McIlwain, Jennifer; Laurie Raymundo

Final Report: Measuring the degree of connectivity between remnant staghorn papatch at risk of Anthropogenic impacts

2015

NOAA, NOS, Office of Ocean and Coastal Resource Management; NOAA, NOS, Office for Coastal Management

"Utilising a suite of techniques which combined population genetics with biophysical and larval dispersal modelling we test the relative influence of connectivity in remote island populations of a Staghorn coral, Acropora pulchra. This species is confined to the shallow lagoons around the reef flats of Guam and adjacent islands in the southern end of the Mariana Archipelago. Total population size of this species has been reduced over the past decade through a combination of bleaching events and pollution, and its long term future in the archipelago remains unclear. Mapping of staghorn corals on Guam revealed a total area of 33 ha across 14 reef-flat and lagoon locations. Acropora pulchra was the dominant species making up 90% of the total area. More than 50% of all Guams staghorn corals were found in Tumon Bay, followed by West Agana (19.3%) and Agat (14.7%). The analysis of 8 microsatellite loci from 430 staghorn colonies collected across multiple sites within Guam and Saipan, revealed high levels of asexual fragmentation at distances greater than 30m ; one of the widest spreadsamong other coral species similarly analyzed. Bayesian clustering analysis and migration modeling uncovered a putative dispersal barrier between staghorn corals in Cocos Lagoon at the southern tip of Guam and staghorn patches along Guams entire West coast. The most parsimonious migration model indicated that the Cocos Lagoon population does not contribute enough migrants per generation to the remainder of Guam to maintain genetic homogeneity. A secondary barrier was also evident between Guam and Saipan to the north, albeit not at the same level as the Cocos Lagoon. Fish species assemblages inside and outside of Acropora thickets were similar across the three locations with the greatest coral cover; Tumon, East Agana and Agat.Our biophysical modelling showed larvae spawned at the two locations with greatest A.pulchra cover (Tumon Bay and Agana Bay) were most likely to settle long the northwest coast with no chance of settlement south of Orote Point. Similarly those spawned at Agat were retained along the southwest coast or advected westwards. For the Cocos Lagoon population the dispersal modelling supports that of the genetics results whereby larvae spawned from Cocos do not settle elsewhere on Guam. The small A.pulchra population on the east coast at Togcha make no contribution to larval settlement on the west coast with most larvae advected to the south of the island. A recent bleaching event during the summer months of 2014, at the latter stages of the project, gave us the opportunity to run our anticipated modelling scenarios as real events. We found that when adult mortality was reduced by up to 70%, larval settlement was heavily impacted and proporitional to the loss of adults. To determine whether marine preserves act to reseeed adjacent non-protected areas we found only one Marine Preserve on Guam, Tumon Bay acts as a net exporter of Acropora staghorn larvae. We argue that while asexual propagation (the dominant mode of reproduction for A.pulchra on Guam) is a successful strategy for responding to storm disturbances, a lack of external larval supply leaves local populations prone to extinction, especially under a regime of increasing SSTs and greater frequency of bleaching. Such restricted gene flow within an island only 35 miles long and 5 miles wide was surprising but is consistent with the apparent lack of resilience being displayed by extant staghorn populations."

FY2012; CRCP Project ID: 198; Project Title:Domestic Coral Reef Conservation Grant Programs; Principal Investigator: Jenny Waddell; CRCP Grant Number: NA12NOS4820071