The point is if the disturbance is intentional or not. The important part of the law also states ..
any spawn or spawning fish may be may be being the important thing.
If I said to you .. don't wade there, there may be spawning fish.. and after my warning you continue ..then the law would see this as wilful
see here
the same of course could be applied to canoeists, who, if they were warned by signs would be committing an offence. The interesting point would be if the people who were encouraging the law to be broken by erecting signs to canoe and selling tickets to fish and wade are actually encouraging people to break the law for their own gain, as they are custodians of the river one would expect them to be aware of the areas where and when fish spawning takes place and to warn people accordingly.