Abstract Summary
Assisted migration of corals over latitudinal gradients has been proposed as a strategy for increasing coral thermotolerance, but is currently considered high-risk, due to the potential for the unwanted introduction of pathogens or invasive species, and genetic risks such as outbreeding depression. A pragmatic way of minimizing these risks is to identify thermal heterogeneity in seascapes over much smaller scales and use corals from locally warmer conditions as source populations for restoration. We investigated this approach in Miami, Florida, by testing whether corals collected from different sites spanning a natural thermal break in Biscayne Bay varied significantly in their thermotolerance. Thirty genotypes of staghorn coral (Acropora cervicornis) were collected from sites spanning this break and maintained in common-garden conditions in three nurseries for 2-10 years. We then used a laboratory heat stress experiment to test the thermotolerance of these genotypes, based on reductions in symbiont density (measured as cell counts and as symbiont-to-host cell ratios), photochemical efficiency (Fv/Fm), and chlorophyll. We grouped genotypes by thermotolerance and found higher thermotolerance was associated with the southern collection locations, with an increase of just 0.25oC in maximum monthly mean (MMM) temperature selecting for genotypes that were twice as thermotolerant (measured as time spent at 32.5oC for a similar decline in response variable) compared to susceptible genotypes. However, thermotolerant genotypes also grew ~25% more slowly at nursery sites compared to susceptible genotypes. When interpreted using high-resolution climate models, these data indicate that assisted translocation of genotypes from southern sites to northern sites – a distance of < 15km – could extend bleaching horizons (measured as time to annual severe bleaching) by >20 years compared to outplanting nursery corals at their original collection sites. Assisted translocation within local restoration areas may help restore reefs which grow more slowly but are more climate resilient, and help buy time for the development of additional interventions.
