Substrate enhancement focuses on making the natural reef structure more suitable for coral reef recovery, particularly the settlement on new coral larvae. One way that substrate can be enhanced to promote reef recovery is by removing algae that can cause coral mortality and disease, reduce coral growth, and restrict coral settlement.
Herbivorous fish and urchins are critical players in coral reef ecosystems as they control macroalgae that can compete with and smother reef-building coral colonies or new coral recruits. Protecting herbivores is a key management strategy for protecting coral reefs and helping them be more resilient to climate change. Protecting herbivores through fishing regulations or MPAs is a passive restoration technique aimed at restoring ecological processes that favor coral-dominated reefs.
There may be instances where herbivores are not controlling algae despite being protected. Alternative methods that have been or are currently being utilized by managers include manual removal with community members, use of technology to remove algae, and cultivation and restoration of urchin populations.
Manual algae removal: Large-scale manual removals of algae have occurred with success on the island of Hawaiʻi. In Hawaiʻi, numerous invasive algae species proliferated, causing significant negative impacts to reef ecosystems. In 2003, The Nature Conservancy (TNC) in Hawaiʻi began working to remove invasive algae by hand with diverse groups of community volunteers, including neighborhood residents, school groups and educators, local business owners, and university students. TNC trained community members to continue monitoring and removal efforts through kits that included 1) instructions on monitoring local areas for invasive algal species; 2) waterproof cards for identifying alien algae; 3) contact information when invasive algae are discovered; 4) information on removal methods; and 5) methods for using removed algae for fertilizers or food consumption.
Algae-removing technology: Manual removal efforts are likely to only occur in shallow-water environments or with SCUBA diving. To remove algae from deeper depths but still achieve success removing large amounts of algae, a technology called the ‘Super Sucker’ has been developed and employed by TNC staff in Hawaiʻi. The Super Sucker consists of a large tube that sucks algae off the reef and transports it onto a barge where it is sorted by trained staff. Invasive algae are then bagged and sent to local farmers to use as fertilizer. This method has been particularly successful at strategically restoring high-priority areas that are more remote, or where manual removal efforts would be impractical or impossible.
Cultivation of urchins: Some managers are actively cultivating and transplanting urchins to enhance natural herbivory processes on coral reefs. These include the long-spined urchin, Diadema antillarum, in the Caribbean (ISER Caribe webinar) and urchin, Tripneustes gratilla, in Hawaiʻi. Laboratory cultivation helps to bypass the settlement and recruitment stage where many urchins can die. Cultivation techniques generally include collecting settlers, growing settlers into young adults in the lab where conditions can be closely monitored, and restocking urchins onto natural reef sites. 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. Much of this work remains in the research phase. ref