Researchers at the Florida International University (FIU) found that shark fin sold in Hong Kong often contained high levels of toxic mercury exceeding the limits set by the government’s Centre for Food Safety.

“The mercury levels are, on average, six to 10 times higher than what is considered a safe level of mercury,” says FIU student marine biologist Laura Garcia Barcia, one of the study’s co-authors.

The team, comprising researchers from the US and Hong Kong, examined 267 shark fin trimmings from nine common shark species in the fin trade: blue shark, silky shark, blacktip shark, scalloped hammerhead, smooth hammerhead, bull shark, shortfin mako shark, great hammerhead and oceanic whitetip shark. The samples were randomly selected from 10 shops in Sheung Wan district on Hong Kong Island, an area known for selling dried seafood.

The researchers tested for levels of total mercury and methylmercury, the organic, highly toxic form of mercury. The concentrations in most of the shark fin tested exceeded the maximum legal limit in Hong Kong of 0.5 parts per million. The highest of 55.52 parts per million came from a great hammerhead while the lowest level sampled was 0.02 parts per million from a blue shark, the most common species found in the trade, the study said.

Exceeding these limits raises both health and legal red flags, says FIU marine scientist Demian Chapman, another co-author of the study.

“The Hong Kong Food Adulteration legislation states anyone who sells a food product with levels above the legal limit can face fines and criminal charges,” Chapman says.

Under Hong Kong laws, any person selling food with metallic contamination above the legal limit face up to a HK$50,000 fine and six months jail.

Prolonged exposure to mercury can lead to damage to the brain and central nervous system. It can also interfere with fetal cognitive development.

The research fills an important gap in highlighting that shark is a dangerous choice for human consumption, says David Baker, assistant professor at the Swire Institute of Marine Science, part of the University of Hong Kong’s School of Biological Sciences.

“Irrespective of the catastrophic loss of sharks due to over-harvesting for the fin trade, shark products should be avoided out of concern for one’s personal health,” said Baker, who is not affiliated with the FIU study.

“It’s widely known that methylmercury contamination is a serious issue with respect to human health. It reflects the bioaccumulation and biomagnification of toxic contaminants in the ocean.”

Bioaccumulation is when an organism gains more contaminant in its body as its ages, while biomagnification is when predators consume contaminated prey, and the contaminant is concentrated further in their bodies, Baker explains.

“For these reasons, large, long-lived ocean predators like sharks are a risky choice with respect to methylmercury toxicity and is why pregnant women are advised to reduce consumption of large fish, such as tuna,” he says. Methylmercury can be damaging to a developing nervous system and can also lower fertility.

More than 100 million sharks are caught worldwide every year for their fins and other products, while more than 50 per cent of the world’s shark fin is traded through Hong Kong alone, according to the Hong Kong Shark Foundation.

The Centre for Food Safety says while recent testing of shark fin had been satisfactory, it would, in light of the study, enhance surveillance of products on the market.

“The main food-safety concern for consuming shark and other predatory fish such as tuna, alfonsino, swordfish, marlin, orange roughy and king mackerel is the accumulation of mercury, especially methylmercury, in these fish. Methylmercury is the most toxic form of mercury affecting the nervous system, particularly the developing brain,” a spokesman said.

Global conservation organisation WildAid says despite signs of a declining industry, shark fin is still in demand in China and Hong Kong, particularly among the elderly.

“Those elderly [people] who still insist on shark fin as a dish at large-scale family and company banquets, such as weddings and Chinese New Year celebrations, are risking their health and the health of their children when they eat what so many of their peers regard as an unsustainable, wasteful and cruel luxury,” says Dorothy Cheng of WildAid Hong Kong.

It is not surprising that many shark species harvested for the fin trade, such as the blue shark or silky shark, could have fins containing these levels of mercury as they are at the top of the food chain, says Gloria Lai Pui-yin, senior conservation officer for sustainability at environmental group WWF-Hong Kong.

“Heavy metals such as mercury can enter marine food webs and accumulate up the food chain,” she says. “Many sharks and rays can live for a few decades, some longer, which can increase the accumulation of such chemicals.”

Climate, diet and habitat can also affect chemical levels in sharks, meaning that – as the FIU paper suggests – species-specific risk assessment and proper labelling is essential, Lai says, adding that 30 per cent of the more than 1,200 species of shark and rays are threatened with extinction.

“The majority of fins sold in Hong Kong come from unsustainable and untraceable sources, which is why we ask the public to say no to shark fins and urge the government to take decisive action and list wildlife crime offences.”

Companies, hotels and restaurants need to formulate policies banning shark and ray products, and air and sea freight carriers need to pledge to not carrying them, Lai says.

Ignoring this urgent issue not only risks the future of vital oceans system, but risks exceeding the safe level of mercury intake, she adds.