Large scale field trial of a selective coral breeding intervention (RRAP ECT 2.1)

Testing the thermal tolerance of selectively bred coral stock under simulations of stress events is essential for determining the efficacy of selective breeding as a viable tool for large-scale coral restoration. While aquariums provide controlled environments that can isolate specific variables, such as temperature, they cannot account for the complex ecological variables and interactions which naturally occur on reefs (e.g., competition, predation, environmental fluctuations, etc.). Field testing of selectively bred coral offspring is therefore essential to understanding various trait and environmental trade-offs that can influence coral performance post intervention.

To test whether the heritability of heat tolerance is linked to the deployment environment we selectively bred Acropora spathulata colonies based on their performance under a rapid thermal stress test then deployed the offspring of heat-tolerant and control corals to nine sites along the lagoon, back, and flank of Davies Reef (central, midshelf, Great Barrier Reef).

Gravid, adult Acropora spathulata colonies (N = 40) were collected (hammer and chisel) in October 2023 from Davies Reef in Bindal Sea Country (S18.81958&deg;, E147.63129&deg;; GBRMPA permit G21/38062.1) then brought onboard an AIMS research vessel. A rapid heat stress test was conducted 37 days prior to the full moon using an onboard temperature control experimental system (AIMS SeaSim in a box; following Naugle et al. 2024 and Denis et al. 2024). The experimental temperatures were set according to the local maximum monthly mean (MMM) of 28.38&deg;C for Davies Reef using sea surface temperature data from the NOAA Coral Reef Watch Operational Daily Near-Real-Time Global 5-km Satellite Coral Bleaching Monitoring Product (Version 3.1, 1985&ndash;1990+1993; following Liu et al. 2003). Corals were exposed to the MMM and +4&deg;C, + 7&deg;C, and +10&deg;C treatments.

Photosynthetic efficiency (dark-adapted yields or maximum quantum yield, Fv/Fm) measurements were used as a proxy for the extent of heat stress experienced by the coral holobiont. Imaging Pulse Amplitude Modulated (IPAM) fluorometry was used to measure Fv/Fm two hours post temperature ramp down to MMM.  Photosynthetic efficiency (dark-adapted yields or maximum quantum yield, Fv/Fm) ED50 values, the temperature that induces a 50% decline in Fv/Fm in a colony compared to its Fv/Fm at MMM, were calculated and used to detect variation in heat tolerance between fragments. The colonies were then phenotyped from most to least heat-tolerant based on their Fv/Fm ED50 performance and served as the broodstock to selectively breed coral offspring.

The broodstock corals were transported to the AIMS National Sea Simulator facility for coral spawning.  Three main gamete crosses were used to create the selectively breed coral offspring. (1) Gametes from five heat-tolerant colonies, ranked in the top 25% percentile of the population, were used to produce the Resilient X Resilient (RR) group. (2) Gametes from four heat-sensitive colonies, ranked in the bottom 25% percentile of the population, made the Sensitive X Sensitive (SS) group. (3) Gametes from 24 colonies, with mixed heat tolerance rankings, produced the Bulk offspring group. The reared larvae were settled onto preconditioned (i.e., early successional mixed crustose algal and bacteria biofilms) concrete settlement tiles (~ 200 mm X 200 mm). The tiles yield 400 individual tabs that are compatible with coral-seeding devices being tested and developed by the Coral Aquaculture and Deployments team in the Reef Restoration and Adaption Program at AIMS (Whitman et al. 2025; Jurriaans et al. 2025; Page et al. 2025; Ramsby et al. 2026).

675 coral seeding devices with live corals were deployed to Davies Reef. A single device contained slots for three tabs, one tab from each of the three offspring groups (RR, Bulk, SS). At the deployment, devices were linked in chains of five then hand-delivered to the reef by divers. Each site (N=9) contained three device plots, five device chains, and 75 devices in total. Three deployment sites selected across the back reef (5-9 m depth), lagoon (2-5 m), and reef flank (5-7 m).

Organization: Australian Ocean Data Network