Restoring Damaged Coral Reefs Using Mass Coral Larval Reseeding

Restoring Damaged Coral Reefs Using Mass Coral Larval Reseeding

Chapter One of Restoring Damaged Coral Reefs Using Mass Coral Larval Reseeding

INTRODUCTION

BACKGROUND TO THE STUDY 

Reef corals are critically important because they build the primary reef framework, supply essential habitats for thousands of fish and other species, and provide direct energy and other nutrient inputs to the ecosystem (Gardner et al, 2003). But the loss of substantial live coral cover on reefs – fundamentally through overfishing, eutrophication and other disturbances arising from human activity – has seriously disrupted their ecology. Continued human pressures on damaged reefs leads to further reef decline and ecosystem collapse. But this situation may be alleviated through mass reseeding with coral larvae. The aim of this study is to quantify the effectiveness of mass larval reseeding to restore damaged coral communities on reefs in Nigeria, and to use this assessment as a case study for future global reef restoration management strategies.
The continuous degradation of coral reef ecosystems on a global level, the disheartening expectations of a gloomy future for reefs’ statuses, the failure of traditional conservation acts to revive most of the degrading reefs and the understanding that it is unlikely that future reefs will return to historic conditions, all call for novel management approaches. Among the most effective approaches is the “mass coral larval reseeding” concept of active reef restoration, centered, as in silviculture, on a two-step restoration process (nursery and transplantation). In the almost two decades that passed from its first presentation, the “mass coral larval reseeding” tenet was tested in a number of coral reefs worldwide, revealing that it may reshape coral reef communities (and associated biota) in such a way that novel reef ecosystems with novel functionalities that did not exist before are developed (Chen et al, 2011). Factors such as such as elevation of seawater temperature, extreme weather events, ocean acidification and intensifying tropical storms that cause, for example, enhanced frequency and intensity of mass coral bleaching has directly or indirectly influence coral survival, coral growth rates, reproduction efforts, larval development and settlement, and post-settlement survivorship/development of corals, damaging reef ecosystems’ health and resilience and reducing species abundance.
Graham et al (2014) further attests to the decimation of key reef-building coral populations, to a dramatic shrinkage in global reef structural complexity and that many reefs experience phase shift phenomena in addition to reefs that are continuously changing in unprecedented ways towards new ecosystem configurations and novel reef compositions that did not exist before. As the major emerging sources of global reef degradation, such as coral bleaching, seawater acidification impacts and coral diseases, interact synergistically and also in concert with local/regional anthropogenic specific stressors, among them pollution, eutrophication, sedimentation, coastal development and overfishing, augmentation of existing climate change impacts is anticipated. Large-scale rearing of coral larvae during mass spawning events and subsequent direct introduction of competent larvae onto denuded reefs (larval seeding) has been proposed as a low-tech and affordable way of enhancing coral settlement and hence recovery of degraded reefs.
While some studies have shown positive short-term effects on settlement, to date, none have examined the long-term effects of larval seeding for a broadcast-spawning coral. However, this study will test whether mass larval reseeding significantly increases coral recruitment rates both in the short and longer term towards restoring damaged coral reefs. Mass coral larval reseeding plays a critical role in the persistence and resilience of reef coral populations (Richmond 1997) but its relative importance in coral population and community dynamics can vary according to species, habitat and reef location (Connell et al. 1997). Recent data indicate quite different patterns of reseeding and post-reseeding mortality occurring in reef crest coral communities along the length of the reef (Hughes et al. 1999). Characterizing the site-specific nature of processes and mechanisms influencing the arrival and survival of corals onto reefs is necessary for sound reef management.