I suppose that would technically be in violation of the stay at home order,
Well, you are supposed to stay at home unless essential but the rules in detail
https://www.ontario.ca/laws/regulation/r21265 allow
Exercising, including, ... using an outdoor recreational amenity that is permitted to be open.
I believe fishing is permitted, if you consider it
exercise however there are other restrictions ...
"Do not travel outside your region"
so, the way I read this is if you have access to "reasonably close by" fishing, you can do it, but you cannot travel to go fishing. Again, a grey area on how to interpret "outside your region".
My personal interpretation is that " region" reasonably maps to the health unit you live (ie the area they report covid for where you live) so for me its the boundary for Ottawa Public Health since I live in Ottawa. So I should not travel to outside that (say to renfrew health or leeds health unit) but otherwise fishing as exercise seems reasonable. This is me, not advice. This means this years lake ontario trib opener was a bit too far to make the run... See ya next year Ganaraska
I have co-workers in India and they are getting slammed, most have already lost family members and its getting worse. Weeks ago they reported no restrictions, all is "good" while we locked down. So, to me, being cautious is worth it.
Stay safe