Aquaculture Biodiversity Biology Conservation Ecology Human impacts

Stop Clowning Around: Cyanide Fishing in the Indo-Pacific

Madeira, Diana, et al. “Synergistic Effects of Ocean Warming and Cyanide Poisoning in an Ornamental Tropical Reef Fish.” Frontiers in Marine Science 7 (2020): 246.

According to National Geographic, around 10 million tropical saltwater fish are imported to the US each year for stocking aquariums. While the majority of ornamental reef fish come from the Philippines and Indonesia, they can come from over 45 countries around the world. Of those imported, experts estimate that 10 to 30 percent were caught using cyanide fishing, despite being illegal in many Indo-Pacific countries. The authors of this recent paper explore what cyanide does to marine ecosystems and the consequences of cyanide fishing as the oceans warm.

Why Cyanide?

Cyanide (CN) is a chemical compound that is highly toxic not only to people, but many other animals as well. In many tropical reefs in the Indo-Pacific, fishers involved in cyanide fishing create highly concentrated solutions of cyanide and spray them over the area where they plan on fishing. This practice damages reefs, causes organ damage to fish and threatens marine ecosystems. You can read more about cyanide fishing here. Scientists wanted to see how much cyanide damaged reef ecosystems and understand how the effects could be amplified when also considering the effects of climate change. As sea surface temperatures rise, the authors warn that the combined effects of cyanide fishing (and overfishing in general) could have devastating impacts on the future of coral reef ecosystems.

The Big Picture:

The Indo-Pacific region is especially at risk, since sea surface temperatures are expected to rise by up to 3.3°C by 2100 according to a 2019 Intergovernmental Panel on Climate Change (IPCC) report. The region has also experienced a string of marine heat waves in recent years, which has exacerbated the already stressed tropical reef ecosystem. While the reef faces threats from pollution and overfishing, adding increased SST to the mix will likely lead to marine species becoming even more susceptible to the effects of toxins (like cyanide) and pollutants in the environment.

Figure 1. The Indo-Pacific region is shaded in dark blue. From

In order to understand how the ecosystem would respond to these stressors, the authors performed three experiments. They focused their experiments around the fish from the family Pomacentridae, which includes clownfish and damselfish, since they are among the most common fish in the aquarium trade. They first examined how species of the family Pomacentridae responded to a cyanide exposure under normal temperature conditions, which allowed the scientists to create a baseline for the other experiments. The second experiment examined how the fish responded to varying concentrations of cyanide, while the third examined the combined effects of cyanide and higher water temperatures meant to simulate rising sea surface temperature.

The Experiment:

For the first baseline experiment the researchers collected nine or ten fish from eight different species in the family Pomacentridae. Each group of species was subjected to a 60 second “cyanide pulse”. The “cyanide pulse” was a tank of cyanide solution in which the researcher held the fish for 60 seconds (see figure 3 for more detail). Following the pulse, the fish were cleaned three times before being placed in individual 1 liter jars to observe their responses. The two most common responses in the fish were immobilization (being unable to move or swim for a time) or mortality after being exposed. From the monitored responses, the authors concluded that the clownfish A. ocellaris (aka Nemo) fared the best after being subjected to the first cyanide pulse.

Figure 2. A. ocellaris, commonly known as the clownfish. From

For the second experiment, twelve groups of ten clownfish were subjected to cyanide pulses of varying concentrations. The researchers hypothesized that fish size had an impact on both immobilization and survival rates following the pulse, so the fish were then divided by size. Like the first experiment, the fish were cleaned three times before being placed into individual jars. The researchers’ hypothesis was confirmed, with larger fish having a higher survival rate than the smaller fish. The third experiment only used larger clownfish, this time divided into six groups of ten each. Each group was subjected to a different treatment of varying temperature and cyanide pulse strength before being cleaned and separated like in the previous two experiments.

Figure 3. The design for the three experiments. A) Experiment #1: Eight species with groups of n=9 or 10 subjected to a cyanide pulse for 60s. B) Experiment #2: Twelve groups of A. ocellaris (n=10) subjected to six differing pulse concentrations, separated by size. C) Experiment #3: Six groups of A. ocellaris (n=10) subjected to three different temperatures, with cyanide and no cyanide. From Madeira et al. 2020. For the full-size image, click here.

Cyanide is No Joke:

Madeira and her team came to several conclusions after observing the responses of the fish. Of the species of the Pomacentridea family, only two of the eight species tested survived more than 50% of the time. They also concluded that smaller fish are more susceptible to cyanide than larger fish, due to the higher mortality rate and longer immobilization time. When combined with increased temperatures, the cyanide caused up to 80% mortality. This stat was especially alarming for the researchers and indicated that temperature and cyanide have a “negative synergistic effect.” This means that the combination of these two stressors has a much worse effect than either of them separately.

Unfortunately, if cyanide fishing continues on the same scale, Nemo’s future isn’t looking very bright. Madeira and her team hypothesized that the clownfish in the experiment could have had a higher survival rate due to their mucus covering, but it’s no guarantee that it will protect them from stronger cyanide pulses in a warming ocean. Scientists warn that many species of tropical fishes are especially vulnerable to the negative synergistic effects that will likely arise from cyanide fishing and increased SST. In order to prevent the degradation of tropical reef ecosystems the authors call for more sustainable fishing practices and the involvement of local and national governments for implementing conservation measures. Without ramping up conservation efforts and protective legislation, the destructive practice of cyanide fishing could prove too damaging for tropical fish in a warming ocean.

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