Some 250,000 seals will be killed over the next few days in a yearly hunt in Canada, sparking big arguments.
The Canadian government allows people to cull the harp seals as they say it's important to let them make money from the creatures' oil and fur.
But animal rights activists say the hunt is cruel, and should be stopped.
America has already banned Canadian seal products, and there are moves to stop seal fur and oil being sold across Europe in the future.
The hunt will start on Friday, off Canada's north east coast.
"This is the largest mammal slaughter in the world. It is just a beautiful, icy landscape. The seals go there to give birth to their young because they are safe," said Wayne Pacelle, the president of the Humane Society Of The United States.
But a Canadian government spokesman defended the cull, saying that allowing people to hunt the seals meant lots of small communities could survive.