Do you ever wonder why the flags are upside down?
That is the signal that fish were successfully released.
White Flag with blue colored marlin in the center is Blue Marlin catch and release.
White Flag with blue sailfish is the same thing.
Flying flag on fishing boats goes back to the old days before radios were common in boats, this would signal the others on the fishing grounds and in the harbour what fish were caught (before catch and release was common) so the weigh master could get the scales ready.