Cuing oyster recruitment with shell and rock: implications for timing reef restoration

Abstract

Reducing uncertainty surrounding the biological responses of degraded habitat is key to providing confidence and efficiency in its restoration. Many coastal habitats are so extensively degraded that organismal responses to restoration interventions are entirely unknown. Among the most degraded coastal ecosystems are oyster reefs, whose restoration typically occurs where oysters are degraded to the point of functional extinction. This loss creates uncertainty on the fundamental processes for reef recovery; the timing of oyster recruitment and their preferred substratum for settlement. Such knowledge can inform restoration strategies to accelerate habitat recovery. Near the site of Australia's largest restoration of native oyster reef, we quantified temporal variability in recruitment of the native flat oyster (Ostrea angasi) and assessed their preference between the settlement substrata deployed for the reef restoration. Combining half a decade of environmental data with oyster recruitment data, we provide a model that identified distinct peaks in oyster recruitment which correlate with food availability and seawater temperature. In addition, oysters preferentially settled on oyster shell relative to other materials used in local restoration. In combination, these results suggest that there are opportunities to augment recruitment through addition of shell substratum synced with recruitment peaks. Our recruitment model likely represents a minor investment with large returns, providing opportunities to capture peak recruitment and greater confidence in utilizing natural recruitment as a restoration resource.

Multi‐species restoration accelerates recovery of extinguished oyster reefs

“…Prior to reef construction, only sparse O. angasi were observed in the area, attached to Pinna bicolor shells which provided the only hard substrate on the sandy seafloor. Locally, O. angasi spawning is documented from mid‐October to early April (Austral summer) when water temperatures cross a thermal threshold for spawning (>17°C, McAfee & Connell, 2020).…”

Repairing recruitment processes with sound technology to accelerate habitat restoration

“…The best use of this technology may center on synchronizing its use with other restoration strategies that aim to provide conditions (e.g., settlement substrate) that maximize recruitment. For example, synchronizing soundscape enrichment with the timing of recruitment or interventions to provide competitor‐free substratum (McAfee and Connell 2020, McAfee et al. 2021) may increase the success of restoration initiatives.…”

Spatial variation in the biotic and abiotic filters of oyster recruitment: Implications for restoration

“…At sites with suitable abiotic conditions for oyster recruitment but high predation (i.e. downstream sites in higher‐latitude estuaries, with low sediment loads), complex substrates such as shell or crushed rock may be used to provide protective microhabitats for recruits (McAfee & Connell, 2020; O'Beirn et al., 2000; Soniat et al., 2004) and, where the scale of restoration is small, could be combined with predator exclusion cages. Additionally, at sites with high predation, projects that restock oysters might transplant larger individuals to reduce effects of predation.…”

“…At each of the 15 sites, five caged and five uncaged substrate units were interspersed at each of the three tidal elevations (see Figure ) in February and March 2020—the peak S. glomerata settlement period (Lee et al., 2012). Concrete Besser blocks (19 × 19 × 19 cm) were used as substrate because concrete releases a positive settlement cue for oysters and is used in oyster reef restoration (McAfee & Connell, 2020). Cages were 25 × 25 × 25 cm and constructed of 1 cm diameter non‐toxic plastic mesh (Buono‐Net) that excluded large vertebrate and invertebrate predators.…”