water plants which have the ability to out compete corals for space, nutrients, and other resources. Another anthropogenic force that degrades coral reefs is bottom trawling; a fishing practice that scrapes coral reef habitats and other bottom substrate dwelling organisms off the ocean floor. Bottom trawling results in physical wreckage and stress that leads to corals being broken and zooxanthellae expelled. Managing coral reefs In an attempt to prevent coral bleaching, scientists are experimenting by "seeding" corals that can host multiple types of zooxanthellae with thermally-resistant zooxanthellae. MPAs have begun to apply reef resilience management techniques in order to improve the 'immune system' of coral reefs and promote reef recovery after bleaching. The Nature Conservancy has developed, and is continually refining, a model to help manage and promote reef resilience. Although this model does not guarantee reef resilience, it is a comprehensible management model to follow. The principles outlined in their model are: Representation and replication: Coral survivorship is ensured by representing and replicating resilient species and habitats in an MPA network. The presence of resilient species in management in MPAs will help protect corals from bleaching events and other natural disturbances. Critical areas: Conservation priority areas provide protection to critical marine areas, such as sources of larvae for coral reef regeneration or nursery habitats for fish spawning. Connectivity: Preserving the connectivity between coral reefs and surrounding habitats provides healthy coral communities and fish habitat. Effective management: Resilience based strategies are based on reducing threats to maintain healthy reefs. Measurements of effective management of MPAs allows for adaptive management. References Further references Oliver, Thomas Andrew (2009) The role of coral's algal symbionts in coral reef adaptation to climate change ProQuest. . External links Reef resilience – coral reef conservation site of The Nature Conservancy