Density of coral larvae can influence settlement, post-settlement colony abundance and coral cover in larval restoration

Abstract

Successful recruitment of new individuals is essential for recovery of degraded coral reefs. Enhancing supply of coral larvae increases initial settlement, however post-settlement survival can be influenced by density-dependent processes. We investigated the influence of larval density on settlement, colony abundance and growth to 24 months for Acropora tenuis in the northwestern Philippines, to determine whether larval supply can be optimised to maximise successful recruitment. Thirty different densities of coral larvae were enclosed for five days around settlement tiles and highest total settlement occurred on tiles with highest larval densities. After 12 months, however, colony abundance and coral cover was lower on high density tiles (supplied with ~2,500-5,000 larvae) than tiles supplied with ~1,000-2,000 larvae. Coral cover at 24 months remained highest on tiles supplied with ~1,000-2,500 larvae. Larval density influenced larval substratum selection, with proportionally fewer larvae settling in typically preferred locations as density increased. We conclude that larval density can influence post-settlement colony abundance and coral cover to 12 months, with coral cover trends persisting to 24 months. We show that optimising larval densities can maximise coral recruitment and growth, however oversupply of larvae at very high densities can have negative outcomes for larval restoration.

Variability in Fitness Trade-Offs Amongst Coral Juveniles With Mixed Genetic Backgrounds Held in the Wild

“…However, survival between population crosses was variable, and when we examined purebred survival rates for juveniles outplanted to their natal reef (DRxDR juveniles), rates also were high (∼20%) and provide an important ground-truthing for baseline survival rates. Decreased survival rates from the literature relative to ours may be attributed to higher densities of juveniles on substrates, in which overcrowding may lead to high mortality (Cameron and Harrison, 2020). A vast majority of juveniles in this study were found on single tiles with adequate space for growth and low competition and all were singles (no clumps).…”

Transient amplification enhances the persistence of tropicalising coral populations in marginal high latitude environments

“…Intriguingly though, we report that the survival of coral larvae following successful settlement appeared highest in the subtropics. Whilst consistent with expected density dependant patterns in the survival of newly settled larvae (Cameron & Harrison 2020), our finding disagrees with previous reports of extremely high annual post-settlement larval mortality within a subtropical environment (Wilson & Harrison 2005). Seawater temperatures at the time of settlement influence the survival of coral larvae (Randall & Szmant 2009).…”

A framework for selectively breeding corals for assisted evolution

“…Our results show an average of four corals per SU 10 days after settlement, resulted in at least one colony after four months ex situ husbandry rearing. Despite coral post-settlement survivorship being influenced by settlement density and husbandry conditions (Conlan et al, 2017; Cameron and Harrison, 2020), our results provide a useful guideline for optimal settlement densities when using SUs for CLP. For SUs outplanted at different ages, similar analyses are needed to provide information on the minimum number of larvae to settle to obtain one colony per SU at the age of outplant.…”

“…Settling more larvae per SU than needed wastes larvae, whereas, not settling too few larvae per SU lowers yield and increase costs. Evidence thus far indicates that low larval settlement densities can compromise the production of SUs with at least one coral colony at the time of outplanting due to high post-settlement mortality, while high settlement densities can compromise survivorship due to density dependent effects (Doropoulos et al, 2017; Cameron and Harrison, 2020). Additionally, high larval densities promote the formation of chimeras, a factor that needs further research as their implications on colony growth and survivorship are still understudied (however see Rinkevich, 2019).…”