Just some errant thoughts about the "Big Fish"-
(1) the fish was a run-away - Josephus was a "run-away", he had rebelled against Rome in the First Jewish War, and he later ran away from his fellow Jews by surrendering to Vespasian, instead of committing suicide, as he had told his men that he would do.
(2) that had long feasted in Caesar's fishponds; - Josephus had lived off the bounty of the Flavians for about 20 years, at the time of this council meeting of Emperor Domitian.
(3) escaped from thence, - Josephus had escaped from being a prisoner/slave of Rome, by prophesying that Vespasian was the future ruler of the world. The fish himself wanted to be caught- Josephus himself had wanted to be "caught" by Vespasian, into the "net" of his entourage, by this foretelling of the glorious future in store for him.
(4) he must needs be restored to his former master - did Domitian think that Josephus was getting "too big for his britches", with his propaganda work for Rome, and needed a dressing down by the Emperor, to put him back into his place?
(5) This monster the master of the boat and line designs for the High Pontiff - As the master of the boat (structure) and line (propaganda), Josephus had created a "monster" of a religion, designed for an universal Emperor/High Pontiff/Pope, of what was to become the future Roman Catholic Church.