It was intended as a Buddhist act of mercy and compassion, but ended in a criminal conviction and significant environmental risk. The release of hundreds of alien lobsters and crabs into the sea off Brighton has highlighted the perils of a ritual that takes kindness to animals too far.

Two London Buddhists, Zhixiong Li, 30, and Ni Li, 33, pleaded guilty last week at Brighton magistrates court to breaking the Wildlife and Countryside Act 1981 by releasing non-native species into the wild, causing “untold damage” to marine life. The pair were ordered to pay a total of more than £28,000 in fines and compensation.

They, along with more than 100 other Buddhists, were taking part in a rite known as fangsheng, or “life release”, as the highlight of a visit to the UK of Taiwanese Buddhist master Hai Tao two years ago. The group hired three boats from Brighton marina, emptied their load into the sea a mile off the coast and returned to shore – presumably full of good karma.

The ritual dates back to the third century, but has seen a resurgence in recent years. Hai Tao, a champion of animal rights, advocates fangsheng – saving the lives of creatures destined for slaughter – as a way for Buddhists to demonstrate compassion, create good fortune and earn merit.

According to Humane Society International (HSI), hundreds of millions of birds, fish, monkeys, turtles and other animals are involved in acts of fangsheng every year. But these days, it says, “mercy release has become an industry built on the capture and supply of wild animals, for whom there are devastating consequences of injury, illness or death”. In Taiwan alone, 200 million wild animals are used every year in release rituals, according to HSI in 2012. Fangsheng is also practiced in China, Hong Kong, Singapore, Nepal, the United States and the UK.

Chinese people release baby turtles into the sea in Indonesia. Photograph: Anadolu Agency/Getty Images

The organisation says many animals are fatally injured in the ritual, and those that survive release often die soon afterwards from exhaustion, injury or disease, or else become prey to other species. Some are re-captured after the ritual and re-sold. Release can also cause environmental harm, it adds. Animals “may be released outside their natural habitats and in groups large enough to establish breeding populations, often wreaking havoc on local ecosystems. Some are invasive species that may threaten the survival of the native species.”

A few days after the fangsheng ritual off the south coast of the UK, a Brighton fisherman was puzzled to find non-native lobsters and crabs among his catch. He alerted the Marine Management Organisation (MMO), which chartered boats and offered local fishermen a £20-a-head bounty for every alien crustacean they captured. But only 323 out of a total of more than 700 were recovered, and there was evidence some lobsters had begun breeding in their new home.

Still, the Brighton incident is far from the worst case of fangsheng gone wrong. In 2012, followers of Hai Tao released hundreds of snakes in a mountainous area of Taiwan not native to them, leaving locals to deal with regular house visits from the serpents. “Releasing captive animals is good practice, but releasing snakes into the wild hurts society and contravenes the intention of mercy,” Hai Tao said at the time.

In comparison, dumping a few stray crabs and lobsters really does seem merciful.