TMX Group Ltd's CEO on Thursday brushed off signs that the grip of Canada's dominant stock exchange operator on overall trading volumes was slipping, saying recent market share losses were mostly due to its rejection of certain listings.

TMX has shied away from listing marijuana-related companies with U.S. investments or operations, so many of them have turned instead to the smaller Canadian Securities Exchange, or CSE.

TMX is wary of legal liability as more U.S. states legalize the drug while it remains illegal under federal U.S. law, industry participants and analysts say.

July data from industry body IIROC shows the market share in Canada of TMX's main Toronto Stock Exchange and its junior TSX Venture Exchange slipped to 54.8 per cent from 57.5 per cent in June. It had 61.9 per cent of all trading volume a year earlier.

Meanwhile, CSE's share rose to 7.9 per cent in July from 6.4 per cent in June and 2.9 per cent a year earlier, the data showed.

"The bulk by far of that is coming from the additional listings on CSE, for the record, most of which would not qualify to be on our venues," TMX Chief Executive Officer Lou Eccleston said on a call with analysts.

One such company is CannTrust, a medical marijuana producer that has licensed a cannabis-infused single-serve coffee and tea product for U.S. markets and plans to list on CSE next week.

He said U.S.-based Nasdaq and local rival Aequitas, which both began operating Canadian venues in recent years, did not appear to have been a factor, but added that volumes were also hurt by growing traffic on Match Now, a dark pool alternative trading system.

Eccleston said the TMX expected to start a dark pool venue, which allows big players to place and match buy-and-sell orders anonymously to avoid tipping off rivals and moving share prices, in the fourth quarter to address that demand.

Nasdaq's share of trading volume was 15.7 pe rcent in July, up from 14.9 in June and 14.6 per cent a year earlier. Aequitas' share rose to 6.3 per cent from 5.3 per cent in the previous month and 6.2 per cent a year earlier.

Shares of TMX (X.TO) were up 3.5 per cent at $68.18 in midday trading after it posted higher-than-expected quarterly earnings and revenue late on Wednesday, helped by lower costs.