Many grilling aficionados don’t try grilling large fish such as salmon or tuna as they’re afraid the fish will stick to the grill. Culinary site Chow.com recommends using the 70/30 rule when grilling fish: cook the flesh side down first for 70 per cent of the total cooking time, then flip the fish over to the skin side for the last 30 per cent.
The extra time spent grilling on the flesh side will help the flesh develop a crust over the entire surface of the fish. That way when you pull the skin side off the grill it should keep the skin and flesh sides from tearing.
How to Keep Fish from Sticking to Your Grill [Chow.com]