Does fishing make corals sick?

Coral reefs, teeming with life, are home to vibrant fish communities — which makes them extremely attractive places to go fishing. However, fishing in reefs may injure corals, which in turn allows disease to fester, a recent study on Australia’s Great Barrier Reef reports.

Coral disease is a major problem for reefs around the world. When a coral is battling an infection, its growth rate, ability to reproduce, and resistance to other diseases and climate-driven stressors like bleaching are all weakened.

Many countries have established protected areas to keep corals and other marine life safe from fishing and other human interference. How effective are these areas in protecting corals from disease? A handful of studies have offered conflicting evidence. Joleah Lamb, a marine biologist now at Cornell University, sought to find out whether marine protected areas have lower levels of reef disease than unprotected areas.

Lamb and her collaborators visited reefs in the Whitsunday Islands, a group of islands off the coast of Queensland in Australia. They chose 21 sites designated Marine National Parks where fishing is banned and 20 sites where fishing is permitted, in order to determine whether the park indeed protects reefs from disease.

The team visited more than 80,800 coral colonies and surveyed whether the corals were healthy or diseased. They also made other observations — on the species of fish, the presence of any discarded fishing gear lying around, whether the reefs were damaged, bleached, or eaten by snails and other predators.

They found that the protected sites had rates of coral disease four times lower than unprotected sites. The most common disease, affecting 60 percent of sick corals, was one called skeletal eroding band, which appears as a black or dark grey band that progresses across corals, leaving dead tissue in its wake. Next, affecting 16 percent of sick corals, were white syndromes, which appear as white lesions of exposed coral skeleton. Third up, affecting 15 percent of sick corals, was brown band disease, which appears as a golden-brown band with a slightly jellylike consistency that kills coral tissue as it progresses; with time, algae overrun the corals.

The researchers considered 31 factors that might explain the difference in disease incidence between protected and unprotected reefs. Protection from fishing emerged as the main explanation. Three indicators of poor coral health and intensive use by people — bleaching, coral damage, and the presence of derelict fishing lines — were much higher in unprotected sites than in protected ones.

The authors conclude that when anglers fish on reefs, the fishing equipment can scar corals, providing an entry for disease pathogens to infect the corals.

“Given that colony damage and the abundance of derelict fishing line were the major factors driving dissimilarities between reserves and non-reserves, we conclude that it is the activity of fishing itself, rather than changes in fish or other benthic communities caused by fishing, that accounts for the striking differences in disease levels between reserves and zones open to fishing,” the authors write in the paper.

The notion that fishing causes trauma, which in turn favours the spread of disease makes sense to Garriet W. Smith, a microbial ecologist specializing in coral at the University of South Carolina Aiken, who was not involved in the study.

“Damage or change to the ‘normal microbiota’ of corals is the first step toward the disease condition (regardless of the specific disease). This change can come about by physical damage, temperature rise (bleaching), or chemical change,” Smith told mongabay.com in an email.

However, some coral scientists are not convinced of the link between coral injury and disease. “If fishing-gear induced trauma is truly the cause of coral disease, what could be the mechanism for this? Surely not trauma alone. After all, corals are pretty tough and get traumatized all the time by predators and wave action yet they recover fine, so why should fishing gear be any more damaging?” Thierry Work, a wildlife disease specialist at the US Geological Survey ‘s Honolulu Field Station, told mongabay.com.

“The mixed results of studies looking at effects of marine protected areas on coral disease suggest to me that the answer is more complex. Until we can get a handle on what is actually killing corals, all these ecological studies will be simply associative,” said Work, who was not involved in the study.

Outside the Marine National Parks, the researchers compared two kinds of unprotected sites: ones where the kind of fishing gear people could use was restricted and ones where any gear was fair game. They expected to find more sick coral in areas where fishers could use whatever gear they wanted – and possibly inflict greater damage on the coral. Instead, they “unexpectedly” found about twice as much disease, as well as more coral damage and abandoned fishing gear, in areas with gear restrictions.

This could be because fishers were actually targeting the sites with gear restrictions, perhaps because fishers perceived them to be less depleted of fish or perhaps because they were more accessible from adjacent boat moorings, the authors surmise.

Smith agreed. “I have observed fishermen preferring restricted areas over non-restricted areas in the Caribbean, believing the fishing is better there (and it often is),” he wrote in an email. The finding calls into question whether gear restrictions are a useful reef-conservation strategy.

This is the first study to attempt to show a relationship between human activity, coral injury, and disease. It also points to another potential positive effect of marine protected areas: decreasing rates of coral disease.

As the authors write in the paper, “no-take marine reserves played a significant role in mitigating coral disease on heavily fished… reefs in Australia, suggesting an additional conservation tool for reducing coral diseases promoted by physical injury.”

 

Citations:

Lamb, J. B., Williamson, D. H., Russ, G. R., & Willis, B. L. (In Press). Protected areas mitigate diseases of reef-building corals by reducing damage from fishing. Ecology.