Many coral reefs worldwide have undergone phase shifts to alternate, degraded assemblages
because of the combined effects of over-fishing, declining water quality, and the
direct and indirect impacts of climate change. Here, we experimentally manipulated
the density of large herbivorous fishes to test their influence on the resilience
of coral assemblages in the aftermath of regional-scale bleaching in 1998, the largest
coral mortality event recorded to date. The experiment was undertaken on the Great
Barrier Reef, within a no-fishing reserve where coral abundances and diversity had
been sharply reduced by bleaching. In control areas, where fishes were abundant, algal
abundance remained low, whereas coral cover almost doubled (to 20%) over a 3 year
period, primarily because of recruitment of species that had been locally extirpated
by bleaching. In contrast, exclusion of large herbivorous fishes caused a dramatic
explosion of macroalgae, which suppressed the fecundity, recruitment, and survival
of corals. Consequently, management of fish stocks is a key component in preventing
phase shifts and managing reef resilience. Importantly, local stewardship of fishing
effort is a tractable goal for conservation of reefs, and this local action can also
provide some insurance against larger-scale disturbances such as mass bleaching, which
are impractical to manage directly.