Sea-weeding enhances early coral survival on seeding devices, but benefits of seeding diminish after one year

Abstract

Borrowing from principles of aerial seeding in terrestrial reforestation, coral seeding utilises “devices” designed to increase coral spat survival. However, device-assisted coral survival has not been compared to natural survivorship, nor have devices been trialled in environments with strong competitors such as macroalgae. Herein, we deployed seeded devices alongside terracotta tiles, a proxy for natural coral recruitment dynamics. Tiles and devices were deployed to plots examining ongoing macroalgae removal (“sea-weeding”), and survival was monitored over two years. First-year coral survival was enhanced on devices compared to natural survival, and devices conferred the greatest survival benefit when deployed in areas where “sea-weeding” was undertaken. However, over the second year, the benefits of sea-weeding for device-assisted survival were lost, with no significant difference in survival on devices in control versus weeded plots. On average, devices retained 1.3 surviving colonies at two years, which was lower than naturally-occurring juvenile density in removal plots, but higher than control plots. Several factors influenced survival, including the starting spat density, orientation of deployment surface, and site. After two years, 50 % of devices yielded one live coral, but site-based survival varied between 37 % and 93 %. The estimated cost per surviving coral varied when accounting for site-based survival (high survival: $334 coral⁻¹; low survival: $577 coral⁻¹), which could be reduced via future efficiencies. The results of this study inform the potential outcomes of coral seeding on macroalgae dominated reefs, and highlight that assessing survival up to one year is not sufficient to measure long-term restoration goals.