al. 2013). The use of river

systems by these apex predators sometimes

extends 1,000 km or more from a river’s mouth:

Bull sharks have been recorded in the upper

Amazon River of Peru, the upper Mississippi

River Basin of the United States, and in other

freshwater bodies such as the Zambezi and

Ganges Rivers, of Africa and the Indian subcon-

tinent, respectively (Gaussman 2018). Recent

records of freshwater incursions have expanded

to include the islands of Indonesia (Iqbal et al.

2019a, 2019b).

Bull shark fossil remains are known from the

Miocene of North and South America (Cameron

and Boreske 1972; Apolín et al. 2007), though

fossil teeth similar to the ‘bull [shark] group’ of

species may extend the North American record

of similar animals to the Eocene (Cappetta

2012). Similarly, both the Mississippi and Ama-

zon Rivers have geological histories spanning 10

million years or more. Given these observations,

it is likely that far upstream exploration of river

systems by these sharks is a behavior with a long

history as well. However, it is not presently clear

if the history of upstream exploration in the Mis-

sissippi River Basin is well-evidenced.

MISSISSIPPI RIVER BULL SHARKS IN

DEEP TO RECENT TIME

In the Miocene and Pliocene North American

fossil record, bull shark teeth are reported in

marine depositional environments and marginal

marine depositional environments, especially in

North and South Carolina (Cameron and Boreske

1972). Their appearance in what were once mar-

ginal, near-shore ecosystems suggests a potential

for past movement into fresh waters, but there are

no definitive reports of bull shark teeth in Mio-

Pliocene freshwater deposits.

The Pleistocene to early Holocene record of

freshwater bull sharks is equally limited. In the

Pleistocene, remains from a possible estuarine

deposit reported from Florida are known (Hulbert

and Morgan 1989), but all similarly aged bull

shark localities near the mouth of the Mississippi

River are most certainly marine (Ebersole et al.

2017). Historic accounts of shark teeth from

Pleistocene to Holocene Mississippi River grav-

els in western Illinois lack the identification of

teeth to species level or sketches/photographs of

the teeth that would aid current researchers in

identifying the specimens (Worthen 1882).

The Holocene archaeological record of shark

teeth found at inland Indigenous American sites is

fairly rich, especially in the Ohio River Valley

portion of the upper Mississippi River Basin. But

here bull shark teeth occur alongside marine

sharks such as the white shark (Carcharodon car-

charias), or occur alongside the teeth of Miocene-

Pliocene sharks, such as Hemipristus serra,Isu-

rus (or Carcharodon)hastalis, or Carcharocles

megalodon, suggesting their current context

resulted from extra-regional trade or travel (Mur-

phy 1975); which is well accepted for other

exotics such as marine snail shells and alligator

teeth (Colvin 2011).

One exception to the trend of non-contempora-

neous shark tooth utilization by Indigenous peo-

ples associated with the Mississippian material

culture is the report of a non-fossil shark tooth

from an archaeological site (Troyville and Coles

Creek cultural stages) in Louisiana (Springer

1980). The tooth in question, however, was nei-

ther identified to the species level nor figured in

sketches or photographs, and this record is from

the lower Mississippi River Basin, so it does not

shed light on the problem at hand.

Historic bull shark catches or sightings in the

upper Mississippi River Basin seem absent prior

to the 20th century, though confirmed catches of

bull sharks occurred in Alton, Illinois in 1937

264 MARINE AND FISHERY SCIENCES 34 (2): 263-267 (2021)

(Thomerson et al. 1977) and St. Louis, Missouri

near Rush Island Power Station in 1995 (Burr et

al. 2004) (Figure 1).

Excluding the aforementioned accounts, the

bull shark has not been confirmed to travel the

Mississippi River Basin north of Louisiana, let

alone the upper Mississippi River Basin. This is

in spite of this species’ capture in the Atchafalaya

and Red Rivers. While archaeological records

generally reveal an interest and use of shark teeth

by Indigenous people during pre-Columbian

time, there is no evidence this ever included the

recovery of bull shark material from local fresh

waters as opposed extra-regional trade (Murphy

1975; Colvin 2011). Finally, the North American

fossil record also appears to lack confirmed allu-

vial deposits of bull shark remains in the Missis-

sippi River Basin.

