Stream Capture Events and the Range of the Robust Redhorse Sucker Fish (Moxostoma robustum)

Edward Cope first described the robust redhorse sucker fish in 1870, but it was not rediscovered until 1980, and the newly discovered populations in the Savannah and Pee Dee Rivers were not correctly identified until 1991.  It’s a rare fish listed as endangered by the state of Georgia.  Populations of this species are known from the Savannah, Oconee, and Ocmulgee Rivers in Georgia, and the Pee Dee River in North and South Carolina.  Researchers are in the process of introducing the fish to the Santee River, South Carolina where it is thought to have become extirpated.  The robust redhorse sucker fish lives in rocky pools and the slow runs of small rivers.  It spawns in shallow water with gravel bottoms.  Such environments are now far less common than in days of old because sediment from cleared agricultural and suburban development covers the gravel bottoms, making them unsuitable for spawning sucker fish.  This species spawns in late spring and early summer.  It feeds on freshwater mussels and insects, and the most common species in its present day diet is the invasive Asian clam.  The fish crushes the shells with teeth located in its throat. It grows to over 2 feet long and up to 18 pounds, and it can live for up to 25 years.

Robust Redhorse drawn by Joe Tomelleri

Robust redhorse sucker fish drawn by J. Tomalleri.

There are at least 10 species of redhorse sucker fish in Georgia including the silver redhorse (Moxostoma anisurum), river redhorse (M. carratum), notched redhorse (M. collapsom), black redhorse (M. duquesne), golden redhorse (M. erithrurum), harelip redhorse (M. lacerum), greater jumprock (M. lachner), blactailed redhorse (M. poecilurum), and striped redhorse (M. rupicates).    There may be an additional 3 species not yet given scientific names.  Sucker fish are often labeled as trash fish, but some people know better.  Fish fries featuring sucker fish are held in various locales, and the people who eat them seem to enjoy the fish.  I’ve never had the opportunity to try sucker fish.

The study of redhorse sucker genetics referenced below determined the population of  M. robustum found in the Pee Dee River diverged from the Savannah River population 1.5 million years ago.  There is a considerable distance between these 2 river drainages.  This fact inspires the obvious question: How did this species come to occupy such disparate ranges?  There is a simple explanation.  River drainages are separated by high ridges of land.  Streams that flow into major rivers often erode backwards into the ridges and occasionally erode into the stream of another river system.  These backward flowing streams then capture the other stream, causing it to reverse and flow into the other river drainage.  This is how fish species spread into different river drainages.  The Savannah River drainage is too far away from the Pee Dee River to have ever shared a stream capture event.  However, the robust redhorse sucker fish formerly occurred in the river drainage between these 2 distant river systems.  The Santee River drainage served as a conduit between the 2.  Various stream capture events between the Savannah and Santee and between the Santee and Pee Dee facilitated the exchange of many species of fish.

South Carolina Lakes and Rivers Map

Map of South Carolina river systems.  A series of stream capture events in the Southern Appalachians likely led to the colonization of the robust redhorse sucker fish and many other species of fish into adjacent river systems.  Populations of this species have been found in the Savannah river system and the Pee Dee river system.  Note the considerable distance.  The intervening population has become extinct.

Effects of stream capture on species-richness of adjacent drainage basins based on dispersal, vicariance, and extinction

Figure showing how stream capture events can occur and result in the spread of a fish species into another river system.

Streams erode backwards more rapidly in overgrazed landscapes.  Pleistocene megafauna overgrazed many areas, thus increasing the frequency of stream capture events in the past.


Darden, T.L.; and C.M. Tarpey

“Genetic Characteristics of the Savannah and Pee Dee River Populations of Robust Redhorse (Moxostoma robustum) with Conservation Implications”

Copeia 2014

Matthews, William

Patterns in Freshwater Fish Ecology

Springer 1998


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