Updated recovery potential assessment for Northern Madtom (Noturus stigmosus) in Canada, 2012-2021

Abstract

The current Northern Madtom (Noturus stigmosus) distribution (Figure 1) is limited to four distinct localities in Canada: St. Clair River, Lake St. Clair, Thames River, and Detroit River. The species is likely extirpated in the Sydenham River. Evidence of significant differences in population genetic structure between the Detroit and St. Clair rivers suggests that Lake St. Clair may represent a natural barrier to movement between these two riverine locations. The genetic structure of the Thames River has not been evaluated. Adults occupy a wide range of habitats with clear to turbid water in large rivers, with moderate to swift current, and occasionally lakes. Little is known about young-of-the-year (YOY) and juvenile habitat, but it is likely similar to adult habitat. Coarse woody debris, complex rocky substrates, or other overlaying objects have been used for cover and nesting. Northern Madtom occupies residences during the breeding and rearing parts of its life cycle. Spawning begins when water temperature reaches approximately 20 °C and takes place in cavity nests; males guard the eggs then young for approximately one month post-hatch (July to August) in Canada. It is uncertain when Northern Madtom start building nests. To achieve ~99% probability of persistence over 100 years, a minimum viable population (MVP) requires ~97,000 (CI: 29,000–230,000) adult and juvenile Northern Madtom (assuming age at maturity of 3 and maximum age of 5). Given evidence of population structure (reproductive isolation), each population needs an abundance consistent with the MVP to achieve long-term sustainability. Given current density estimates, an MVP abundance for each population would require ~1,900 ha of suitable habitat in the St. Clair River, ~1,600 ha in the Thames River, and ~1,700 ha in the Detroit River. Time to recovery was assessed with a simulation of an initial population at 10% of MVP. The median time to reach recovery (i.e., MVP) was 17 years, and 95% of populations reached MVP in 58 years or less. The greatest threats to the survival and recovery of Northern Madtom in Canada are invasive species, climate change, pollution, and shipping channel works. Population growth rate and abundance are most sensitive to perturbations in juvenile survival, followed by fertility and YOY survival, and are least sensitive to adult survival. There remain numerous sources of uncertainty related to Northern Madtom biology and life history, population abundance estimates and trends through time, status in the Sydenham River and Lake St. Clair, and total habitat extent. A thorough understanding of the mechanisms of impact of numerous threats affecting the Northern Madtom populations is also lacking.