I wish it was per fish.
It is an annual stamp. You purchase it in conjunction with your saltwater fishing license. It costs $6 and it allows you to retain the pacific salmon that you catch, within regulated limits, of course, set by DFO. Some fisheries are closed. Some species are closed and others are open.
As long as you're fishing within the limit, you may retain your Pacific salmon. If you think about the value of a Pacific salmon in terms of what you might pay at the supermarket, $6 for an entire year to retain a lot of salmon is—