I dont know whether to report this as a bug or unintended but welcome consequence.
I have noted that ships with the extra cargo trait appear to fill in the same time as those without when fishing. This gives an extra 20 crates per vessel but with no extra time needed to fish. I dont think this was intended but do think it should remain as an extra bonus when such ships are set to fishing.
Of course this could be resolved by adoption of my other suggestion to restrict fishing to a dedicated new vessel (the fabled Dogger of blessed memory) ...
*walks away whistling...