Depletion of herbivore populations, especially in conjunction with elevated levels of nutrients from fertilizers or sewage, can shift the balance from coral to algal dominance. ref This change in community structure and composition, e.g., from a coral-dominated to an algal-dominated state, is called a phase shift. Once a phase shift has occurred, it can be difficult for the system to revert to coral dominance as mature macroalgal communities are often characterized by species with physical or chemical deterrents that make them less palatable or digestible to herbivores. Preventing phase shifts by protecting herbivores from over-exploitation (in combination with reducing input of nutrients) is now recognized as a crucial consideration for coral reef management.
Coral reef managers can play a key role in herbivore protection by regulating herbivore removal in MPAs over which they have jurisdiction, and by working with fishers and fishery managers to protect the viability of herbivore populations in the wider reef ecosystem. Fisheries legislation can provide important legal frameworks for protecting herbivores, but, in most instances, existing fishery management strategies have not been designed to protect functional roles such as herbivory.
A range of conventional fisheries management tools and strategies are available for protecting herbivores. These include:
- Area closures — Prohibiting removal of herbivores (or general bans on fishing) in portions of habitat or at sites important for herbivores (such as aggregation sites) can help maintain herbivore populations.
- Gear restrictions — Herbivores generally are not caught in hook and line fisheries, and instead are targeted using traps, nets or spears. Some herbivorous fishes, such as parrotfishes, are especially vulnerable to nocturnal spearfishing and spearfishing on SCUBA. Limiting use of certain types of fishing gear or fishing times can reduce pressure on herbivores.
- Species bans — The most effective way to protect herbivores is to place a total ban on collection of key herbivore species. This has now been implemented in several locations, including Belize. Market-based approaches, such as prohibiting the sale of herbivores, can prevent commercial fishing for herbivorous species although subsistence fisheries can still be a significant source of pressure in many locations.
- Temporal closures — The role of herbivores can be especially important after disturbance events that kill corals, such as hurricanes or mass coral bleaching. Managers can consider a temporary restriction on harvesting important herbivore species to maximize the chance that coral populations will be able to recover without the added pressure of excessive competition with algae.
- Active restoration — In cases where herbivore populations have been reduced through overfishing or disease, active restoration may be the most feasible way to rebuild populations to the level required to prevent or reverse a phase shift. Depleted urchin populations have been the focus of assisted recovery trials in some places where they are one of the main sources of herbivory (which in itself may be symptomatic of depleted fish herbivore populations), but there have not yet been any examples of successful broad-scale restoration.
Enhancing Urchin Populations
In some instances, managers are taking action to increase urchin populations to support reef recovery especially areas where populations of other herbivores (e.g., parrotfishes and rabbitfishes) have been depleted. For example, in the Caribbean, researchers are assessing the potential of growing sea urchin in labs and relocating them to the wild. In Hawaii, managers have raised and released collector urchins (Tripneustes gratilla) to graze the invasive algae and promote reef restoration.
Other actions have been explored to increase sea urchin populations, such as the building of artificial reefs which provide niches for urchins to hide from predators, and restrictions on fishing of urchin predators (e.g., triggerfish and larger wrasses) which could support the recovery of urchin populations. While a range of urchin enhancement projects have been attempted, the sustainability of these efforts remains in question. ref
Research and Practice
While diverse herbivorous reef fishes play a role in reversing coral-algal phase shifts, two recent studies demonstrate phase-shift reversal by an “unsuspected” single species. ref Both studies used large-scale experimentally induced phase shifts on the Great Barrier Reef and the use of underwater cameras to document the species responsible for the transition from macroalgal domination to epilithic algae and coral.
One study ref revealed that the single species, Naso unicornis, was almost solely responsible for the removal of Sargassum biomass. The second study ref documented recovery of the reef (removal of Sargassum) was primarily due to the batfish, Platax pinnatus. The results of both studies reveal the importance of protecting those groups that underpin the resilience of the local ecosystem. Management and conservation strategies may need to look beyond species diversity, and also focus on the maintenance of ecological processes and protection of key species in functional groups.
To control the overabundance of marine algae on coral reefs in Maui, Hawai‘i, the Kahekili Herbivore Fisheries Management Area was established. It was designed to increase the local abundance of certain herbivorous fishes and sea urchins through fisheries management methods. For example, hatchery-raised juvenile urchins (Tripneustes gratilla) have been released onto the reef to graze the invasive algae and promote recovery of the reef.
Video: Using Herbivores to Save a Reef (2:39)
Darla White, State of Hawaii Division of Aquatic Resources, desribes how fisheries are managed for reef resilience.
Natural controls of marine algae are intended to help the marine ecosystem in the area return to a healthy balance. The management area prohibits the fishing of any fish in the following families: Kyposidae (sea chubs), Scaridae (parrotfish) or Acanthuridae (surgeonfish) or any sea urchins.