It's a really great question as well.
The old subsection 36(3) and the way it's currently worded, if the exceptions weren't given, was to prevent the deposit of a deleterious substance into waters frequented by fish. It did prevent cumulative impacts because it didn't allow the proponent to sample from the receiving waters. It had to sample from the actual discharging waters.
The idea was the precautionary principle was built into it. As long as no one put in deleterious substances—those that kill fish in 96-hour acute lethality tests or whatever else you want to use, such as Daphnia magna—and you didn't discharge water that was acutely toxic, then there wouldn't be a cumulative impact problem.
Now, with the new changes to the act, the government is going to have to find new ways to take into account cumulative impacts, how it's going to measure it, and how it's going to monitor it. With self-regulation it's going to be even harder, because the government somehow needs to be in charge to keep an eye on what's happening generally, aggregating that information and ensuring that the laws are put in place to prevent real destruction of fish and fish habitat. We shouldn't wait until that happens before we act.