River redhorse (Moxostoma carinatum) assessment and update status report: chapter 2

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Species Information

The river redhorse (Moxostoma carinatum) is one of seven species of the genus Moxostoma in Canada. It is similar in appearance to the shorthead redhorse (M. macrolepidotum) and greater redhorse (M. valenciennesi). It can be distinguished from other redhorse sucker species by its moderate-sized head, entirely plicate lips, and caudal peduncle scale count.

Distribution

The river redhorse is found throughout the central and eastern Mississippi River system and the Gulf Slope from Florida to Louisiana. Several disjunct populations of river redhorse are found in southcentral Ontario and southern Quebec. Since the previous status report, river redhorse populations have been reconfirmed at some historical locations. New populations have been identified in Ontario and Quebec.

Habitat

In Canada, the river redhorse has been captured in both river and lake environments. However, its persistence is dependent on access to suitable riverine spawning habitat: moderate to swift current, riffle-run habitat and clean coarse substrates. Outside of the spawning period, reduced abundance in these habitats suggests that deeper run/pool habitats are used during other periods of the year. In the Richelieu River, young-of-the-year are found along vegetated shores where average depth is 1.5 m (maximum ≤ 3.0 m), the bank slope is shallow (≤ 20º) and the substrate is fine (silt, clay and sand).

Biology

The river redhorse is a late-maturing, long-lived species that requires large interconnected riverine habitat to fulfill the need of all life-stages. Generally, mature specimens are greater than 500 mm TL, and can exceed 700 mm TL. The maximum age reported for river redhorse in Canada is 28 years. Spawning occurs during late spring in areas with fast-flowing water and gravel or cobble substrate. Except for the few Quebec rivers supporting copper redhorse (M. hubbsi), the river redhorse is the last of the Moxostoma species to spawn. River redhorse feed primarily on benthic invertebrates including molluscs, insect larvae and crayfishes.

Population Sizes and Trends

Given the lack of baseline data, population size trends cannot be addressed directly. Populations in the Mississippi River, Ottawa River, and the Richelieu River identified in the last status update are still present. In some watersheds, the species has likely disappeared (Châteauguay and Yamaska) or suffered substantial declines (St. Lawrence River). Population sizes for the Gatineau River and Mississippi River have been estimated. However, low numbers of recaptured fish and violations of model assumptions (e.g. high immigration rates) indicate that these estimates have a high degree of error. Identification of population trends would benefit from continued standardized sampling.

Limiting Factors and Threats

Tolerance for a narrow range of habitat characteristics and a limited amount of suitable habitat restrict the distribution of river redhorse. It is an inhabitant of medium- to large-sized rivers and intolerant of high turbidity levels, siltation, and pollution. Rivers supporting river redhorse are generally fragmented by hydroelectric, navigational and flood control dams. Dams can adversely affect populations by altering upstream and downstream habitat conditions, restricting the movements of individual fish, and limiting gene flow between populations. Due to restrictive spawning habitat (water depth and substrate) preferences, river redhorse recruitment is vulnerable to changes in the flow regime and siltation of spawning habitats.

Special Significance of the Species

River redhorse was historically a food source for native people and first European settlers and a species of minor commercial importance near Montreal. This fishery no longer exists. It is one of the few freshwater fishes that feed extensively on molluscs, thereby performing a unique ecological function.

Existing Protection or Other Status Designations

The river redhorse was first designated as Special Concern (Rare) by COSEWIC in 1983 and was reconfirmed as such in 1987 and 2006. In Ontario, it has been ranked as Special Concern by the OMNR. The river redhorse will be designated Vulnerable in Quebec.