Fishing for Answers– Cyanide Detection in Aquarium Fish

Publication: Breen, N. E., Bonanno, J. A., Hunt, S., Grossman, J., Brown, J., Nolte, H., & Rhyne, A. L. (2019). On the half-life of thiocyanate in the plasma of the marine fish Amphiprion ocellaris: implications for cyanide detection. PeerJ, 7, e6644.

It only takes ten minutes to feel the positive effects of watching a fish tank. Scientists have shown that watching fish is calming enough to lower one’s blood pressure and heart rate while improving their mood. So, it’s no wonder that fish tanks have earned their way into waiting rooms, restaurants, and living rooms across the world. Each year about ten million tropical fish are imported to the US for aquariums, a multimillion-dollar industry that only seems to be growing. Limited fishing regulations coupled with high demands for exotic fish has created a serious environmental problem- many fishermen have turned to illegal and harmful fishing practices as a way to boost productivity.

Figure 1. A saltwater Mandarin Gobe nestled in its aquarium habitat. Source: Wikimedia Commons
In Troubled Waters


The “Coral Triangle” is an oceanic region between the countries of Indonesia, Papua New Guinea, Malaysia, and the Philippines. Known for its variety and abundance of corals, 76% of the world’s coral species can be found within Coral Triangle. Many fishermen within the Coral Triangle harvest fish for the aquarium trade market as their major source of income. To keep up with demands, fishermen have developed a variety of techniques to increase fish catch.

Cyanide fishing (“CF” for short) is a technique used in the Coral Triangle to stun fish for easier catchment. Fishermen crush cyanide salt tablets into water bottles and dive down to coral reef habitats. By squirting this mixture onto reefs, the release of cyanide will temporarily stun the fish. Stunned fish will float upward from their hiding spots which makes them easier to catch.  Once back at the boat, the fish are stored in clean seawater which allows them to recover from cyanide poisoning.


This technique was outlawed for fish caught for human consumption and is largely banned worldwide for the hazardous effects on both fish and coral reef ecosystems. Cyanide is poisonous to all living things, including humans. It is estimated that over 1,000 tons (1,000,000 kg) of cyanide have been used in the Pacific Ocean alone. And many degraded reef habitats are only further harmed by the effects of CF. More than 85% of the reefs within the Coral Triangle are threatened by local stressors including CF and other fishing techniques. There is no question that the practice of CF is harmful, but enforcing the ban is difficult. One of the difficulties is that it’s impossible to tell certain if a fish was caught using cyanide. To solve this, Dr. Nancy Breen of Roger Williams University (Rhode Island, USA), along with cohorts from both Roger Williams University of Massachusetts at Boston (Massachusetts, USA) are working to develop a rapid response test for the detection of cyanide in saltwater fish.


Sofishticated Detection


One of the major problems in identifying fish caught with cyanide is a poor understanding of how cyanide breaks down once inside a fish. “Metabolic pathways” is a term used to describe the series of chemical reactions that will occur to a particular compound in a living organism. Recent advancements from Breen et al., show the metabolic pathway of cyanide in certain fish will breakdown and create thiocyanate (SCN). High levels of SCN within fish indicate initially high levels of cyanide– cluing us into whether a particular fish was exposed to cyanide poisoning.


These experiments were conducted on clownfish (A. oscellaris), perhaps best known for their starring role in Finding Nemo. Clownfish bred at Roger Williams University (Bristol, Rhode Island, USA) were exposed to cyanide in their fish tanks to mimic cyanide fishing scenarios. Each tank was assigned a specific concentration of cyanide and a different amount of cyanide exposure time. After exposure, fish were placed in tanks of clean water and their blood was drawn for further analysis. Plasma from the fish blood was then analyzed for SCN.

Figure 2. A clownfish peers out from his hiding spot. Source: Wikimedia Commons

When fish were held in cyanide water, the SCN levels in their plasma increased with the time spent in that tank. But when the fish was removed from the cyanide water, the SCN levels continually decreased. Meaning, the best time to test a fish for CF effects is before it reaches a fish tank. And the longer the fish is removed from cyanide water, the harder it is to tell if a fish was caught with CF. Even so, the findings from Breen et al., suggest that even small levels of SCN in fish could be used to detect CF.


What does this all mean? Linking SCN levels to the illegal practice of cyanide fishing could help fish importers determine if their fish were illegally caught. Having a quick and reliable indicator test could greatly improve the ability to stop CF trade and hold fishermen accountable for their techniques. Some of the big questions remaining include how SCN levels change from fish to fish, or how much cyanide a fish is exposed to during CF. Filling in these knowledge gaps will help bring protect all living things within the Coral Triangle. And let’s face it– no fish tank will ever compare to what lies beneath the sea.

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Michaela Cashman

Michaela Cashman is a Ph.D. student at the University of Rhode Island and an ORISE participant at the US EPA Atlantic Ecology Division. Her current research focuses on microplastic isolation from marine and estuarine sediments. Other research interests include soil science, hydrogeology, and emerging contaminants. When not in the lab, Michaela enjoys creating stained glass windows, watching live music, and playing the clarinet in her community band. Follow her on twitter for research updates and occasional animal GIFs. twitter: @EnviroMichaela

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