Combined impacts of natural recruitment and active propagation for coral population recovery on the Great Barrier Reef

Abstract

Coral populations on the Great Barrier Reef (GBR) are experiencing long-term shifts in size structure, including steep declines in small colonies, driving major concerns for recovery through the supply of new recruits. Whilst coral restoration began on the GBR in 2018, the combined influence of natural recruitment and outplanting for coral population recovery has not been evaluated. Here, we assessed 2 sites (Rayban and Mojo) at Opal Reef that were subject to intensive outplant efforts over a 3 yr period (2018-2021). Coral cover did not change significantly, with a baseline of 15% in 2018 and a cover of 28 and 25% in Rayban outplant and control areas, respectively, in 2021, while Mojo exhibited a coral cover of 38% in 2018 and 52% (outplant area) and 29% (control area) in 2021. Natural recruitment in 2021 did not vary by site and was characterised by a settlement rate of 5.5 and 3.7 recruits tile-1 at Rayban and Mojo, respectively. Juvenile coral abundance and diversity were similar for control and outplant areas at each site. Over the 3 yr period, coral cover as a metric did not identify differences between control and outplant areas; however, size-frequency distributions of key coral taxa revealed a higher frequency of small to mid-sized colonies in outplant communities compared to controls. Given that no differences were observed in recruitment rates or juvenile abundances, variations in population structure appear to be driven by planting efforts rather than natural recovery. Our results demonstrate the need for combined monitoring of natural versus intervention-based rehabilitation to understand the impact of coral propagation efforts for local site recovery.

Bacterial communities associated with corals out-planted on the Great Barrier Reef are inherently dynamic over space and time

“…obs. ; see also Roper et al 2022) that was initially characterised by higher ow rates, light attenuation, and lower ) in contrast to the 0-35% survival after 12 months observed across plots within the same site in our current study. Further, similar bacterial community composition and low overall survival were observed at sites Rayban and Sandbox (0-43%) despite different initial environmental conditions ( ow, temperature, and light attenuation), indicating the need for ner resolution spatiotemporal monitoring of environmental conditions and associated bacterial communities within-sites as well as between sites.…”

Fish-based herbivory and the natural maintenance of algal fouling on coral nurseries used across the Great Barrier Reef

“…Acropora was the most abundant genus across the two sites. Percentage coral cover near Mojo was consistent with later survey findings that directly evaluated coral cover at the Mojo nursery site (Roper et al 2022). Coral nursery frames at both sites comprised a 2.0×1.2 m aluminium floating frame (Howlett et al 2021) This low-cost design is commonly employed across CNP sites.…”

Cost‐effectiveness of tourism‐led coral planting at scale on the northern Great Barrier Reef

“…The wide variation in PO across CNP trips, reefs and/or tourism operations is likely the result of a conflation of operational and environmental variables that can both impact different stages in the coral restoration workflow. Environmental variables include underlying coral cover and composition (ranging 3-52% across CNP sites; Howlett et al 2022;Roper et al 2022) and the presence of herbivorous fish communities that remove biofouling algae from nursery frames (see also Frias-Torres et al 2015). Operational and logistical factors include distance to reef from port, tourism operation size, the number of nurseries installed at sites, staff turnover rate, other site stewardship requirements (e.g., reef surveys, COTs removal), and the tailored adoption of restoration activity to operations (e.g., less frequent and intensive planting vs. regular, less intensive approaches as described in Howlett et al 2022Howlett et al , 2023.…”

“…For example, Humanes et al (2021) and Baria‐Rodriguez et al (2019) determined that extending coral nursery rearing periods for sexual recruits resulted in enhanced survivorship over the long term, thereby negating any additional costs associated with longer husbandry periods. In the context of the CNP, although typically more expensive, “training days” and “dedicated days” are not only critical to evaluating efforts, trialing new techniques, training staff, and improving practice, but also concentrating efforts at more degraded sites where coral population recovery is most needed (Roper et al 2022; Howlett et al 2023).…”

“…Ultimately, the realized costs of restoration efforts are dynamic over space and time, and are highly dependent upon stochastic disturbances on reefs (Boström‐Einarsson et al 2020), and importantly how “effectiveness” is defined and measured, and for how long. For example, employing ecological changes (e.g., live coral volume—Morand et al 2022; or population structure—Roper et al 2022), ecosystem service values (Abrina & Bennett 2021) or socioeconomic benefits (Hein et al 2017) in cost evaluations rather than outplant survivorship, would likely deliver vastly different, and arguably more informative (Suggett et al 2023) assessments of cost‐effectiveness. Furthermore, as climate‐driven disturbances increasingly drive coral mortality, measures of ecological and social resilience will become essential to justify return‐on‐investment in iterative cost–benefit analyses (Shaver et al 2022).…”