Evaluating pump-assisted larval transfer for scaling coral larval restoration interventions

Globally, multiple techniques are being trialed to accelerate ecosystem recovery in shallow coastal habitats. One technique aims to boost propagule supply in areas where the current supply is insufficient to re-establish depleted populations to ecologically significant levels. However, transferring large quantities of propagules requires large-scale methods to be tested to ensure safe and efficient collection and deployment into degraded habitats. This study aims to determine if the mass transfer of coral larvae through pumping techniques affects their survival, locomotion and settlement potential. A pilot experiment tested the effects of pumping at two flow rates, and a gravity-fed control, on Acropora cf. tenuis larval survival and settlement across four larval ages. Following pumping at days 2-5 following spawning, larval mortality rates were typically low (~0.8% and ~3% for low and high flow, respectively) and settlement rates similar. A subsequent experiment then investigated the effects of pumping on a mixed larval assemblage following in situ wild spawn slick collection and cultivation. Findings were similar to those of the pilot, confirming low average mortality rates (0.6-1%) with highest mortality (1.3%) for 3-day old larvae. At larval ages of 4-, 5- and 6-days post-spawning, pumping had no significant effects on locomotion abilities or settlement among larvae that were retrieved after transfer. For 3-day old larvae, locomotion following high flow pumping was marginally compromised, especially in the first minute following pumping, but locomotion increased significantly 5-10 minutes later. Larvae that were 3-days old and subjected to pumping (low or high) exhibited approximately 50% lower settlement rates compared to the control group. In contrast, no effects on settlement due to pumping were observed for 4-, 5-, or 6-day old larvae. Larval transfer, regardless of the technique employed, resulted in losses ranging from 21-27%. Losses were likely caused by some larvae becoming trapped in the fine mesh filter nets after transfer or lost through the aquarium system, with the extent of loss varying with the developmental stage of the larvae. Overall, results highlight that pumping coral larvae with a diaphragm pump can effectively facilitate their mass transfer when conducted from 4-days post-spawning onwards, at which point larvae are fully developed, motile and acquiring competency. Pumping techniques can therefore be utilized to facilitate increasing the rates and scales of larval restoration operations.