The role of reef sharks in structuring predatory fish communities after recovery from illegal fishing

Blog written by Mark Meekan & Conrad Speed. Read the full paper here.

Predators structure prey communities both through consumption and by altering the behaviours of their prey. Prey that is wary of a predator is less likely to use a risky habitat and will spend more time being vigilant, which can be costly as it can mean that there is less time for foraging and reproduction. Such influences of predators on prey communities are well documented in terrestrial and some aquatic ecosystems but remain largely unstudied on coral reefs.

Although sharks are the most obvious and ubiquitous large predator in reef environments, research into their effects on fish communities is both limited and controversial. Because sharks can move over large distances and are slow to reproduce, experimental studies that seek to examine the role of sharks as structuring agents by excluding or removing them from reefs are fraught with logistic and ethical difficulties. However, due to fishing, populations of reef sharks in many areas have been in steep decline.

Some researchers have used this situation as a “natural experiment” to compare fish communities of reefs where sharks have been removed to communities on other reefs where populations remain intact. Such studies suggest that sharks may affect the abundance, trophic role and morphology of mesopredatory fishes at lower or equivalent positions in the food chain.

These conclusions are controversial, because fish communities can also differ between reefs due to habitat quality, productivity and the history of natural and anthropogenic disturbances. To avoid these issues, our study used a different approach. Instead of comparing communities between reefs, we looked at how fish communities have changed on the same reef where the enforcement of a no-take Marine Reserve and cessation of illegal fishing has allowed reef shark and fish communities to recover through time. As the reader might imagine, given the current state of the world’s oceans, such opportunities are very rare. 

We worked at Ashmore Reef, a remote atoll located hundreds of kilometers northwest of Australia. The reef is a no-take marine reserve that has had near-fulltime protection of its borders by Australian Customs and Border Force since 2008. Enforcement was essential due to the numbers of vessels fishing illegally in the area, many of which targeted sharks for their fins.

Our earlier work (Speed, Cappo and Meekan 2018) has already shown that shark numbers at Ashmore Reef have recovered at a remarkable rate – up to six times the pace predicted by demographic models. Today, shark abundances on Ashmore are 4.6 times the numbers we recorded before protected status was enforced. But what happened to the rest of the predatory fish community? To find out, we compared data collected by baited remote underwater video stations (BRUVS) in 2004 (prior to enforcement of the reserve), with a more recent BRUVS survey in 2016, which occurred after eight years of strict protection of the reef in 2008. The 2016 survey was conducted as part of the Global FinPrint Project (https://globalfinprint.org/).

The illegal fishing that targeted sharks also caught other large predatory fishes, so it is not surprising that these also recovered once fishing was stopped. In comparison to 2004, large mesopredatory fishes (> 100 cm in length) – species such as the big snappers and large trevallies (jacks) – increased in numbers by a factor of 2.3. The story was similar for medium-sized mesopredators (50 – 100 cm in length) which were mostly coral trout, snappers and emperors. These increased in abundance by a factor of 1.5. However, outcomes were very different for small mesopredators (< 50 cm in length). These species declined in abundance by a factor of 2.5 between 2004 and 2016. Today, abundances and size structures of the shark and fish faunas of Ashmore Reef are comparable to those of other “pristine” reefs in the region nearby, suggesting that fish and shark communities have largely recovered from exploitation.

The changes in abundance of mesopredators at Ashmore Reef we recorded are consistent with theories and observations of how large predators structure trophic pyramids. When large predators are removed, smaller mesopredators become abundant, a phenomenon known as “mesopredator release”. Conversely, when populations of large predators recover, abundances of smaller mesopredators are suppressed. It is difficult to separate the relative contributions of reef sharks and the other large mesopredators to our result. However, it is interesting that we found only a small change over time in the numbers of smaller mesopredators in the near-reef habitats (areas of sand and rubble adjacent to the coral reef) we sampled with BRUVS. In this habitat there was a much smaller increase in shark numbers, but an increase in the abundance of larger mesopredators (by a factor of 2.3) similar to the one that occurred on the reef. This implies that abundances of reef sharks were more influential than large mesopredators in determining changes in numbers of smaller mesopredators.

Why did these changes in numbers of small mesopredators occur? The obvious answer is that predation by larger predators was a key driver, but the story is not likely to be so simple. For example, we found some evidence that certain species of smaller mesopredators moved to different habitats when predation pressure changed. Although total numbers of the spot-cheek emperor (Lethrinus rubrioperculatus) did not vary between surveys, the species occurred in highest numbers in reef habitat in 2004, when there were few sharks and large mesopredators. In 2016, the same species occurred in highest abundances in the near-reef habitat, where there were fewer sharks.

Overall, our results are consistent with earlier studies in NW Australia that used spatial comparisons among reefs with and without sharks to examine the role of these predators in fish communities. These studies also found evidence for mesopredator release in smaller size categories of reef fishes. Although the mechanism(s) underlying the changes we observed in some parts of the fish communities remain to be documented, our study does provide a very positive message for managers and researchers working toward the conservation of coral reef ecosystems. The predatory fish communities of coral reefs, which are invariably the primary targets of exploitation and are so often over-fished in many places around the tropics, can recover at an unexpectedly fast pace. Our study shows that well-enforced marine reserves could play a key role in making this happen.

References

Speed CW, Cappo M, Meekan MG. 2018. Evidence for rapid recovery of shark populations within a coral reef marine protected area. Biological Conservation 220:308-319.

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