Sharks prefer healthy reefs, healthy reefs need sharks

Stop fishing, and there will be more fish. That’s the idea, at least. Indeed, sharks were more abundant in no-fishing zones in Australia’s Great Barrier Reef Marine Park (GBRMP) than in spots where fishing is allowed according to a new study just published by a group of Australian researchers. But the story is actually more complicated than that.

The Great Barrier Reef is one of the most important hotspots of marine biodiversity on the planet. Within it, elasmobranchs – sharks, rays, and skates – are a particularly diverse group. The reef is home to 134 species from 31 families, and 60% of those are sharks. Despite the fact that some sharks are eaten themselves, the main role that sharks play in balancing their ecosystems is because they’re predators. When apex predators are disturbed – such as by intentional fishing or fisheries bycatch – the entire ecosystem can crumble. That’s what happened, famously, when wolves were driven out of the Yellowstone ecosystem, for example.

Around a third of the GBRMP has been designated as “no-take” zones, closed to all forms of fishing. For species that stick close to home, like the grey reef shark, protecting them is as easy as protecting the reefs on which they spend their lives. But other species are more mobile, which makes them harder to protected. It’s thought that networks of marine protected areas (MPAs) offer protection for those species by reducing their chances of encountering a fishing line, trawl, or gillnet. Historically, it’s been difficult to quantify the effects of marine protected areas on the species they intended to protect, but James Cook University researcher Mario Espinoza and colleagues have ten years of data from a network of thousands of underwater cameras along the Great Barrier Reef.

In all, they recorded 21 shark species from five families and two taxonomic orders. Four species were responsible for 64% of their observations: Grey reef sharks, silvertip sharks, tiger sharks, and sliteye sharks.

Do MPAs within the GBRMP have their intended effect on sharks? In some sense, they do. “This study demonstrated that shark abundances were significantly higher in non-fished sites, highlighting the conservation value of the GBRMP zoning for sharks,” though the effects varied from species to species. However, protection from fisheries alone wasn’t sufficient. Even within protected areas, abundance was highest in spots with healthy coral. Over the last two decades, the GBR has seen coral cover decline by half, thanks primarily to more tropical cyclones, an increase in coral predation by the crown-of-thorns starfish, and coral bleaching. It could even be that where MPAs are successful, that’s because of healthy reefs, rather than the reverse, with MPAs leading to increased reef health.

The researchers speculate that may be the case, writing, “our results showed that hard coral cover had a significant effect on the abundance of reef-associated sharks at non-fished sites while the effect of [the amount of time since the MPA was re-zoned as no-take] was variable, suggesting that coral cover may be an important driver in the success of MPAs.” The reverse can also be true: if reef sharks are removed, the entire ecosystem can become unbalanced, resulting in loss of coral. The relationship between coral health and shark populations is, like all things in ecology, more complicated than it may first appear.

At least one thing is clear, though: the re-zoning of the GBRMP as no-take has been beneficial, at least for some species.

More generally, the study underscores the importance of detailed ecological data in assessing conservation measures. Until now, shark abundance data has been lacking for the Great Barrier Reef, and this study “provided a valuable contribution to the understanding of species-specific habitat associations in response to a range of drivers,” says Espinoza. To predict shark distribution patterns requires understanding the factors that underlie their distribution; in this case, hard coral cover. Only with that level of detail can effective conservation and management procedures be implemented. – Jason G. Goldman | 12 September 2014

Source: Espinoza M, Cappo M, Heupel MR, Tobin AJ, Simpfendorfer CA (2014) Quantifying Shark Distribution Patterns and Species-Habitat Associations: Implications of Marine Park Zoning. PLoS ONE 9(9): e106885. doi:10.1371/journal.pone.0106885.

Header image: Tiger shark, shutterstock.com

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