Sea urchin diseases are on the rise; figuring out why is important, partly because these epidemics can threaten tropical reefs, which depend on urchins to graze the seaweeds that can otherwise overgrow and kill corals. New research suggests that sea urchin populations are more vulnerable to diseases when their predators are overfished: there were four times as many urchin epidemics in fished areas than in unfished areas off southern California.
“In other cases where urchin diseases have been reported, urchin predators have been heavily fished, and urchins reach high densities that may have created opportunities for diseases to invade,” says Kevin Lafferty of the USGS Western Ecological Research Center at the University of California at Santa Barbara in the October 2004 issue of Ecological Applications.
Reports of sea urchin epidemics have risen over the last 30 years, and diseases have decimated urchin populations in many parts of the world. Notably, in the early 1980s, an epidemic killed more than 95 percent of the long-spined sea urchins (Diadema antillarum) in the Caribbean. After the urchins died, prevalence of seaweeds increased dramatically; today, many coral reefs there are dead. Some biologists have suggested that overfishing urchin predators such as toadfish (Opsanus sp.) and queen triggerfish (Balistes vetula) may have played a role in this epidemic.
To test the idea that sea urchins are more vulnerable to epidemics when their predators are fished, Lafferty compared the frequency of urchin epidemics in fished and unfished areas of Channel Islands National Park, off the southern California coast. Purple sea urchins (Strongylocentrotus purpuratus) there have had epidemics of an unidentified but often fatal disease since 1992, and California spiny lobsters (Panulirus interruptus), a major urchin predator, are heavily fished in the region. Lafferty used 20 years of existing data that included urchin and lobster densities as well as urchin disease prevalence at 16 sites near five islands (San Miguel, Santa Rosa, Santa Cruz, Santa Barbara, and Anacapa). Fishing is allowed at all of the sites except two on Anacapa Island that have been in a marine reserve since 1978.
The results supported the hypothesis that fishing their predators can make sea urchins more vulnerable to epidemics. Fished areas had only one- fifth as many lobsters but four times as many purple sea urchins and four times as many urchin epidemics. “Fishing top predators can indirectly favor disease transmission in prey populations,” says Lafferty.
Besides providing insights into the rise in sea urchin epidemics, this work also has implications for fisheries. “Managing marine resources increasingly requires knowledge and understanding of indirect effects,” says Lafferty. In this case, fishing spiny lobsters could also indirectly increase epidemics in red sea urchins (S. franciscanus) in southern California, thereby inhibiting the recovery of this once-lucrative fishery.
Lafferty, K. 2004. Fishing for lobsters indirectly increases epidemics in sea urchins. Ecological Applications 14:1566-1573.
Purple sea urchins (Strongylocentrotus purpuratus) Photo by Kevin Lafferty