It's a Saturday in the Don Valley in the early 1920s. These boys have brought their dog along to go fishing at Riverdale Park (they're probably on the east side of the river according to the caption, with the photo looking south toward the footbridge).
People have been fishing in the Don for thousands and thousands of years, of course — and you can still do it today. There are salmon and pike and carp and plenty of other species in the river, despite the pollution that flows into it (most notably from rain and snowmelt washing our pesticides, road salt, fertilizer and other crap into the water). And unless you're a kid or pregnant, you can safely eat the fish you catch — at least, in limited quantities: the City says about four times a month.
This is a detail of a larger photo I came across on the Toronto Public Library website here.