The Prime Minister's Office made the announcement in a statement Wednesday afternoon.
"I would like to wish Minister Keith Ashfield a quick recovery to full health and look forward to welcoming him back to Ottawa," said Prime Minister Stephen Harper in the statement.
In his own statement, Ashfield said he suffered the heart attack at his home in Fredericton on the weekend.
"Thanks to prompt medical attention, I am now in stable condition and awaiting bypass surgery. While I am assured of a full and complete recovery, my doctors have advised me that I will require several weeks of rest and recuperation."
Ashfield said he recommended Shea as his temporary replacement.
Ashfield thanked his family and friends for their support.
Ashfield, 60, is the MP for Fredericton and a former minister in the New Brunswick government of Bernard Lord. He was appointed fisheries minister in May 2011.
Shea retains her duties as Minister of National Revenue, the PMO release said.