Diving enthusiasts have seen shrimps, abalones and razor clams, which live on the seabed, being thrown into open waters.
"During one dive, I saw a Sabah grouper in local waters," says Allen To, a conservationist at WWF and co-author of a new book, Hong Kong Reef Fish Photo Guide.
The Sabah grouper, a hybrid fish produced by Malaysian scientists, could compete with local species for resources and pose a threat to Hong Kong's marine biodiversity, To says.
He has observed this worrying trend while exploring Hong Kong's underwater world, he says.
It is also possible people who release these captive-bred species will indirectly kill the fish because they might not be able to hunt and reproduce in the wild.
Last month To spoke at the first Talk Ocean lecture - a forum for discussing marine life conservation in Hong Kong - held at the University of Hong Kong,
His book, Hong Kong Reef Fish Photo Guide, features 270 types of reef fish - 300 species live off the coast of Hong Kong - including 15 that are thought to be new to local waters.
The book was compiled by To, Stan Shea, of the ocean environmental group BLOOM, and veteran ecologist Ken Ching. At first, they intended to create a catalogue for fellow divers of the results and pictures collected by a biodiversity survey at Port Shelter harbour in Sai Kung. But as their database expanded, and knowing many divers have trouble identifying reef fishes underwater, they decided to create a more comprehensive guide available to the public.
Most guide books in Hong Kong are either outdated, written in English or show only caught fish, Shea says; the most recent book of this type, Reef Fish of Hong Kong, was published in English, in 2000.
To says it's often hard to identify fish out of the sea.
"For example, a dead mottled spinefoot looks dull brown on the benches of wet markets," To says. "But under water it has more vibrant colours - spotted deep green scales and a silver belly."
Apart from the threat caused by the religion-related release of animals, another disturbing trend is that the population of Hong Kong groupers - classified as critically endangered by the IUCN Red List - might be declining.
When the trio last visited Shelter Island in 2008, they tagged 28 examples of this species. But only one or two remained when the men returned this year.
On a happier note, To is hopeful that local corals will restore themselves, and the population of reef fish will increase following the trawling ban, which began this year.