Prime Minister Justin Trudeau today announced four more byelections will be held on April 3, the first major electoral test for the Liberal government since its election more than 16 months ago.

The byelections will be held in the ridings of:

The races are to replace a number of high-profile members of Parliament, including former ministers Stéphane Dion and John McCallum, who recently took diplomatic postings, Mauril Bélanger, who passed away after a battle with amyotrophic lateral sclerosis (ALS), former prime minister Stephen Harper, who retired from politics, and Jason Kenney, who is now running for the leadership of the Progressive Conservative Party of Alberta.

Trudeau announced the Ottawa–Vanier race last Sunday, as a byelection must be called within 180 days of a riding becoming vacated. (Bélanger's seat was officially declared vacant as of Aug. 23.)

The April 3 votes will be the last series of elections fought by interim Conservative Leader Rona Ambrose before she hands over the reins to one of the 14 contenders vying for the party's leadership at the May 27 convention.

The ridings in contention are all considered some of the safest in the country for the two respective parties as their candidates won the seats with more than 55 per cent of the vote in the last federal election.

Braeden Caley, the Liberal Party's director of communications, said the party has candidates in place in three of the five ridings: Mona Fortier for Ottawa–Vanier, Haley Brown for Calgary Midnapore and Scott Forsyth for Calgary Heritage.

A senior staffer in the Prime Minister's Office, Mary Ng, recently left her post to run in McCallum's old Toronto-area seat, although she has not yet secured the contested nomination. A former Quebec Liberal cabinet minister, Yolande James, hopes to run on the Liberal ticket in Saint–Laurent after Dion's departure.

Yolande James, Quebec's former minister of Immigration and cultural communities, is officially seeking the Liberal nomination in the federal district of Saint-Laurent. (Jacques Boissinot/The Canadian Press)

"While the Conservative Party and its leadership candidates are continuing to demonstrate that they are out of touch with Canadians, only Justin Trudeau and the Liberal team have a positive plan to grow Canada's middle class, and to support every family working hard to join it," Caley said.

Conservatives have nominated small business owner Bob Benzen to replace Harper as the candidate in Calgary Heritage and Stephanie Kuzie, a former diplomat, is running for the party in Calgary Midnapore. Adrian Papara, a parliamentary assistant to Calgary MP Tom Kmiec, has been nominated to run in the Ottawa-area seat.

Cory Hann, the Conservative party spokesperson, said "many others" are interested in carrying the Conservative torch in the other two byelections, and they're "ready to challenge the Liberal government on their high-debt, high-tax policies that are making life more expensive for all Canadians."

"As we've clearly seen in years past, it's generally the governing party with the advantage in byelections but our party will work hard to ensure voters know it's the Conservative Party that will always stand up and be the voice for taxpayers," Hann said in an emailed statement to CBC News.

There was a byelection in the riding of Medicine Hat, Alta., last October that the Conservatives easily won after the death of MP Jim Hillyer, although the Liberals posted their best results in the southern Alberta riding in a generation.