de Breuyn M et al. (2023), Diurnal predators of restocked lab-reared and w...

ECB-ART-52561

PeerJ 2023 Jan 01;11:e16189. doi: 10.7717/peerj.16189.

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Diurnal predators of restocked lab-reared and wild Diadema antillarum near artificial reefs in Saba.

de Breuyn M , van der Last AJ , Klokman OJ , Hylkema A .

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The long-spined sea urchin Diadema antillarum controls reef dynamics by grazing on algae and increasing coral recruitment. Populations of Diadema never recovered after a mass-die off in 1983 and 1984, and numbers were further reduced by a more recent die-off in 2022. To restore grazing pressure and thereby the resilience of Caribbean coral reefs, multiple Diadema restocking efforts have been performed. Although results vary, relatively low retention is one of the reasons restocking is not considered more often. If causes for the low retention can be identified, suitable measures may be able to increase restocking success. In this study, we monitored restocked lab-reared and wild juvenile Diadema on artificial reefs around Saba, Caribbean Netherlands. To assess the retention of Diadema over time, we conducted diver surveys and used underwater photo time lapse during daylight. Retention of uncaged lab-reared and wild Diadema decreased steadily with less than 30% surviving after 10 days. In total, 138 predator-prey interactions were recorded, of which 99% involved the queen triggerfish Balistes vetula, although other potential predators were present in the area. None of the recorded predator-prey interactions was successful, which suggests that artificial reefs with incorporated shelters may be suitable for juveniles as daytime refuge. However, Diadema that were more often attacked during the day were more likely to be absent the next morning. Because queen triggerfish often visited the experimental site in the first or last hour of daylight, it could be that they were more successful in their attacks when it was too dark to see anything on the photos and when Diadema came out to feed or to look for better shelter opportunities. If Diadema migrated off the artificial reef, they were probably predated during the process, because no Diadema were found on surrounding reefs. Wild Diadema were attacked significantly more often than lab-reared Diadema, possibly because the wild urchins were larger, but this did not significantly affect retention. Future restocking should be performed on natural or artificial reefs with deeper shelters, so Diadema can retract farther into their crevice, and should include night-time monitoring to identify the remaining unknown factors that cause low retention, including migration and nocturnal predation. This knowledge is urgently needed to coral reef managers so they can increase Diadema restocking success by selecting reefs with a lower predator density, protect urchins during an acclimatization period and/or conduct temporary predator control measures.

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	Figure 1. Location of Saba in the Caribbean.Experiments were performed at Big Rock Market (white dot) and wild Diadema antillarum were collected at Diadema City (black dot). Map created with ArcMap 10.8 using data from Esri, HERE, and Garmin.
	Figure 2. Moreef artificial reef module.Front view of Modular Restoration Reef (Moreef) module with incorporated shelters.
	Figure 3. Experimental setup.(A) Schematic overview of the experimental setup. Shown are artificial reefs on bare sand with restocked lab-reared Diadema antillarum (white circle) and artificial reefs with restocked wild Diadema (grey circle) of which two reefs on each outer end were caged (squared circle). Cameras (square box) were installed to monitor the artificial reefs. Distances in between the artificial reefs are indicated with arrows. (B) Photo of the experimental setup.
	Figure 4. Codes to categorize actions of predators and Diadema antillarum.Codes used in this study to categorize actions of predators (circled) and Diadema antillarum: (A) Code 1: Diadema predator outside a 50 cm virtual sphere around the artificial reef. (B) Code 2: Diadema predator less than 50 cm from artificial reef, but less than 10 cm from a shelter entrance. (C) Code 3: Diadema predator less than 10 cm from a shelter entrance. (D) Code 4: Interaction between Diadema predator and Diadema on the artificial reef. (E) Code 5: Diadema predator within a 50 cm virtual cylinder around Diadema outside shelter. (F) Code 6: Diadema predator attacks Diadema outside shelter. Code 7 (Diadema predator feeds on Diadema outside shelter.), Code 8 (Diadema outside shelter and within 50 cm of artificial reef.) and Code 9 (Diadema outside shelter and more than 50 cm from the artificial reef. No Diadema predator present.) are not shown. Pictures were only attributed to the highest level code describing the action.
	Figure 5. Diadema antillarum retention.Mean (±SE) Diadema antillarum retention on artificial reefs over the 10-day experiment. Artificial reefs had the following treatments: caged wild Diadema (red circles, dot-dash line, n = 2), caged lab-reared Diadema (light pink circles, dotted line, n = 2), uncaged wild Diadema (light blue circles, solid line, n = 4) and uncaged lab-reared Diadema (dark blue circles, dashed line, n = 4).

References [+] :

Bellwood, Confronting the coral reef crisis. 2004, Pubmed