Gravel chub (Erimystax x-punctatus) COSEWIC assessment and status report: chapter 7

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Population Sizes and Trends

The gravel chub has been reported at only two localities in Canada, both of which were in the Thames River system. The earliest collection was of six specimens from the Thames River, approximately 70 km northeast (downstream) of the Moravian Indian Reserve, in 1923 (Holm and Crossman 1986). Royal Ontario Museum (ROM) collections at or near the same site in 1941 failed to record any specimens of this species. Nine individuals of the species were collected in 1958 from a site southwest of the Moravian Indian Reserve at Muncey (Holm and Crossman 1986). Six of these were catalogued as ROM 20018.

Subsequent attempts to collect this species in the early 1970s by the National Museum of Natural Sciences [now Canadian Museum of Nature (CMN)], ROM and the Ontario Ministry of Natural Resources (OMNR) were unsuccessful as were the efforts of B. Parker and P. McKee in 1971-1980 (Parker and McKee, 1980). The scarcity of collected material indicates that populations were localized. Parker and McKee (1980 1981) suggested that the failure of these attempts specifically directed to locating specimens at previously known sites left the continued existence of Canadian populations in doubt (Scott and Crossman 1998). McAllister and Gruchy (1977) considered the gravel chub to be endangered in Canada and this listing was confirmed by COSEWIC in 1985.

Consequently, staff from ROM undertook two field trips in 1985 (22-26 July and 20-23 October) specifically to sample at or near the two previously known sites. Other suitable habitats were seined and/or electrofished along a 17-km reach, above and below the previous Thames River collection sites (Holm and Crossman 1986). No gravel chub were captured. COSEWIC re-assessed the status of the species in April of 1987 and at that time listed the species as Extirpated from Canada. Sporadic collection efforts between 1985 and 2000 also failed to produce any records of the continued existence of the species in Canada, and the extirpated status was re-examined and re-confirmed by COSEWIC in May 2000.

Substantial sampling effort (mostly seining and electrofishing) was conducted between 2003 and 2006 along the lower Thames River, near former gravel chub collection sites. Using a variety of passive and active gears, Edwards and Mandrak (2006) report that a total of 71 species (including 7 species at risk) were captured in 2003 (51 sites) and 2004 (41 mainstem and 28 tributary sites) from a variety of wadeable and non-wadeable habitats between London and the mouth of the Thames River. No gravel chub were captured. Trawls were only attempted downstream of Chatham, near the confluence of Baptiste Creek. Seine hauls from 13 sites along the Thames River, adjacent to Muncey, yielded 28 species (including two species at risk), but no gravel chub (Marson et al. 2006). In 2006, 128 sites were seined between London and Chatham. More than 50 species were captured, but no gravel chub. Sites were characterized by a range of dominant substrate types (silt to cobble), and included both erosional and depositional habitats. Additional seine hauls in faster flowing, gravel runs and riffles also failed to capture gravel chub (A. Dextrase, OMNR, Peterborough, Ontario; personal communication 2007).

There is little likelihood of immigration from populations in the United States, as the former Canadian sites were approximately 300 km from the nearest American populations of the Ohio River basin (Mississippi River drainage).