The customer relations/assault side of things I will leave to one side.
You say that fish was caught across the road .... landed there, perhaps, caught? unlikely.
You do not know the weight of the fish.
You do not know the price per kilo.
You do not say what kind of fish it was.
and then you
You decide to pay only what you think the meal is worth !
A restaurateur may have some cause to be aggrieved by customers who make up their own bills, so to speak.