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River Redhorse (Moxostoma Carinatum)

Limiting Factors and Threats

The river redhorse is at the northern limit of its distribution in Canada with low numbers and disjunct distributions. Tolerance for a narrow range of habitat characteristics, and a limited amount of suitable habitat restricts the distribution of river redhorse. It is an inhabitant of medium to large-sized rivers and intolerant of high turbidity levels, siltation and pollution (Trautman 1981; Jenkins and Burkhead 1993; Mongeau et al. 1986, 1992; Vachon 2003a) and likely disappeared from watersheds highly developed for intensive industrial agriculture, such as the Yamaska and Châteauguay rivers in Quebec.

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. Large increases in discharge during the spawning period have been observed to prevent the spawning of other redhorse species (Bowman 1970; Cooke and Bunt 1999). River redhorse is also a late spring spawner and as such is significantly smaller at the end of the first growing season than earlier spawning redhorse species (Vachon 1999a). As over-winter survival of YOY is size-selective (Sogard 1997), YOY river redhorse are less likely to survive than earlier spawning species. Lastly, river flows during spawning are lower compared to earlier spawning redhorse species and the river’s capacity to dilute chemicals is reduced. In the Richelieu River, late spring spawning coincides with period of peak pesticide application. In the Yamaska and Richelieu rivers, Gendron and Branchaud (1997) provide evidence that the final steps of sexual maturation by copper redhorse (another late spawner) in these rivers is disrupted by exposure to agricultural, urban and industrial toxins as they congregate to spawn. In the mid-1990s, poor river redhorse gonadal condition was observed in specimens collected from this area.

In addition to adverse effects on spawning habitats, siltation can also result in decreased production of benthic macroinvertebrates and freshwater molluscs, the primary components of the river redhorse diet (Waters 1995; Vachon 2003a). French (1993) suggested that declines in mollusc populations caused by pollution and siltation of habitat will likely factor in the decline of mollusc–feeding catostomids such as the river redhorse.

Canadian populations of river redhorse are regionally isolated and locally fragmented. Rivers supporting river redhorse in Canada are generally fragmented by hydroelectric, navigational and flood control dams. Dams and impoundments have been identified as potential limiting factors for Canadian river redhorse populations (Portt et al. 2003). Recent evidence from the Trent River suggests that large reproducing populations of river redhorse are only found in river fragments of sufficient size with shallow fastwater habitats (Reid 2002). The decline of the river redhorse in the St. Lawrence River coincided with the beginning of works in support of hydroelectric production and shipping (Dumont et al. 1997). In the United States, impoundments have been shown to have a strong negative impact on the distribution of river redhorse (Etnier and Starnes 1993; Quinn and Kwak 2003). In Virginia, dams have also been implicated in preventing the re-establishment of river redhorse populations after fish kills (Jenkins and Burkhead 1993). The influence of hydroelectric energy production on downstream flow levels (i.e. peaking) may impact downstream river redhorse populations. Maintenance of sufficient flows during spawning, egg incubation and through nursery habitats is necessary for successful recruitment and population persistence of the river redhorse. Further dam construction would adversely affect river redhorse populations by altering upstream and downstream habitat conditions; restricting the movements of individual fish; and limiting gene flow between populations.

River redhorse populations may be at risk due to recreational angling activity, in particular the Grand River population where angling for redhorse is reported to occur (Portt et al. 2003). During spring spawning runs, congregative behaviour likely increases the susceptibility of river redhorse to recreational angling or spearfishing. The river redhorse is not afforded protection by catch limits, minimum size restrictions, or spearfishing regulations (McAllister et al. 1985; Ontario Fishing Regulations 1989). Due to lack of regulation and vulnerability during the spawn, populations may be affected to a degree. Additionally, confusion with other sucker species may result in unknown harvesting and be a factor in decline of local populations (Parker and McKee 1984). This potential threat has not been quantified. In Quebec, to prevent accidental catch of the copper redhorse and river redhorse, sportfishing is prohibited for sucker species in the sectors of the Richelieu, des Mille Iles, Yamaska and Noire rivers where both species cohabit. Commercial catch of these two species is also prohibited in Quebec.

Most field biologists have difficulty correctly identifying river redhorse. Past biological studies (e.g. creel censuses, fish community inventories) in Ontario often failed to report the presence of redhorses to species due to problems with species identification and lumping of fish into the category of suckers or coarse fish (Cooke and Bunt 1999). A lack of long-term population monitoring data in conjunction with difficulties associated with correct field identification limits our ability to protect remaining populations. In Quebec, this problem was partly resolved for specimens longer than 250 mm by the production of a poster facilitating the identification of the two Catostomus and the five Moxostoma species occurring in the province (Mongeau 1984; Mongeau et al. 1986). This poster has been distributed to the scientists and consultants working in the St. Lawrence lowlands. Recently, genetic-based Moxostoma species identification tools (Branchaud et al. 1996; Harris et al. 2002; S. Reid unpubl. data) and new taxonomic keys (Holm and Boehm 1999; Vachon 2003b) have been developed.

Introgression among catostomid species due to habitat alteration has been identified as a conservation concern in western North America where hybridization between catostomid species is often common. However, hybridization is unknown or rare in most eastern suckers (Jenkins and Burkhead 1993). Of thousands of Moxostoma specimens examined, only two cases of hybridization have been reported (Jenkins 1970; Jenkins and Burkhead 1993). This included a single river redhorse hybrid with either a shorthead or greater redhorse. Barriers to hybridization among Moxostoma species include aggressive behaviour and differences in spawning time, temperature and habitat (Curry and Spacie 1984; Kwak and Skelly 1992).