"Chicken Haddie is the term for canned haddock, cod, cusk or hake, or any combination thereof, that has not been ground and does not include dark or sow hake."
Basically, flaked lean white fish, usually served creamed and a white sauce. The recipes I found all called for the fish to be made into something flaked in a cream sauce, patties like salmon patties, etc.
Your substitute would be a steamed or poached fillet of any of those fish.