The International Coral Reef Initiative recently released a report detailing the status of Caribbean coral reefs from 1970-2012. Based on data from 88 location around the Caribbean, the report found that average coral cover declined by over 50% while algal cover is over three times higher. This pair of opposite trends is called a “phase-shift” where the habitat transitions from one major type to another.
While human impacts, coral disease, ocean acidification, and coral bleaching are all threats to coral reefs, the ICRI report states that the loss of grazers is one of the main contributors to the loss of coral cover and this phase shift to algae-dominated reefs. Grazers are organisms that eat algae and keep the algal cover on the reef under control. Without grazers, algae can outgrow and shade corals, outcompete corals for space on the reef, and even contribute to the spread of coral diseases. The main two grazers on coral reefs are urchins and herbivorous fish like parrotfish. Unfortunately, there was a major die-off in the 1980’s of the once-abundant Diadema urchins, leaving parrotfish and other herbivores as the sole grazers left to control algae. However, as fish like snapper and grouper began to disappear from reefs due to overfishing, fishermen “fished down the food chain” and turned their spears and nets toward parrotfish. This fishing pressure cause parrotfish populations to decline around the Caribbean, leaving reefs vulnerable to algal-domination.
Pretty sad story, huh? Now time for a bit of good news: Caribbean countries that have banned the fishing of parrotfish and other herbivorous fish are seeing positive results on their reefs. Although, there is no one “cure-all” for coral reef degradation, the conservation and recovery of gazer populations can reduce the stress from this coral to algae phase shift so that coral reefs may be less susceptible to other potential stresses in the future.