Hi,
The Rideau is literally crawling with crayfish. I was fishing behind Carleton U., and the crayfish were as thick as blackflies in season. A couple even crawled up my leg - and they were pretty damn big.
I later learned they're an invasive species of crayfish - the rusty crayfish. They're big enough to make it worthwhile to trap them and do a crayfish boil shore lunch!
Cheers,
Neil