The conservation measure was announced Tuesday by Minister of Fisheries and Oceans Gail Shea in response to dwindling salmon numbers.
The no-retention policy has already been in place on some parts of New Brunswick's Miramichi river system and in Prince Edward Island.
Shea's announcement extends the measure to all waters of New Brunswick and Nova Scotia where salmon angling was permitted.
Fishing for salmon will still be allowed, but any fish caught must be released.
The conservation measure was supported by the Atlantic Salmon Federation, the New Brunswick Salmon Council, the Nova Scotia Salmon Association, and the Miramichi Salmon Association, which petitioned the minister for a one-year moratorium on the issuance of salmon tags in response to declining salmon numbers.
In 2014, the number of salmon returning to the Miramichi River in New Brunswick was about 12,000, which is about half the number recorded annually over the previous three years.
In the first 10 years of this century, about 53,000 salmon returned to the Miramichi annually. In the 1990s, the number returning to spawn was about 82,000.
"These are frightening numbers," Miramichi Salmon Association chairman David Wilson had said when the 2014 salmon counts were released last September.
Shea's office states the number of Atlantic salmon at sea has dropped 69 per cent over the last 42 years, from about 1.7 million in in the mid-1970s to about 600,000 fish now.
The stark drop in salmon numbers led Shea to announce the creation of a ministerial advisory committee on Atlantic salmon in March. The implementation of a Maritime-wide catch-and-release policy for 2015 stems from the advisory committee's first series of recommendations.