Taronga Zoo made the sad announcement that Casey, the world's only captive leopard seal, had to be put down due to terrible health condition.

"Casey's health and condition had been deteriorating over a lengthy period of time despite intensive veterinary treatment and care.

"Although every effort was made by both Taronga's Marine Mammal and veterinary teams, Casey's health had shown no acceptable sign of improvement and staff made the heart-breaking decision to put Casey to sleep on Friday," Tarangoo Zoo said in a statement.

Casey is the only leopard in the world that had experienced being taken care of by humans, away from Antarctic. Casey was rescued ashore in 2007 near Sydney with severe wounds in his abdomen due to fresh cookie-cutter shark attack. He recovered from severe wounds and had remained in the Zoo. He cannot be returned back to his home for scientists fear that he can possibly transferred unknown diseases to other leopard seals in the wild. Leopard seals are fragile animals.

Casey had been instrumental to Australian Marine Mammals Research Centre (AMMRC's) study of leopard seals. Leopard seals are not kept in the zoo anywhere else in the world.

"...working with and learning from Casey was a unique opportunity for our Marine Mammal team. Taronga's Marine Mammal team would like to thank everyone involved in Casey's management and care over the past few months, particularly our vets.We hope that Casey's interactions with our staff and visitors contributed to an awareness of and appreciation of this amazing Antarctic species," Taronga Zoo said.

Casey had been found to be singing back to the sounds of saxophone played to him, as what male leopard seals do when wanting to attract mates.

"At certain times of year they really react to the noises and sing back. They are certainly aware of new noises, so something like the saxophone was a great way of giving our Leopard Seal some different environmental enrichment. Ranging from some very high pitched sounds to a wide variety of music, we think Casey was having a good time with Steve and his saxophone. It was not only the music that seemed to interest Casey but the visual effect of Steve with an impressive looking instrument caught his attention," Marine Mammals Supervisor, Ryan Tate, explained when a zoo employee played a saxophone to Casey.

Steve, the saxophone player, said that he playing with Casey was the biggest audiences he had performed to.