Study site characteristics

This study was conducted on reef flats within no-take marine protected areas (MPAs) adjacent to Votua, Vatuo-lailai, and Namada villages along the Coral Coast of Viti Levu, Fiji. These reserves are scattered along 11 km of fringing reef and are separated by ~3-8 km. The reserves have high coral cover (38-56%), low macroalgal cover (1-3%), and a high biomass and diversity of herbivorous fishes (Rasher, Hoey, and Hay 2013; Bonaldo and Hay 2014). The reef flats range from ~1-3 m deep at high tide, extend ~500-600 m from shore to the reef crest, and are typical of exposed reef flats occurring throughout Fjii.

Except during low tides in calm weather, waves push water over the reef front, and water then flows laterally across the reef flats to discharge through channels bisecting the flats. This creates a relatively predictable current direction at most locations.

Survival experiments
To test whether juvenile corals experienced distance-dependent mortality near adult conspecifics, we collected ~5 mm tall fragments of P. damicornis and S. hystrix, selected suitable adult focal colonies (defined below), and attached conspecific fragments 3, 12, 24 and 182 cm up- and down-current from each focal adult. We deployed fragments around focal colonies in Votua village’s MPA, which supports a diverse assemblage of corals covering about 50% of hard substrates (Rasher, Hoey, and Hay 2013). We used fragments from older colonies as proxies for ~6 month old juveniles (Sato 1985) because, despite these species reproducing monthly in some locations (Fan et al. 2002; Kuanui et al. 2008), neither species planulated at our site during the months of this study (August through October 2013).

We used pliers to clip 16 fragments of 30-40 polyps each from the tips of each of 24 large P. damicornis and 24 large S. hystrix colonies in the Votua village MPA. The fragments from each of four source colonies for a species were collected in six rounds over two days. Each round was taken to shore and four fragments (one from each source colony) were epoxied (Emerkit epoxy) onto the unglazed side of 16 2.54 x 2.54 cm tiles. Thus, each tile had fragments from four different colonies and sets of 16 tiles had fragments from the same four colonies. After epoxying, tiles were held in a tub of seawater for ~1 h, allowing the epoxy to harden. Tiles were then cable-tied onto metal racks at ~1 m deep in the MPA and allowed to acclimate for two weeks before deployment in the experiment. Survivorship during acclimation was 100%, producing 384 fragments on 96 tiles for each coral species.

Within the MPA, 10 adult P. damicornis and 10 adult S. hystrix colonies served as focal colonies. Focal colonies: i) were >10 cm at their smallest diameter (10 to 35 cm for P. damicornis and 10 to 75 cm for S. hystrix), ii) had no conspecific colonies within 4 m (so as not to confound effects of the focal colony with effects of nearby conspecifics), iii) were 5-40 cm deep at low tide, and iv) had space for 190 cm PVC pipes to be placed roughly east and west (the predominant current direction was west) without disturbing other corals. Focal colonies were photographed from above and their size determined using ImageJ (Rasband 1997).

Twenty mm diameter by 190 cm long PVC pipes served as platforms to which we attached the tiles. Pipes were anchored to the reef by driving steel rebar through pre-drilled holes and cementing the rebar to the pipe. Notches 2.54 cm long allowed us to cable-tie tiles onto the pipes at distances of 3, 12, 24 and 182 cm from focal colonies. This approach secured all pipes and tiles throughout the experiment. These distances and this scale were chosen to match a previous experiment in the Caribbean that had detected distance dependent mortality of newly settled recruits for a broadcast spawning coral (Marhaver et al. 2013).

Tiles were randomly assigned to positions on pipes. Thus, fragments at each distance and around each conspecific focal colony were random with respect to source colony. Unassigned tiles were kept on the rack as spares (64 fragments on 16 tiles for each coral species).

Every 1-2 d after deployment, we examined all fragments, recording survivorship, consumption, overgrowth by algae, bleaching, or other changes in status.

On some P. damicornis tiles, three or four of the fragments disappeared within a 24 h period between checks on their condition, appearing to have been bitten off. To determine the agents of this localized mortality, we replaced tiles whose four fragments had been eaten with spare tiles holding four healthy fragments around three of the focal colonies that had experienced localized mortality and videotaped the tiles (GoPro II HD) from about 1 m away during the following high tides. Cameras were retrieved at the next low tide and the videos watched.

We evaluated survival patterns using mixed-effects Cox proportional hazards survival models (coxme package, Therneau 2012) in R (R Core Team 2013). Distance and direction from focal colony were fixed effects and focal colony and tile nested within focal colony were random effects because fragments were blocked by tile and focal colony. The size of the focal colony and the depth of the tiles were included as random effects.