Developing and scaling settlement, deployment and monitoring of wild cultured larvae for reef restoration

Chelsea Waters

doi:10.25918/thesis.505

The recruitment and subsequent recovery of coral populations following large-scale and repeated climate-induced disturbances is being compromised by a combination of declining larval supply and early-post settlement survival bottlenecks. As widespread coral mortality threatens recruitment on reefs through declining propagule supply and habitat fragmentation, active intervention methods are being developed and tested to restore coral assemblages. In particular, larval-based coral restoration methods are undergoing significant research and development in preparation for their application at multi-hectare scales, considering the magnitude of environmental impacts. Whilst the collection and culture of coral larvae in situ has been refined and can now be replicated at scale, the transfer of competent larvae and their deployment onto damaged reefs without constraining them under nets that limits the spatial scale of operations, remains a significant challenge. This research in this thesis aimed to address current bottlenecks within the transfer and deployment of mixed larval assemblages from high density in situ cultures to their deployment onto degraded reefscapes. Once larvae cultured in situ reach competency to settle, there is only a narrow timeframe to transfer them to the reef before they begin metamorphosing. This transfer bottleneck can hinder the scalability of larval-based coral restoration, as large quantities of competent larvae must be moved efficiently within this critical period. Chapter 2 tested the use of industrial diaphragm pumps for transferring large quantities of coral larvae during production. Pumping of coral larvae resulted in low mortality irrespective of flow rate. These results highlight that using a diaphragm pump for the transfer of coral larvae, regardless of flow rate, can effectively facilitate their mass transfer (with some loss) when conducted from 4-days post-spawning onwards, at which point larvae are fully developed, motile and acquiring competency. Thus, this technique appears effective for overcoming transfer bottlenecks. Another approach to scaling up larval settlement, is to seed settlement devices with coral larvae prior to their deployment onto degraded reefs. However, the effects of the substrate that devices are made from and submersion duration in high-density larval cultures on reseeding outcomes remain unknown. Chapter 3 tested the use of settling competent larvae onto two types of settlement devices, CaCO3 travertine limestone tiles and engineered ceramic devices. Both types of settlement devices had very high settlement densities even following short submersion timeframes of 24h within larval culture pools, which resulted in the hypersaturation and crowding of settled larvae. Hypersaturation, in combination with heavy sand deposition after deployment and the reef site habitat being composed of unconsolidated reef substrata led to low proportional coral survival, and an overall low yield of 9% of devices having at least one recruit after 21 months. Therefore, this technique is not suitable in unconsolidated and highly sedimented habitats. Finally, owing to the influence of site-specific environmental conditions (i.e. sedimentation) affecting reseeding outcomes following findings from Chapter 3, Chapter 4 aimed to understand how localised characteristics of restoration sites and the biological conditioning of tiles at these sites may influence coral survivorship following their seeding on tiles in larval culture pools. Results revealed that recruit survival after 9-months was highest on tiles that i) had the lowest initial settlement density, which occurred on unconditioned tiles, ii) were located at a sheltered inshore reef flat, and iii) had minimal changes in the percent cover of bare substrate and red macroalgae. Thus, characterising reef conditions prior to deployment is essential for maximising recruitment potential. Results from this thesis have highlighted the use of diaphragm pumps for effectively transferring larvae from culture to damaged reefs with minimal larvae mortality (Chapter 2). When using settlement devices in dense larval culture pools to transfer larvae, short submersion windows (Chapter 3, 4) and minimal conditioning (Chapter 4) yielded the best recruitment outcomes. In addition, the environmental conditions at the reef sites where the settlement devices are deployed will influence recruitment outcomes, emphasising the need to assess site characteristics prior to deployment (Chapter 3, 4). Ultimately, this work provides important information for improving the effectiveness of larval-based restoration at larger spatial scales.

Developing and scaling settlement, deployment and monitoring of wild cultured larvae for reef restoration

Files and links (1)

Metrics

Abstract

Details

Developing and scaling settlement, deployment and monitoring of wild cultured larvae for reef restoration

Files and links (1)

Metrics

Abstract

Details

Southern Cross University Social media