[@Inkdrop] I imagine the first ships that went through after Cascadia tightened her grip on the region would have been told to surrender themselves, and if one did it's likley her crew were executed after a mock 'trial'.  After that I can only imagine other vessels refused and were fired upon, just as you say.

It's worth noting that the Cascadian attitude to abhumans is pretty influenced by the Grogar, so Cascadian ships might even fire first in what they see as preemptive action as to them it just follows that all abhumans are violent or scheming.