You may notice that around the world fishing boats return to shore, flying various flags ( know as hanging out the laundry ) . This is not just for bragging rights but is a long held tradition that pre dates two way radios . In days of old, fishermen would go to sea and fly the brightly coloured flags as they returned to shore to identify their catch to their wives ( fishwives ), who would then offer the fish for sale . This way they could sell the fish before the boat reaches the dock .
A more modern day variation of this, is a flag flown upside down , this denotes a fish has been caught and released alive ( usually sharks or billfish ) . Watch out for boats returning to see if you can identify their catch .