River redhorse (Moxostoma carinatum) assessment and update status report: chapter 9

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Population Sizes and Trends

Populations appear to have been lost from the Ausable, Châteauguay,Noire and Yamaska rivers. Given the lack of baseline data, population size trends of extant populations cannot be addressed directly. Populations are still present in the following rivers identified in the previous status report to support river redhorse: Mississippi, Ottawa and Richelieu. Evidence of successful reproduction (YOY and juveniles) has been collected from the lower Grand River (S. Reid unpubl. data) and from the Richelieu River (Vachon 1999a, 1999b, 2002). Large aggregations of spawning river redhorse (50+ individuals) have been identified along the Grand River, Trent River and Gatineau River, and in the Vianney-Legendre Fish Ladder on the Richelieu River. Rather than an increase in range and population sizes, the increase in number of known rivers supporting river redhorse is interpreted to be the result of more intensive sampling of large rivers in Ontario, the use of more efficient gear (boat electrofishing units), and greater interest in the ecology of the species.

Considered common in the 1940s in the St. Lawrence River around Montréal (Vladykov 1942), river redhorse is now very rare. The decrease is likely related to reduced access to the Soulanges Rapids located between Lac Saint-François and Lac Saint-Louis. As well, from 1929 to 1961, the discharge of the St. Lawrence River was almost completely diverted to the artificial Beauharnois canal for hydroelectricity production and navigation (Morin and Leclerc 1998). A series of four dams block access to four sections of rapids.

Campbell (2001) provided the only attempt at population size estimation. Summer mark-recapture-based population estimates for the Mississippi River population ranged between 623 and 830 individuals. Mark-recapture population estimates for the Gatineau River population during the peak of spawning was 1012 individuals (Campbell 2001). However, low numbers of recaptured fish and violations of model assumptions (e.g. high immigration rates) indicate that these estimates should be interpreted with caution. Identification of long-term population trends would benefit from continued standardized sampling. For future monitoring, it is recommended that relative abundance be measured during the spawning period (early-mid June) when large numbers of adults are aggregated in habitats effectively sampled by boat-electrofishing. Standardized nearshore fish community trap-net surveys in the Ottawa River and the Bay of Quinte, Lake Ontario during summer and fall may also provide useful data for future assessments.