Tessa N. Hempson, Nicholas A.J. Graham, M. Aaron MacNeil, Nathalie Bodin, and Shaun K. Wilson
Predator populations around the world are declining. One of the key threats to these species is that their prey numbers are dwindling as the habitat they rely on becomes increasingly degraded. Many predatory species live much longer than their prey, and they can adapt their diets when their favourite food is not available. This means that they may be able to survive despite struggling to maintain their energy supplies, making it difficult for scientists and managers to detect that there is something wrong.
In coral reef ecosystems, increasing water temperature are causing coral bleaching, which can kill extensive tracts of coral, allowing algae to thrive in its place if the coral does not recover. We call this a ‘regime shift’, because the entire appearance and function of the ecosystem changes. Corals are responsible for building the complex structure of reefs, so when they die the structure erodes. Many small-bodied fish species disappear from the reef, as they have nowhere to shelter, and those that feed on live coral no longer have food. These small fish are the prey that larger predatory fish, like peacock grouper (Cephalopholis argus) rely on to stay healthy. So, when they disappear, peacock grouper are in trouble. Also, because peacock grouper hunt by hiding in dark caves in the reef, waiting for an unsuspecting fish to swim by, and then ambushing it, they find their strategy isn’t very effective when the reef structure degrades after a regime shift.
We found that 16 years after a large coral bleaching event in the islands of Seychelles, the numbers and diversity of large predatory reef fish have dropped. And, using several different techniques, including analysing stable isotopes and fat in tissue samples, we found that peacock grouper on regime-shifted reefs are feeding lower down the food chain (i.e. eating grazers and detritivores rather than fish that eat coral or plankton) and have lower stores of energy. This could mean that these fish don’t have enough energy reserves to reproduce as effectively as they normally would, or might be less able to survive periods of stress, which in the long term could mean that their populations shrink.