Growth and survival among Hawaiian corals outplanted from tanks to an ocean nursery are driven by individual genotype and species differences rather than preconditioning to thermal stress

Abstract

The drastic decline in coral coverage has stimulated an interest in reef restoration, and various iterations of coral nurseries have been used to augment restoration strategies. Here we examine the growth of two species of Hawaiian Montipora that were maintained in mesocosms under either ambient or warmed annual bleaching conditions for two consecutive years prior to outplanting to determine whether preconditioning aided coral restoration efforts. Using coral trees to create a nearby ocean nursery, we examined whether: (1) previous ex situ mesocosm growth would mirror in situ coral tree nursery growth; and (2) thermal ex situ stress-hardening would predict future success during natural warming events in situ for corals moved from tanks to trees. For Montipora capitata, we found that variation in growth was explained primarily by genotype; growth rates in the mesocosms were similar to those in situ, irrespective of preconditioning. Variation in M. flabellata growth, however, was explained by both genotype and culture method such that an individual M. flabellata colony that grew well in the tanks did not necessarily perform as well on the coral trees. For both species, previous exposure to elevated temperatures in the mesocosms provided no benefit to either growth or survival during a warming event in the coral tree nursery compared to those grown in ambient temperatures. Overall, M. capitata performed better in the tree nursery with higher net growth, lower mortality, and was subject to less predation than M. flabellata. Our results show little benefit of the additional cost and time of stress-hardening these corals prior to outplanting because it is unlikely to aid resilience to future warming events. These results also suggest that selecting corals for restoration based on long-term genotype growth performance may be more effective for optimal outcomes but should be weighed against other factors, such as coral morphology, in situ nursery method, location, and other characteristics.

Keywords: Acclimatization; Acropora; Adaptive potential; Assisted evolution; Bleaching; Coral farm; Coral nursery; Coral tree; Reef restoration; Resilience.