Investigating fragment size for culturing reef-building corals (Porites lobata and P-compressa) in ex situ nurseries

Culturing small propagules of coral has the potential for high yield with low environmental impact, provided that mortality is low. This study investigated the size-specific mortality and growth of minute fragments (as small as 5 polyps) of two of the most abundant reef-building corals in Hawaii (Porites lobata and P compressa). Two ex situ nursery systems differing in cost, design complexity, and labor intensity were compared. The first nursery experiment lasted for four months in a large tank with high water motion/surge and extensive manual cleaning to remove competitive algae. The corals were then transferred to a simple low-flow tank containing sea urchins (Tripneustes gratilla) and reduced cleaning, where they were grown for six more months (Nursery II). 'Nursery I' resulted in 92% of P. lobata and 73% of P. compressa fragments surviving and nearly doubling in area, in spite of a brief infestation by a nudibranch (Phestilla sibogae) that primarily fed on larger P compressa fragments. There was a significant positive relationship between fragment size and growth rate, and survivorship was significantly different between donor colonies (genets), but there was no evidence of size-specific mortality. 'Nursery II' on the other hand had clear size-specific mortality and higher urchin damage for smaller fragments, resulting in moderate survivorship (78% for P. lobata and 76% for P. compressa), and only a slight increase in the total area covered by coral tissue. Fragments larger than 3 cm(2) were undamaged and had the highest survival and growth rates. This study illustrates how size-specific mortality can be reduced by ex situ nursery conditions. (c) 2006 Elsevier B.V. All rights reserved.