Replace the question mark with a number

This fisherman goes to the river to check an illegal fish trap that he owns. He looks around to make sure there are no Fishing Inspectors about and proceeds to pull the fish trap out to check it.
An Inspector steps out of the bushes, “Ahha!” he said and the fisherman spun around and yelled “Shiiiit!”. The Inspector, who wasn't expecting such a response said “Settle down, I'm the Fishing Inspector”. “Thank God for that” said the fisherman, “I thought you were the bugger who owned this fish trap”.