LAUSR

Coral reefs are predicted to be vulnerable to the Allee effect due to the density dependence of broadcast spawning fertilisation success. Verifying such effects in corals, however, is hindered by their annual or biannual nocturnal multispecific synchronous spawning events, the lack of colony spatial distribution data, and the difficulty of manipulating coral colony density in the field. These obstacles were overcome by simulating scleractinian coral spawning events at various distances between colonies. While controlling for density, and including the influence of colony size and current flow speed, we investigated the distance at which reproductive contact became unlikely (critical intercolonial distance). The relationship between fertilisation success and colony density was also examined by simulating broadcast spawning events at different densities, with the aim of investigating whether Allee effects can occur at population densities observed in the literature. Fertilisation events rarely occurred between simulated coral colonies that were more than 30–40 m apart in the conditions tested due to sperm dilution and insufficient mixing between gametes from different colonies. Higher flow speeds improved mixing for distant colonies but generally resulted in low fertilisation success; larger colonies reduced the impact of dilution due to increased reproductive output. Fertilisation success exhibited a logarithmic relationship as colony density increased (e.g. 99.0% at 3.36 × 10−2 colonies m−2 for colonies with 1 m2 surface area), with low fertilisation success (21.0% at 2.70 × 10−3 colonies m−2 for 1 m2 colonies) at densities comparable to post-disturbance populations. These findings suggest the potential for strong component Allee effects due to broadcast spawning. Our critical intercolonial distance results support existing evidence that coral populations within a reef may consist of multiple breeding groups instead of a single homogeneous one. The inclusion of spatial data when surveying for coral colony density should be considered when attempting to predict density dependent effects.