RE:Monday NAV was back onsideGood point Obscure...
Back in the day when the difference between the bid and the ask was an eighth and later a sixteenth, there were no high speed traders as the math simply didn't work. Now with the difference between the bid and ask can be a fraction of a cent there is lots of scope for high volume trades to make a miniscule fraction on a lot of trades and hence a lot of money.
To put all this into perspective, you mention a volume of 9 million shares traded on ENB alone. In the early 80s, the total volume on the TSX on a busy day was less than that.