The River Redhorse... a Species at Risk

This species has been identified as Special Concern by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC). It is listed under the federal Species at Risk Act (SARA) and was afforded protection under the SARA as of December 2007. Additional protection is afforded through the federal Fisheries Act. Under the SARA, a management plan must be developed for this species.

General Description

Moxostoma carinatum

© J. R. Tomelleri

The River Redhorse (Moxostoma carinatum) is a large sucker in the family Catostomidae. Its common names include Big-sawed Sucker, River Mullet, Greater Redhorse, Redfin Redhorse and Redhorse Sucker. It has the following characteristics:

• Large, laterally compressed body;
• Adults are generally over 500 mm in total length, sometimes exceeding 700 mm;
• Heavy pharyngeal arch with molariform teeth;
• Subterminal mouth;
• Deeply folded lips with transverse ridges and no bumps;
• Straight or slightly concave single dorsal fin; forked caudal fin;
• Red-tinted caudal and dorsal fins; orange to reddish lower fins;
• White belly, brown or olive green back and brassy, yellowish-green or coppery sides;
• Dark, crescent-shaped spots on each scale;
• Tubercles present on the snout, anal and caudal fins of spawning males; and
• Caudal peduncle scale count is usually 12; along the lateral line it is 42 to 47.

Distribution

The River Redhorse occurs throughout the central and eastern Mississippi River system and the Gulf Slope from Florida to Louisiana. In Canada, its distribution is characterized by disjunct populations in southern Ontario and Quebec. This species has declined considerably over much of its range in the last 100 years. Populations still occur in the Grand, Trent, Thames, Mississippi, Gatineau and Richelieu rivers, and recent data suggest a wider distribution in the Ottawa River than previously documented. However, this fish appears to no longer exist in the Ausable, Châteauguay and Yamaska watersheds and has declined dramatically in the St. Lawrence River.

River Redhorse  Distribution

Habitat and Life History

In Canada, the River Redhorse lives in medium- to large-sized rivers. In the late spring, fish migrate from pool habitats with slow currents and abundant vegetation to shallow areas with moderate to swift flow, riffle-run habitats and coarse (gravel, cobble) substrates. Spawning occurs at temperatures between 17 and 20ºC and ritualized spawning displays have been observed. Fertilized eggs hatch within five to six days depending on the temperature. Larvae are dispersed by drifting to suitable rearing habitats. The age at maturity is older than in southern populations, ranging between five and ten years. Maximum age is 28 years in Canada.

Diet

The diet of the River Redhorse consists of crayfishes, insect larvae and other benthic invertebrates. It also feeds extensively on molluscs. Its large teeth allow it to crush the shells of these organisms.

Threats

Due to its narrow range of habitat preferences, spawning requirements and intolerance of high turbidity, siltation and pollution, the River Redhorse is susceptible to a number of threats. Hydroelectric development and flood control dams cause habitat fragmentation and alter habitat conditions, resulting in restricted movements of individual fish and limited gene flow between populations. Also, changes in flow regime and siltation of spawning habitats may reduce recruitment. Agricultural and municipal activities that affect water quality (increased sediment load, excessive nutrients) also adversely impact this species.

Similar Species

The River Redhorse resembles all redhorse species especially the Shorthead Redhorse (M. macrolepidotum) and the Greater Redhorse (M. valenciennesi). The River Redhorse can be distinguished, although with difficulty, from most other members of the genus by its heavy pharyngeal arch with molariform teeth. Additional features that may distinguish it from other redhorse sucker species include entirely plicate lips and caudal peduncle scale count.

Text Sources: COSEWIC Status Report 2006.

For more information, visit the SARA Registry Website at www.SARAregistry.gc.ca.