Ahhh of course, thanks. What a dolt I am.

Oh I didn't mean that, it don't think it's at all obvious. I thought your question was reasonable.

I was genuinely thanking you! As soon as I read your answer I thought "oh, that's incredibly obvious, I should think more before posting". Hopefully it all helped someone else anyway.

I still don't understand - what is it about the noise that prevents the whales from crossing it? Do they confuse the noise for an active predator, or does the noise prevent them from navigating properly with their echolocation?

I'd guess it's interfering with their communication and internal mapping in myriad ways.