Serious harm is defined in the legislation. It's essentially “the death of fish or any permanent alteration to, or destruction of, fish habitat”. Fish habitat is also defined in the legislation as the “spawning grounds…nursery, rearing, food supply and migration areas, on which fish depend directly or indirectly”. It's very broad, and that's very consistent with the current definition of fish habitat as well.
On May 28th, 2012. See this statement in context.