Greater redhorse  (Moxostoma valenciennesi)

The greater redhorse has yet to be classified by the IUCN.

The largest of the redhorse (Moxostoma) species (3) (4), the greater redhorse (Moxostoma valenciennesi) is a large, stout freshwater fish with a dark brown to copper back, yellowish sides and a whitish belly (2) (3) (4) (5). The scales on the back and sides have a dark spot at their base (3) (5) (6). The greater redhorse’s tail fin is red, and the other fins are yellow to orange or reddish (2) (3) (4) (5) (6).

The greater redhorse has a large head and a large mouth, with thick lips that are covered in folds (3) (5) (6). In adults, the dorsal fin has a straight or slightly convex edge (3) (4) (6), and the tail fin is large and forked, with two pointed lobes (5). The female greater redhorse is generally larger than the male (2), and breeding males have large, white tubercles on the tail fin and anal fin (2) (3).

Like other fish in the Cyprinidae family, the greater redhorse does not have teeth in its jaws, instead processing food using specialised bones in the throat (7). Known as ‘pharyngeal teeth’, these are thin and bladelike in this species (5) (6).

The greater redhorse occurs in North America, from Ontario and Quebec in Canada and northern Minnesota in the United States, south to Indiana, Ohio and Kentucky. It has been recorded in the Great Lakes-St. Lawrence River, Hudson Bay (Red River) and Mississippi River basins (2) (3) (4) (5).

Although it has also been reported south to the Ohio River in Kentucky (2) (3) (5), the greater redhorse may now have disappeared from this state (3).

This freshwater fish is usually found in medium to large rivers and sometimes also in lakes (2) (3) (5) (8). The greater redhorse generally requires clear water, without much silt (3), and prefers parts of the river with sandy or rocky pools, or areas of moderate to fast current (2) (3) (5) (9).

The greater redhorse typically migrates upstream to spawn over gravel, cobble or pebble substrates in shallow riffles (2) (3) (10) (11) (12).

The greater redhorse usually spawns between May and July, depending on the location (3) (4) (11) (12). On arriving upstream at the spawning grounds, the males congregate and hold territories, generally moving about slowly and often nudging each other. The females visit the males when ready to spawn (3) (12). This species may spawn in groups of two to seven males and one or two females (2) (11), or in trios of two males to one female (3) (12). The male greater redhorses flank a female on either side, rolling over each other and the female while the tail and dorsal fins are vibrated and the eggs and sperm are released over the substrate (2) (11) (12).

The female greater redhorse is estimated to release around 32,000 to 72,000 yellow eggs, which hatch in about 6 to 8 days (11) (13). Both male and female greater redhorses are thought to become mature at around 5 to 6 years old (3) (11). After spawning, the adult greater redhorses move back downstream to areas of deeper water and slower current (10).

The diet of the greater redhorse comprises a variety of invertebrates, including aquatic insects, molluscs and crustaceans (3) (4), as well as some plant material (3). This species is likely to forage mainly on the river bottom (3) (8).

Although it is widely distributed, the greater redhorse occurs in relatively small, disjunct populations and has been declining in parts of its range (3) (5) (8). Its numbers may have stabilised or even in increased in some areas in recent years due to improvements in water quality, but smaller, more isolated populations and those on the edges of its range are more vulnerable (3) (8).

The greater redhorse is particularly sensitive to alterations to its habitat, with the main threats coming from siltation and pollution, which may reduce its prey and also reduce the quality of its spawning habitats. The habitat of this species may be further degraded by dam construction, river channelization, alterations to flow regimes, and the removal of riverside vegetation. Infrastructure such as dams can also create barriers along spawning migration routes (3) (8).

The greater redhorse is not commonly targeted by anglers, but low numbers may be caught in some areas (3) (8).

The greater redhorse is listed as a ‘Species of Concern’ by the U.S. Fish and Wildlife Service (14). There have been few specific conservation measures targeted at this species, but it is likely to have benefitted from general improvements in water quality and from efforts to restore stream habitats and riverside vegetation, as well as to improve fish passage at dams (3) (8).

Recommended conservation measures for the greater redhorse include undertaking habitat restoration, and protecting its habitat from pollution and from erosion problems, which can lead to siltation (3). Detailed information on this species is lacking, so further research is required to determine the extent of its distribution, to monitor its population trends, and to increase the knowledge of its habitat requirements, diet, movements and life history (3) (8).