Cooler weather has released the Salmon to head upstream out of Lake Ontario and into rivers such as the Credit River to spawn. These look to be Chinook Salmon, an introduced species, as they pass through Norval, Ontario - just east of Georgetown. Their spawning cycle runs from August to October. Along the shoreline lie the bodies of fish who struggled hard for the journey against the current and passed on, hopefully they finished their terminal objective. 

From the Salmon's lifecycle, "Adult Salmon spawn in freshwater, where female salmon lay thousands of eggs that are fertilized by male salmon. Spawning can occur in spring, summer, fall, or winter and depends on the salmon species. After spawning, adult salmon die and their bodies provide nutrients for the freshwater ecosystem. Eggs are buried in gravel nests, called “redds,” and salmon can stay in their eggs for several weeks to months until they hatch. Once eggs hatch, the juvenile fish, called “fry,” can stay in the gravel nest to feed for 3-4 months. As juvenile salmon grow, they may remain in the freshwater rivers anywhere from a few hours to several years, depending on the species and other environmental factors. As juvenile salmon swim towards the ocean, they begin the process called smoltification–transitioning from living in freshwater to living in saltwater. The “smolts” may stay in estuaries from a few days to several weeks to feed, adapt, and prepare to enter the ocean."

Moving from the Ocean to the freshwater lake the fish return to the river of birth, confirmed through the chemical smell of the water.
Salmon know when to start the spawning run when the rivers rise due to rainfall and the temperatures are between 5 to 13 degrees Celsius. They also stop feeding during the run which would surprise the few people that are trying to catch these large fish.