In accordance with the 2015 Fishing Reg's Summary (I know we're talking 2016 here but lets work with this for now), on page 70, it states no fishing from Jan 1 to the Friday before the 4th Saturday in April, along with no fishing from Dec 1 to Dec 31.
I'm sure when they came up with these regs, they wouldve been aware that every now and then there will be a 5th Saturday in April.. but I don't see why that would be reason enough to modify the opening dates, or they would have went with a solid date number (ie: Fishing opens April 30, every year), as opposed to the 4th friday thingy.. no?
I believe Rob is correct in that ice out won't affect the opening dates, just lake accessibility.. so technically, there could be cases where ice fishing is not illegal in Algonquin Park (assuming the ice is still around after trout opener) not that you'd want to be out on that thin ice anyway, but in theory, it's possible to ice fish in Algonquin without breaking the law.