Sharks and the Health of Oceans

Scope Correspondent

Shark fishing on coral reefs can threaten herbivorous fish that help to protect the reefs after natural disturbances such as cyclones, according to new study findings.

In areas where commercial shark fishing has occurred, there were declines in parrotfish and other types of herbivorous fish, and increases in carnivores, such as snappers.

Herbivorous fish help to clean and maintain coral reefs by eating algae off of the coral. “They are really essential to the recovery process of reefs,” said Dr. Jonathan Ruppert, a post-doctoral researcher at York University and lead author on the study. “Herbivores help in removing algae from the reef to allow coral to repopulate or reestablish on the reef.”

Researchers looked at the effects of the presence or absence of sharks before and after a traumatic event, either a cyclone or coral bleaching – a process through which warmer water temperatures cause algae to disappear from the corals. They found there was a link between the presence of sharks and populations of other types of fish further down the food chain.

“The data supports this idea that sharks and top predators in general have significant impacts on reef communities,” said Dr. Simon Thorrold, Senior Scientist at the Woods Hole Oceanographic Institute. “The reefs that had sharks on them recovered more quickly from these pulse disturbances.”

In the study, researchers identified two reef habitats off the coast of Northwest Australia. The first was a marine protected area where commercial and other types of fishing were prohibited. In the second area, researchers used data from Australian custom and border patrol flights that recorded the presence of legal and illegal Indonesian fisherman who were targeting sharks.

The setting provided a “unique opportunity where we have the removal of sharks from the top end of the food chain and we’re able to see what sort of ecological interaction these sharks have within the fish community,” said Dr. Ruppert.

To measure the fish populations, divers returned to the same sections of reef each year to count the number of fish they saw. The data was collected as part of a long-term coral reef monitoring program led by the Australian Institute of Marine Science.

The team took data from 1994 to 2008, and reviewed the numbers before and after a cyclone or coral bleaching to see how quickly the reefs could recover. They used footage from underwater cameras to record such disturbances.

In cases where there were fewer sharks, the team found that there was an increase in the numbers of mid-size carnivorous fish, such as snappers, and a decrease in the numbers of herbivorous fish, such as parrotfish.

This is another example in a growing list that have documented this kind of top-down control,” said Thorrold. “Maintaining the functional integrity of the food web is important in terms of the resilience of the reefs to these disturbances.”

Results from the study raise questions as to whether the patterns seen in this context are similar for coral reefs in general. To address that issue, Ruppert and his team will look at data across a number of other islands in the Pacific where shark fishing and other human activities are impacting top-level predators.

For now, says Dr. Michael Heithaus, Director of the School of Environment, Arts, and Society at Florida International University, “We need to catch fewer sharks of the species that have declined,” adding “That involves a lot of complex fishery science and negotiations at international levels, because a lot of these species are moving across international boundaries.”