Chester the false killer whale has died, according to the Vancouver Aquarium.

"We are deeply saddened to announce his passing this morning," aquarium president and CEO John Nightingale said in a statement.

Nightingale did not say why Chester died but did say his behaviour changed Wednesday afternoon, leading staff to deliver "intensive overnight care Wednesday and Thursday."

Chester was rescued by scientists from the Vancouver Aquarium and DFO in July 2014. (Neil Fisher/Vancouver Aquarium)

Chester arrived at the Vancouver Aquarium in July 2014 after Tofino residents discovered the undernourished and dehydrated calf on nearby North Chesterman Beach.

Upon rehabilitation he was deemed non-releasable into the wild by a panel of experts brought together by Fisheries and Oceans Canada.

False killer whales are a species of dolphin, distinct from, though often confused with, killer whales. They are considered to be a tropical or sub-tropical species with the most studied populations living around the Hawaiian Islands.

Chester's death will add fuel to the debate that has raged for years over whether the Vancouver Aquarium should keep captive whales and dolphins (cetaceans).

In November 2016, the two remaining Vancouver Aquarium belugas — Aurora and her grown calf Qila — died nine days apart.

The aquarium said an unknown toxin was responsible for the deaths.

In June a harbour porpoise named Daisy also died at the aquarium.

The death of Chester leaves just a single living cetacean at the Aquarium — a Pacific white-sided dolphin named Helen.

"Today and for the next few days, our team will be dedicating our time with Helen...to help her adjust to the change," said Nightingale. "We encourage you to share your memories of Chester."

Chester was believed to be approximately three and a half years old.