Robert Toonen

Scaling up production capacity for Hawaiian coral reef restoration: few large or many small?

2020

University of Hawaii

"As coral reefs decline worldwide, interest in active coral reef restoration has increased. The micro-fragmentation method is becoming widely used for massive and encrusting species such as those that dominate Hawaiian reefs. Micro-fragmentation takes advantage of the tendency of small (about 1 cm) coral fragments to rapidly spread tissue and fuse to cover a surface, resulting in increases in coral cover at rates that can be orders of magnitude higher than typical growth rates. Micro-fragmentation shows promise and flexibility for reef restoration; however, it is a relatively new method and there are many unanswered questions for optimal application at a particular restoration site. This project sought to explore relationships and tradeoffs between size, nursery residence time, and genotype for a variety of outplanting field sites. We developed coral micro-fragment growth assays to provide data on growth and survivorship for the predominant reef building corals in Hawai'i. We deployed 126 assay modules consisting of 882 small, 378 medium, and 126 large fragments yielding 1,386 coral fragments in total across 3 distinct coral reef habitats (Kane'ohe Bay reef flat, Honolulu Harbor, and a South shore exposed reef). The assays were held in the nurseries and deployed over time in batches (direct transplantation (0 months), 4 months, and 8 months) to examine effects of nursery residence time. Health checks tracking mortality, partial mortality, and tissue discoloration were ranked, and coral tissue cover (cm2) was measured from scaled digital 2D photographs and also estimated from 3D Structure from Motion (SfM) scaled photogrammetric models. The project coincided with a bleaching event in the summer, providing data on growth and survivorship before, during, and after the event. Survivorship was highly dependent on site, size, and genotype (i.e. donor colony). Colony genotype had a major impact on coral growth and survivorship, indicating that selection of genotypes is critical for restoration success. Several genotypes more than doubled in total coral cover and total colony surface area while one genotype was highly susceptible to bleaching and suffered total mortality. Growth data from 2D and 3D measurement methods showed highly similar results, although 3D measurements were less time consuming, while accounting for complex colony morphology. In-situ nursery residence time showed high temporal variability likely due to micro-spatial variation, temperature stress and micro-predator outbreaks. The ex-situ nursery in contrast had almost no mortality and the highest growth rates, but variable mortality when outplanted depending on size and genotype. Mortality was highest on the exposed reef and both harbor and reef sites had the highest mortality for smaller fragments, indicating that a 'size refuge' is highly site specific. Mortality also varied by colony genotype at both harbor and reef sites, with several colonies showing an interaction between genotype and environment. Final growth data at the AFRC sites could not yet be collected due to the COVID-19 shutdown; however, this project demonstrated that the assays successfully provided data on optimal colony size, genotype, and nursery residence time for a variety of reef locations."

coral reef, restoration, micro-fragmentation, growth, survivorship, growth assay, Structure from Motion, Hawaii

NOS

OCM

FY2020 CRCP Project ID 31209; Project Title: FY18 - Domestic Coral Reef Conservation Grants; Principal Investigator: Liz Fairey; Grant NA18NOS4820110.

NA18NOS4820110