Full protection secures greater ecological benefits for coral reefs than partial regulation in an urban Kenyan MPA

Marine Protected Areas (MPAs) are essential conservation tools, yet their ecological efficacy remains understudied in densely populated urban settings. The unique juxtaposition of full protection (no-take marine park) versus partial protection (regulated fishing reserve) within the Mombasa Marine Protected Area, adjacent to a major urban center, serves as an ideal model system to test whether full protection outperforms partial regulation. We evaluated reef conservation outcomes by comparing three key ecosystem indicators: fish assemblage structure (density and biomass), benthic cover, and macro-invertebrate abundance.

Our results demonstrate that the fully protected area supports a three-fold higher biomass of piscivores and omnivores, and a four-fold higher herbivore biomass compared to the adjacent partially protected area. Crucially, critical macro-herbivore functional guilds, specifically browsers, grazers, scrapers, and excavators were almost exclusively restricted to the fully protected area, providing the functional complementarity required to suppress macroalgae. Consequently, hard coral cover was significantly higher under full protection (27.0%) than under partial protection (14.4%). Conversely, the partially protected area exhibited a five-fold higher density of bioeroding sea urchins (Echinometra spp.), signaling a top-down trophic cascade driven by predatory and competitive release.

High macroalgal cover and urchin hyper-densities within the partially protected reserve indicate that partial regulation fails to arrest ecological degradation. We conclude that while full protection must remain the cornerstone of local marine conservation, alternative "functional management" frameworks such as targeted herbivore protections must be integrated into partial-use zones to secure coral reef resilience against the synergistic threats of escalating urban and global climate pressures in the Western Indian Ocean.