Coral reefs usually conjure up images of pristine atolls teeming with colorful fishes and a vibrant community of organisms, including sharks, living in harmony with each other. For this reason, they are often, aptly referred to as “rainforests of the sea”, and although they comprise less than one percent of the ocean, they are home to a quarter of all marine species.
Corals are creatures, living in colonies of soft “polyps”, which contain many finger-like projections called tentacles – what we usually see sticking out on the surface. These poisonous tentacles sometimes grab and feed on tiny fish. Hundreds of polyp colonies comprise what we call a reef. These polyps are attached to a hard surface such as rocks. Most corals derive energy from tiny one-celled photosynthetic algae that live inside them, producing nutrients and oxygen from sunlight. It is these algae that bestow corals with the beautiful colors that we often marvel at.
Corals are vulnerable to increasing threats: occasional natural disturbances from cyclones and bleaching, as well as chronic human disturbances like fishing – particularly overfishing of sharks (a top-down effect). Following natural disturbances such as bleaching (which exert a bottom-up effect on the food chain), coral often lose their tiny food-producing algae and as a result they turn white. Large algae often colonize and smother these white corals, but, they are usually resilient, and can recover through re-growth and recruitment, which can take months.
But, what happens when young recovering corals are also faced with the threat of fishing? In the first study to span more than a decade, scientists investigated the combined impact of both natural and human disturbances on coral reef fish communities. They discovered that chronic fishing of top-order predators such as sharks alters fish communities that hamper coral reef recovery from natural disasters.
Dr Ruppert and colleagues studied two remote reefs located 300km off the coast of north-western Australia, both of which were ravaged by natural disturbances in the late 1990s. The first, Scott reef, suffered from coral bleaching; the second reef, Rowley Shoals, was battered by a cyclone. These disturbances were catastrophic: coral cover declined below 30 percent, leaving the once-thriving reefs barren.
At Scott reef, Indonesian fishermen are permitted to fish, where they often target high-value species like sharks, slashing off their fins and dumping them back into the sea, where they die – a centuries-old practice; however, fishing is prohibited at Rowley Shoals, a marine-protected area.
They sampled sharks during daylight using underwater video stations at both fished and non-fished reefs between 2003 and 2004. The number of fishes and sea-floor organisms were assessed once a year at both reefs between 1994 and 2008. Fishes were identified by species and classified into five groups, according to their diet: carnivores, herbivores, planktivores, corallivores, and detritivores.
The researchers used a series of statistical tests to determine the extent to which differences observed in fish and sea-floor organism community structure were attributed to fishing or habitat (such as coral cover). They also compared shark numbers between the fished and non-fished reefs.
The tests revealed that sea-floor organism composition was largely attributed to coral and algal cover, and fished reefs had more algae and lower coral levels following bleaching than the non-fished reef after the cyclone. Both habitat and fishing largely accounted for the variations in fish community at the two reefs.
Ruppert’s team observed larger numbers of mid-sized carnivores at the fished reef compared with the non-fished reef. Interestingly, they noticed fewer herbivorous algae-eating fish at the fished reef. They spotted sharks more frequently at the non-fished reef – three times more than at the fished reef.
They suggest that a lack of top-predators such as sharks at the fished reef allowed mid-sized carnivores to thrive. These carnivores snapped up the herbivorous algae-eating fishes that play instrumental roles in clearing away algae, allowing young coral more space to grow; lack of herbivorous fish meant algae overwhelmed corals at the fished reef, hindering their recovery.
Overfishing of sharks triggered a cascade-effect down the food chain, ultimately affecting coral recovery by upsetting the intricate structure of fish communities. Even worse, shark populations can take years to bounce back due to their long-maturation times and low-fertility rates. Ruppert’s team suggests designating more marine-protected areas to help conserve reef shark populations and revive our underwater rainforests to their former glory.
Coral Reef (2013, Oct 11)
Ruppert JLW, Travers MJ, Smith LL, Fortin M-J, Meekan MG (2013) Caught in the Middle: Combined Impacts of Shark Removal and Coral Loss on the Fish Communities of Coral Reefs. PLoS ONE 8(9): e74648. doi:10.1371/journal.pone.0074648