Prolonged instability in blast-fished rubble beds impedes coral recovery

Blast fishing has severely degraded Indonesia's coral reefs, reducing biodiversity and leaving rubble beds. In Bunaken National Park (BNP), it peaked in the 1970s and declined after the park's 1991 establishment, yet extensive rubble remains. Unstable rubble hinders coral recruitment and reef recovery. We surveyed four large rubble areas in BNP, including one previously restored site, to assess their potential for natural regeneration. At each site, we quantified rubble bed and fragment features (i.e., slope angle, bed thickness, fragment morphometrics and stability, coral recruitment) and characterized the rubble binding community. We then analysed which biophysical factors drive rubble stability, binding prevalence, and coral recruit abundance. Despite differences among sites in other factors, the rubble stability likeliood was consistently low (<10 %), even at the site where restoration was attempted. Rubble binding likelihoods ranged from 7 to 25 %, with crustose coralline algae being the dominant binding agent across all sites. All rubble was relatively small (∼10 cm), but longer rubble fragments were associated with greater stability, binding, and coral recruit abundance. Thicker rubble beds also drove increased binding, while steeper slopes corresponded with lower recruit abundance. Overall, natural coral recovery remains limited (0.6-2.6 % coral cover) despite the passage of 30 years since blast fishing ceased, and the restoration attempt. Our findings underscore the importance of rubble morphometrics and the biophysical environment in shaping reef recovery potential. Substrate stabilization appears essential to support coral recruitment. Crucially though, restoration interventions must be securely anchored and consistently maintained and monitored to be effective over time.