Given the fossil, archaeological, and historical

rarity of bull sharks in the Mississippi River

Basin of the United States, the historical behavior

of this species to sometimes explore far upstream

(with particular emphasis on occurrences near the

city of St. Louis) in America’s largest river basin

may be described by one of three hypothetical

explanations.

The first hypothesis would be that far upstream

exploration, as a behavior, is very new and may

have only begun in the last 1 million years or

fewer. This appears unlikely (see below), but if it

genuinely represents a true pattern, it could imply

that the bull shark’s ability to persist in freshwater

may also be novel, in an evolutionary sense.

A second hypothesis is that the behavior is

deeply rooted in the evolutionary history of this

species. Given the wide global range of bull

sharks that enter rivers, and their similarity to

fresh water sharks of the Genus Glyphis this

appears the most likely explanation (Fowler et al.

1997).

If either of these two hypotheses are true, it

would indicate that bull sharks are capable of

engaging in cryptic movement. This in turn seems

to suggest that a group of these large bodied,

aquatic apex-predators are capable of moving into

the Mississippi River Basin and avoid being rep-

resented in the fossil record, the archaeological

record, and (mostly) avoid detection by the mod-

ern society living along the waterways. Such cryp-

tic movement appears to have extended past the

placement of locks on the Mississippi River dur-

ing the mid 20th century. The movement of these

265

SHELL AND GARDNER: UPPER MISSISSIPPI RIVER BASIN BULL SHARKS

Figure 1. Map of historical records of bull sharks in the upper Mississippi River Basin. The Mississippi River is highlighted in

light blue. Red dot: Alton, Illinois in 1937; yellow dot: Rush Island Power Station near St. Louis, Missouri in 1995. The

base map was modified from Google Maps imagery derived from Landsat/Copernicus and TerraMetrics.

sharks past locks begs a question on how many

bull sharks enter the upper Mississippi River with-

out detection to this day.

Finally, the third possible hypothesis is simply

that the tendency of the bull shark to move into

the upper Mississippi River is rare and anomalous

to the point of being unprecedented in North

America. It may be that the 1937 and 1995 cap-

tured specimens in Illinois and Missouri were

hoaxes (despite photographic evidence for both)

or totally dependent on random chance; that tem-

perature differences between the Mississippi

River and the Gulf of Mexico prevent bull sharks

from exploring as far upstream as they do in the

Amazon River. However, this does not seem par-

ticularly likely given that both 20th century

records seem legitimate (see above), and that the

average preferred temperature of this species is

experienced or exceeded by the Mississippi River

at Alton, Illinois over a large portion of the year

(Elderkin and Klerks 2004; Kaschner et al. 2016).

Regardless of the reason for the paucity of

data, in the absence of new confirmed catches, or

new fossil and archaeological finds, the history of

far-upstream exploration in the Mississippi River

Basin by bull sharks remains in need of further

research. Key areas for consideration should

include to what extent physical barriers (such as

locks) play a role in limiting bull shark upstream

movement and what motivates or fails to moti-

vate bull shark movement into upstream environ-

ments. How habitats, prey species, and species

potentially competitive with bull sharks interact

with this shark far upstream from the mouth of

the Mississippi River will factor heavily into a

more thorough understanding of these apparently

cryptic predators.

ACKNOWLEDGEMENTS

We would like to express our thanks to col-

leagues who pre-read this report and offered use-

ful suggestions during the final writing and edit-

ing phase, especially C. Bridges, M. Hintermeis-

ter, JP Hodnett, H. Robison, G. Storrs, and D.

Cline. Additionally, we thank two anonymous

reviewers whose comments helped strengthen

this manuscript.

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