I've built a cool little Crabbing vessel and managed to collect a wealth of Mud and Coconut crabs. When I journey to the nearest fish selling point - which is the fishing gantry at the URAN Freight terminal - I place the vacuum point against the hopper and 27 crabs immediately transfer into the header unit of the fish gantry boom (this means that I know the connection between ship and gantry is fine). The vacuum is switched on, but the crab just remain in the cargo hopper of the header unit (on the end of the boom) and don't sell (and because the hopper is full, nothing else will transfer from my boat).

It's almost as though the crab can't transfer along the autocannon ammunition sections that make up the fishing gantry boom.