Will these fish carry any levels of mercury or PCB's once they're eating size? (Assuming the water and rain is unpolluted, of course)
Yes. First of all, you can't assume water & rain are unpolluted, no matter what.
As shown in this article (one of many):
http://www.usatoday.com/news/nation/2005-03-13-pollution-_x.htm Almost every place in the USA has suffered from the effects of imported air pollution, at least occasionally. Some of the most serious impacts:
• Mercury emitted by power plants and factories in China, Korea and other parts of Asia wafts over to the USA and settles into the nation's lakes and streams, where it contributes to pollution that makes fish unsafe to eat.
• Dust from Africa's Sahara Desert blows west across the Atlantic Ocean and helps raise particle levels above federal health standards in Miami and other Southern cities.
• Haze and ozone from factories, power plants and fires in Asia and Mexico infiltrate wilderness spots such as California's Sequoia National Park and Texas' Big Bend National Park, clouding views and making the air less healthy. There's nowhere - nowhere - on planet earth where wild areas aren't affected by this.
Now, here's the other half of the issue - basically, fatty tissues acumulate toxins, and the further up the food chain you go, the more toxins you get. So that nice big trout you got - the one that at a few hundred pounds of smaller critters - it's going to have more toxins than if you made a nice minnow smoothie. The ecosystem of the water is much more connected - nothing does not get eaten by something, and there's lots of stages from the smallest to the biggest. So the stuff builds up!
Sorry to be gloomy on this topic, but for LTS, freshwater fish are NOT the choice to make if that's you're plan. You better get good at identifying nuts and berries and capturing small game.