Recovery potential assessment of black redhorse (Moxostoma duquesnei) in Canada

Abstract

The current distribution of Black Redhorse in Canada is limited to southwestern Ontario, including tributaries of Lake Erie (Grand River and tributaries), Lake Huron (Ausable River, Bayfield River, Maitland River, Saugeen River, Sauble River and Gully Creek), and Lake St. Clair (Thames River and tributaries). Most populations are considered to be in poor status. Black Redhorse generally occupies cool, clear streams over clean pebble, gravel and cobble substrates in 0.6–2.5 m of depth with medium to fast flow and gradients of 1.2–1.5 m/km. Juveniles may occupy shallower runs, riffles or pools with reduced flow compared to adults. Spawning occurs in late spring in shallow riffles with cobble substrates, when water temperatures are between 15 and 21 °C. The greatest threats to Black Redhorse in Canada are pollution from urban wastewater and agriculture, and natural systems modifications, such as dams and water withdrawals, that result in altered flow regimes and groundwater disruptions. Additional threats include impacts from climate change related to increased temperatures and changes to flow, invasive species, indirect impacts from recreational fishing (including incidental bait harvest), and recreational vehicle and vessel use in streams. To achieve persistence, the minimum viable population size for Black Redhorse is estimated to be 1,700 adults (ages 4+) and 3,900 juveniles (ages 1–3), assuming the probability of catastrophe (50% decline) is 0.15 per generation, with an extinction threshold of 50 adults. The minimum area for population viability was estimated to be 14.5 ha of good quality, suitable habitat. Black Redhorse occupies approximately 554 linear river kilometer segments in Ontario; although suitable habitat has not been quantified, there is likely sufficient area to support populations of Black Redhorse in each of the occupied watersheds. Modelled recovery projections that incorporate a 20% increase to both juvenile and young of-year survival resulted in recovery targets being reached within 29–119 years. If a 20% increase in survival of young adults was included, recovery targets were reached in 17–40 years. Increasing fecundity further reduced time to recovery to 11–37 years. Black Redhorse populations are most sensitive to perturbations in annual survival of juveniles and early adults; therefore, harm to these life stages should be minimized. Reductions of juvenile survival greater than 64% when populations are at maximum growth rate are likely to result in population decline; allowable harm to juveniles would be lower with a lower population growth rate, and zero if the population was not growing. There remains uncertainty around population sizes and trends, availability of suitable habitat, the association with groundwater seepages, and impacts of threats related to water quality.