Most marine life depends on sunlight for its continuous existence. The sun's visible light penetrates as much as a mile deep in some parts of the ocean, although most of that light is lost after the first 50 feet or so. However, the sun's electromagnetic output also includes ultraviolet radiation, which has been linked to increased rates of skin cancer in humans. New evidence has shown that marine life may also be vulnerable to the harmful effects of the sun. Melanoma, a serious type of skin cancer, has now been observed in fish which live along the Great Barrier Reef in Australia.
UV light and skin cancer
Living in the ocean depths does not protect upper water column marine life from the harmful effects of the sun. UV light from the sun is known to have adverse effects on some marine life. For example, sunburn, which is caused by UV-B radiation, has been observed in some whales. Paler-skinned whales seem to be more vulnerable to sunburn than their darker-skinned kin, just as lighter-skinned humans burn more easily than darker-skinned humans.
However, the amount of UV-A and UV-B rays which reach the ocean surface has been increasing. Over the past 30 years, NASA has observed a 6% increase in the amount of UV radiation which reaches the Earth's surface at close to Australia's latitude. The amount of UV radiation which enters the water is probably also proportionately higher. As a result, sun damage in marine life has gone beyond a simple sunburn.
In humans, UV rays have been linked to skin cancers, with UV-A rays causing squamous cell tumors and UV-B rays causing melanomas. Thus, the World Health Organization classifies all types of UV radiation as a Class 1 carcinogen.
In 2012, scientists observed melanoma for the first time in wild populations of the coral trout (Plectropomus leopardus), a commercially-important fish which lives in the Great Barrier Reef. As many as 15% of all caught fish had a melanoma. This is also something that has been observed by fishermen in the region.
None of the melanomas in the caught fish were advanced cancers. It is also not yet known if the melanomas in these fish were malignant or benign. However, a fish suffering from advanced cancer would have lost energy and appetite. As a result, its behavior would change so that it could not be caught using conventional methods.
The type of melanoma found in the wild fish was strongly similar to UV-induced melanomas in the laboratory fish model Xiphophorus. Thus, the researchers conclude that the melanomas in the wild coral trout were probably caused by UV exposure.
Why doesn't water block UV radiation?
UV rays don't stop at the ocean surface. Instead, ocean water reflects some UV radiation, scatters some more, and absorbs the rest of it. The deeper the water, the more UV has been absorbed or scattered. By 100 feet depth, most of the visible and UV light spectrum striking the surface of the water has been absorbed or reflected.
The permeability of ocean water to UV rays varies based on its composition and the ecosystem it hosts. Water chemistry plays a major role, because different ions affect UV light in different ways. Other factors which affect the absorption or reflection of UV rays are the amount of particulate matter in the water and the density of phytoplankton in the region.
As well, freshwater acidification has been shown to increase the amount of UV which reaches the upper water column. As acidification also affects the oceans, the same could be true for seawater.
Given the water composition along the Great Barrier Reef, 90% of all UV-B rays striking the surface of the water should be absorbed or scattered in roughly the first 40 feet of depth. UV-A rays can penetrate slightly deeper, but are also increasingly scattered or absorbed with depth.
Yet much of the Great Barrier Reef is very close to the surface of the ocean, and parts of it break the surface as islands. Fish which live most of their lives in these shallow waters are exposed to nearly all of the UV radiation which falls on the surface of the ocean water.
This makes these shallow-water fish extremely vulnerable to increases in UV radiation. Until the rate of UV exposure drops to pre-1977 levels, it is likely that skin cancers will also be found in other wild fish populations in these regions.