First incidence of skin cancer found in fish
3 August 2012 by Tamera Jones
UK-led researchers have identified skin cancer in a wild fish population for the first time.
Coral trout (Plectropomus leopardus)
They found that an unusually high percentage of fish they caught from islands in Australia's Great Barrier Reef Marine Park had dark skin lesions reminiscent of human skin cancer.
The disease is remarkably similar to that found in humans and the scientists think UV radiation from the Sun is the likely cause.
The number of people affected with skin cancer has risen sharply in recent years, and although it has been detected in other creatures like whales, it's never been reported in wild fish until now.
We only saw it here because we looked. This is a massively understudied topic and it's not beyond the realms of possibility we'd find the same disease affecting different fish species in the northern hemisphere as well.
Dr Michael Sweet, University of Newcastle
The findings raise concerns because the species the scientists studied is commercially important. Common coral trout is eaten throughout Australia and is widely exported to China and Hong Kong.
"These fish are exported live and are vetted on quality and looks. If they don't look very healthy, it'll be much harder to export them," explains lead author Dr Michael Sweet, University of Newcastle.
Not just that, but the problem is unlikely to be confined to the small geographical area Sweet and his colleagues studied.
"We only saw it here because we looked. This is a massively understudied topic and it's not beyond the realms of possibility we'd find the same disease affecting different fish species in the northern hemisphere as well," adds Sweet.
If this is true and fishermen are throwing fish back into the ocean because they don't look healthy, this will certainly be affecting their total catch.
"We've already seen photos of fish from the Caribbean that have similar-looking lesions," Sweet says. However it's not yet known whether these abnormalities are down to skin cancer or other causes.
Sweet and colleagues from the University of Newcastle originally went to Australia to study coral diseases, which have increased dramatically in recent years. Some researchers think climate change is behind this upsurge: rising sea-surface temperatures are having a detrimental effect on corals, reducing their immunity and making them much more susceptible to disease.
Bar cheeked trout with sunburn shown by black lesions
The researchers began talking to Australian researchers who'd noticed black lesions on the bodies of fish around Heron Island and One Tree Island at the southern end of the Great Barrier Reef.
"They asked us to check if a fungal pathogen was the culprit," says Sweet.
So Sweet and researchers from the Australian Institute of Marine Science and James Cook University line-caught 136 wild coral trout from around the islands. 20 fish (15 per cent) showed clear signs of skin abnormalities. The dark growths covered as little as five per cent of some fishes' bodies, but in others, the lesions were much more extensive.
Tests revealed that the fish were free of the microbes that have previously been shown to cause numerous other marine diseases. The researchers also ruled out pollution: Heron Island and One Tree Island are both in a marine protected area, and "the likelihood of potential carcinogenic pollutants being the causal factor is low," say the authors in their report, published in PLoS ONE.
"Many fish diseases are caused by either microbial pathogens or pollution, but we're fairly sure that isn't the case here," says Sweet.
But the fish only had surface melanomas, suggesting the cancer hadn't spread any deeper and the fish were basically healthy.
"Once the cancer spreads further you would expect the fish to become quite sick, becoming less active and possibly feeding less, hence less likely to be caught. This suggests the actual percentage affected by the cancer is likely to be higher than observed in this study," Sweet adds.
The next step is to find out if other fish in other locations are similarly affected by skin cancer.
'Evidence of melanoma in wild marine fish populations' - MJ Sweet, N Kirkham, M Bendall, L Currey, JC Bythell, M Heupel, PLOS ONE, published 2 August 2012.