Fish no exception to trend in marine-organism disease


Sea fan corals like this dead one near the Pacific islands of Palua were devastated by a 1998 bleaching event, then suffered mass mortality the next year for unknown reasons. Cornell graduate student Jessica Ward, shown conducting a survey for the Global Environment Facility and the World Bank, is part of a team trying to assess worldwide coral disease threats.
Drew Harvell/Cornell University

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ITHACA, N.Y. -- Disease is increasing among most kinds of marine organisms, according to a long-term study by Cornell University and the National Center for Ecological Analysis and Synthesis in Santa Barbara, Calif. And fish are no exception to the troubling trend, despite fewer reports of fish disease over the years.

The analysis of hundreds of previous studies of marine-ecosystem disease is published this month in the journal Public Library of Science Biology (available without charge at ). The report finds the rate of disease increasing in some taxa, such as in turtles, mammals, mollusks and urchins, but declining in fish.

However, says Jessica Ward, a Cornell doctoral student in ecology and evolutionary biology and lead author of the published study, "Disease in fish populations is decreasing only because their numbers are decreasing, due to over-fishing and other factors."

Says Ward, "Undoubtedly there are fewer and fewer cases of pneumonia among veterans of World War I , but that doesn't mean the veterans are becoming healthier. They are becoming fewer in number, and so are populations of wild fish."

Indeed, she notes, for many populations, there are too few fish left for disease to be observed.

The study was conducted by the 15-member Marine Disease Working Group of the National Center for Ecological Analysis and Synthesis, a Santa Barbara, Calif., ecology think tank. Heading the marine disease group is C. Drew Harvell, a Cornell professor of ecology and

evolutionary biology. She comments: "This is the first quantitative study to provide solid evidence that rates of disease do seem to be increasing in the ocean. It has been hard to tell if disease is increasing because we didn't have proper baselines. It's been much tougher to understand the frequency of marine disease than it is to monitor for SARS , for example, because the ocean is out of sight and out of mind." Harvell points to trends revealed by the study:

  • Sea turtles are suffering from large tumors caused by a herpes virus that has spread over the last two decades.

  • Marine mammals have a variety of viral and bacterial ailments, some of which also seem linked to human activities.

  • Mollusks, such as commercially farmed oysters, could be experiencing increasing stresses from both climate and aquaculture practices.

  • Corals had an increase in bleaching, although not in actual disease.

  • Only sea grasses and sharks display no apparent increases in disease levels; both groups have disease, but it does not appear to be increasing.

Ward worked with collaborators to develop a literature-based research method to test the increasing-disease hypothesis, a widespread concern among ecologists. The analysts used the number of times diseases were mentioned in the literature as a proxy for actual prevalence of disease over time. Besides making statistical adjustments to account for the increase of scientific publications in recent years, the researchers verified their methodology with raccoon rabies, a widely reported disease of land animals. Because mentions in the literature and the cases of raccoon rabies matched, the researchers concluded the literature citations and the actual cases of marine disease should be consistent as well.

One future application of the study method might be as a management tool for marine ecosystems in need of conservation, or as an aid for trend spotting and finding underwater populations that need to be studied, Ward notes.

"We are seeing so many emerging diseases that it can be overwhelming, and certainly we have to be careful about how dire we make the risk out to be," Ward says. "There are so many things we have yet to understand about the sea. My hope is that the methods developed in our paper can be used to identify those groups most in need of conservation."

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