Why do salmon return to their natal spawning grounds?

We went to Etienne Brulé Park on Toronto's Humber River the other day to watch the salmon running the river, hurling themselves up the weirs and fighting against the strong currents and rapids, as they are inexorably called back to their natal spawning grounds.

Surprised that there are salmon in Lake Ontario and its feeder rivers? Well, the native Atlantic salmon were fished into extirpation some years ago, but Pacific Coho and Chinook salmon have been reintroduced in recent years, and, even more recently, some Atlantic salmon too. I'm guessing, from their large size, most of the ones we saw were probably Chinooks. 

Either way, the salmon run has apparently become a very popular family sporting event in Toronto, especially this year, with the pandemic and all, and there were hordes of kids cheerlng the fish on, yelling, "Go, salmon, go!" It was quite endearing, really.

But, it made me wonder how can such a difficult task have any evolutionary advantage? At each weir, we watched fish after fish fail, and only the odd one actually succeed after many attempts. Such a lot of effort expended! (There at least 7 weirs on this section of the Lower Humber alone, built for flood control after Hurricane Hazel in 1954, although notches have been cut since to smooth out the flow and make it a bit easier for fish to navigate). There are no grizzly bears snapping at these salmon at least, but still, there must be really good reasons for them to attempt such an arduous undertaking, no?

Well, it seems that, although there is much we still do not understand about it all, the main reason is that they need a safe place to spawn and, by returning to the same place they themselves were born, they can rely on finding such a place, rather than trying some other random river on the off chance that it has a good spawning spot. They can be sure that their natal river has conditions favouring that particular type of salmon, and they can sure of finding mates of the same species. They navigate both by magnetic inclination and intensity, and by a very keen sense of the distinctive smell of their own river (who knew fish could smell underwater?)

This makes sense, I guess. It's also possible that the very difficulty of the undertaking is enough to ensure that only the fittest and strongest salmon survive to breed new generations.

But I still can't help but wonder if a much greater number of salmon spawning in an easier-to-access, but admittedly less safe, place wouldn't result in greater numbers in subsequent generations. But maybe they would all be namby-pamby, feckless fish, not worthy of the name "salmon"?