Perhaps I mistyped...... it's not about proving who is liable, it's about preventing the next eco disaster.
I think ships (foreign) are supposed to deballast their home port fresh water and reballast with salt water prior to North America fresh water entry, to prevent spreading invaders. Back in the 80's when fleas and ZM's started showing up, you think we'd have started inspecting ships in the SLR Waterway to ensure they had reballasted with salt. And prohibitively fine the ship out of any ship found with fresh water ballast. That was 30 years ago. What has this industry done to minimize the risk of contamination since the introduction of the spiny water flea?
This is way over simplification, but Inspection and testing of ships ballast water should be(should have been) prioritized, and violators should be fined such that it reinforces compliance........