Planetary pole shifting could be the reason the runs come in later and later. If the fish DO find their way home by some built in compass, it makes topical sense. If that's the true deep down reason, who knows at this point. Climate change/global warming is bunk - humans (at least some of us) are so egotistical as to think that in all of history, including before when we were here, that we could have some sort of catastrophic impact on the planet's cycles is laughable. There were ice ages and warm ages long before, and there will be long after man kind has blinked out into nothingness. It will continue that way until the sun goes nova and burns the Earth into vapor with her fusion flames.
Sure, we can cause some nasty temporary (talking geological time here) effects - nuclear winter comes to mind. But to think that an infinitesimal increase in the carbon content of the atmosphere will cause the ice caps to melt, the seas to rise, and the plains to turn into a dust bowl is silly. last time I checked, carbon dioxide was the compound that photosynthetic plants consume and turn into breathable oxygen. That doesn't sound like a problem to me. I'm sure the plants are enjoying it also.
We're simply in the middle of one of the natural cycles - whether it be a cooling cycle or a warming cycle - it's a natural thing.
And here's a thought - maybe, just maybe, mother nature DOES want to destroy us. Maybe she doesn't like us mucking about her planet, over-harvesting fish, trashing her streams, dumping garbage into her oceans, and generally making a mess. MAYBE she wants to kill us off and make room for the next best thing. Sentient cock roaches? Talking rabbits? Or maybe the dolphins will be the "top" of the food chain in the next cycle.
If we manage to survive the next few hundred years, or the next thousand, without going completely insane as a species, our decedents will look back upon us and laugh at our silly notions and wonder how we could've been so full of ourselves. Or maybe they'll spit our names, for creating mountains of garbage, and sludging up the rivers, or damming them up and sucking down the free flowing waters just to feed our greed for expanding into areas we never were meant to expand into (like, oh, I dunno, the desert) in great numbers.
We can change a lot of things on a planetary scale, but the weather isn't one of them.