The catch comes days after 16-year-old high school student Stella Berry was fatally attacked in the river.
The horrific death was the first fatal attack in the Swan River in 100 years.
Stella, a student at Shenton College and the daughter of two high school teachers, was playing with friends in the river in North Fremantle, near the Fremantle Traffic Bridge, on Saturday afternoon when she was attacked.
Emergency services were called about 3pm; however, Stella died at the scene.
It is understood Stella and her friends has been swinging into the water on a rope when the shark appeared.
Horrific screams were heard from the water before anyone noticed the shark.
Witnesses said a heroic bystander jumped into the water to save Stella and bring her back to shore.
Fremantle District acting inspector Paul Robinson said on Saturday the victim’s friends were with her when the “extremely traumatic” attack occurred.
“Unfortunately we have no idea what shark it is,” Inspector Robinson said.
“From what I’ve been advised, her family weren’t there when the attack took place but her friends were,” he said.
“As you can imagine, this is an extremely traumatic incident for anyone to witness.
“It’s unusual for a shark to be that far down river … at this point in time an alert has been put out, a shark warning just to let people know the event has taken place.”
The species of the shark is yet to be confirmed.
Shark attacks are incredibly rare in the Swan River, which is connected to the Indian Ocean through Fremantle Harbour.
The only other recorded fatal shark attack in the Swan River occurred a century ago when a young schoolboy was attacked while swimming in neighbouring Peppermint Grove on January 27, 1923.