The Toronto Zoo is expected to remain closed on Friday after 400 of its employees went on strike Thursday — a situation that could jeopardize this season's breeding program if it's not soon resolved, the union says. 

The striking workers include zookeepers, horticulturists, maintenance and administrative staff and others. The CUPE Local 1600 employees began picketing Thursday morning, turning over the care of the zoo's nearly 6,000 animals to the zoo's management. 

Talks continued until late Wednesday night before they broke off — due to a dispute over  job security, according to the union, and affordability, according to the zoo. 

"We certainly don't want to be out here," union local president Christine McKenzie said. "We have a number of animals who are pregnant right now, we have a brand new health centre opening next week and all of that's in jeopardy right now." 

Toronto - Zoo strike

About 400 Toronto Zoo workers went on strike just after midnight Thursday. They say job security is the main issue. (Linda Ward/CBC)

The union has said they're worried the zoo's board of management — which includes four city councillors — is trying to strip away job security language so it can contract out more work to the private sector. 

McKenzie said the zoo wants to do away with a clause that guarantees a minimum workforce of 150, while the zoo spokesperson Jennifer Tracey said management has presented proposals that address job security. 

Tory says workers have job security

The zoo receives an annual subsidy from the City of Toronto of about $11.6 million.

Toronto Mayor John Tory said Thursday that striking zoo workers have good jobs with excellent benefits, something he said should be kept in mind during the current job action. 

"These are jobs that have a guaranteed pension, generous benefits and more job security than most people who live in the city of Toronto," the mayor said. "We have to be responsible in terms of how we manage the money."

The union local's president, Christine McKenzie, said Thursday the bargaining committee is ready to resume negotiations whenever the zoo wants.

"No one on this bargaining committee wanted a strike, but you cannot have a world-leading research, conservation and education facility by consigning hundreds of workers to precarious, insecure jobs that don't support communities," she said.

Baby African penguins

These African penguin chicks hatched in February and March. Their keepers are on strike. (Toronto Zoo)

​Concerns about animals' care

The timing of the strike also proves difficult.

Not only are several animals expected to give birth in the coming weeks, but other breeding programs are expected to launch for the season as well, McKenzie said.

Tracey said administrative staff will step in "to ensure the animals receive quality care." The mayor also said he's been assured the animals will be properly taken care of during the strike.

Last year, the zoo warned it was dealing with poor attendance. However, the first quarter of this year has seen some improvement despite cold winter weather.

The zoo has already welcomed some 97,000 visitors as of the end of March, while February set an attendance record with 46,000 visits.

McKenzie added said the union's bargaining committee remains on standby, ready to quickly resume negotiations.

"Ultimately, that ball is in the employer's court," she said.

Toronto - Zoo strike 2

Workers have said they're worried that zoo management wants to erode job security provisions, but management says union proposals are 'unaffordable.' (Linda Ward/CBC )

With files from CBC's John Rieti, Adrian Cheung