The current predator/prey balance in Earth's oceans occurs at around a water pH of 8.14. That's normal, and it's in these conditions that fish have developed the ability to detect predators by smell and thereby survive just a little bit better. In the next hundred years or so, however, that pH is forecast to change as the surface waters of our oceans absorb more and more carbon dioxide from the atmosphere and acidify.
By 2100, it's expected that the average surface water pH will have plunged to around 7.8. That might feel like a small difference, but it's enough.
In the past, it's been demonstrated in a lab setting that what happens as acidification progresses is changing fish behavior. Fish become more bold (read: careless), and they become less able to sense predators.