No they are stocked, theres very little natural reproduction.
Its a good conservation/success story though. The river was largely dead for lack of better terms about 15 years ago. Gist of it is some people noticed the river had all the makings of a great Brown fishery (bottom draw dam, insect life) though after rainbow efforts werent promising and the rest is history.
Litterally from rags to riches in 10-15yrs.
if your curious heres a bit about the history and management plan.
http://www.grandriver.ca/FisheriesManagement/Tailwaterplan.pdf
fyi Zach, even the Becks have been drawn to it
😉