Comparison of Coral Growth Rates among Species, Growth Forms and Habitats on Unguja Island, Zanzibar, Tanzania

Abstract

Recovery of degraded reefs depends on how quickly juvenile corals grow and how well they survive, both of which vary among growth forms. In Tanzania, species and habitat- specific performance information remains limited, constraining evidence-based restoration. We quantified juvenile growth and survival across coral species and growth forms: Echinopora lamellosa (foliose), Isopora palifera (sub-massive), Acropora tenuis (corymbose), Echinopora hirsutissima (encrusting), and Porites lutea (massive) on Chumbe Reef (Unguja, Zanzibar), and identified habitat locations and species that best support short-term coral growth and long-term reef survival. Fifty-nine colonies representing five species and five growth forms were monitored at ~6-month intervals from October 2010 to May 2012, across three reef habitats (reef flat, reef crest, and reef slope). We measured Planar tissue area to estimate growth (cm² per 6 months); survival was recorded at each interval. We summarized growth and survival by species, growth form, and habitat; compared performance among habitats; and described inter-individual variability. Mean planar growth ranged from 8.1 cm²·6 mo⁻¹ (P. lutea) to 155.7 cm²·6 mo⁻¹ (E. lamellosa). Overall survival was 68%, with peak mortality in the first interval (Oct 2010-Apr 2011). Explicit growth survival trade-offs emerged in which foliose and corymbose taxa (E. lamellosa, A. tenuis) grew rapidly but survived poorly, whereas submassive, encrusting, and massive taxa grew more slowly yet survived at higher rates. Inter-individual variability in growth was high for E. lamellosa and low for P. lutea. Both growth and survival were highest on the reef slope compared with the crest and flat. Collectively, the results suggest that restoration designs should prioritize slope habitats for nurseries. A mixed taxa strategy can balance goals by deploying fast-growing foliose and corymbose corals to accelerate short-term cover, while integrating submassive, encrusting, and massive forms to secure steadier, long-term recovery.