The book chronicles a three-week fishing trip through central Nova Scotia, and is an excellent account of the unspoiled Nova Scotia wilderness that existed at the time, which has been largely diminished since. The group encounters moose (which Eddie tries to capture and bring back alive), beaver, and numerous trout, the first of which is now very scarce in the region, and legions of mosquitos, moose flies, black flies, noseeums, and midges, all of which are regrettably abundant to this day.
Many of the areas described in the book, then virtually unexplored and uncharted, are now well known to back-country campers in Kejimkujik Park and the Tobeatic Wilderness Area. The descriptions of the central Nova Scotia woods contained in the book are beautifully written and uncannily accurate, and while the trout which brought Paine and Breck to Nova Scotia are less abundant, due in part to acid rain and increased fishing pressure, they still provide good sport for anglers.  
Paine, a famous New England novelist and biographer, initially had some difficulties with the lack of modern amenities in camp life, but soon came to love the rugged beauty and solitude of the woods. As advice to other potential campers, he has this to offer:
"...if you are willing to get wet and stay wet - to get cold and stay cold - to be bruised, and scuffed, and bitten - to be hungry and thirsty, and to have your muscles strained and sore from unusual taxation: if you will welcome all these things, not once, but many times, for the sake of moments of pure triumph and that larger luxury which comes with the comfort of the camp and the conquest of the wilderness, then go!
The wilderness will welcome you, and teach you, and take you to its heart. And you will find your own soul there; and the discovery will be worth while!"

Question: Why are trout less abundant now?
Answer: because of acid rain and more pressure from fishing
Alternative answer: acid rain and increased fishing pressure