Smaller, isolated reefs more vulnerable

By Emma Young 8 July 2010
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If you’re a fish, you’re safer in a bigger, better connected reef, according to new research.

IF YOU’RE A FISH, it might sound like a good idea to live on a small, isolated reef, far from boats, pollution and large numbers of predators, but on Australia’s Great Barrier Reef (GBR), the opposite is the case. In fact, these fish populations are more likely to die off.

This finding could now be used by Great Barrier Reef Marine Park managers to better target limited conservation funds, say the researchers, from the Australian Institute of Marine Science (AIMS) and the University of Adelaide.

The team studied 15 years of fish data collected by AIMS on 43 reefs. A reef was defined as small if it was less than 4 km2 and large when it was bigger than 8.5 km2.

Small and isolated reefs generally support about 30 species, whereas about 75 species live on the larger and better-connected reefs. The researchers then produced a map showing the differences in risk to fish populations across the GBR.

“There are many possible reasons to think that small and isolated reefs would be better places for fish to live,” says project leader Camille Mellin. “But we showed that the size of the fish population on small and isolated reefs is more variable over time than on larger and better-connected reefs.” And this makes smaller reefs more susceptible to losing fish.

If a cyclone or an outbreak of coral bleaching strikes a small reef, the fish populations are much slower to recover because they don’t receive as many new immigrants compared with bigger, better-connected reefs. “These populations are not as resilient to change and are not easily replenished, increasing the risk of extinction,” Camille says.

Safe havens

While bigger reefs generally have more predators, they also tend to be safer havens for individual species, says the team. That’s because if the population of the predator’s favourite fish food drops, it can easily find an alternative meal without wiping out its preferred species.

The map shows a clear north-south trend, with reefs from the northern section, such as those around Cooktown and Lizard Island, being generally larger and better connected, and so less vulnerable than southern reefs.

“Our research suggests that conservation resources should be better allocated to the protection of large, connected habitats that might act as sources for the recolonisation of threatened species,” says Camille.

The team suspects that the pattern on the GBR will hold for other reefs worldwide, but the long-term data needed to investigate this isn’t yet available.

“This work suggests some very interesting and important ideas that need to be taken into account when managing fish populations in the future,” says Ove Hoegh-Guldberg, an expert on coral reefs at the Centre for Marine Studies at the University of Queensland.

However, other studies show geographically isolated reefs are home to more unique species, making these reefs also important for conservation efforts, he says.