‘And he made proclamation and published through Nineveh by the decree of the king and his nobles, saying, “Let neither man nor beast, herd nor flock, taste anything. Let them not feed, nor drink water, but let them be covered with sackcloth, both man and beast, and let them cry mightily to God, yes, let them turn every one from his evil way, and from the violence that is in his hands.”
The king issued an official proclamation calling on the people, along with their domestic animals, to fast and cover themselves with sackcloth. Both man and beast would suffer if the city was overthrown and so both man and beast were called on to take part in the ritual acts. This inclusion of animals in a ritual of mourning is mentioned among others by Herodotus.
And they were then to ‘call mightily on God (Elohim)' and ‘turn from their evil ways', and especially from violent behaviour. There was a recognition in this, no doubt resulting from Jonah's preaching, that Jonah's God was a moral God Who was concerned about man's moral behaviour. Thus the call was both for them to turn to God and for them to genuinely repent of their sins.