Effect of macroalgal reduction on coral-reef fish in the Watamu Marine National Park, Kenya

Abstract

The paper extends by five months the record of effects of reduction of leathery-macrophyte abundance in plots in a coral reef outcrop in the Watamu National Park, a site that had eliminated fishing for >20 years. After one year, leathery macrophytes had not recovered, but articulated green calcareous algae (Halimeda) had, and replaced leathery macrophytes as the dominant cover on the experimental plots. Of the 56 fish species studied, 20 were influenced by the algal reduction. There were increased numbers of individuals and species of herbivorous surgeonfish (4 species) and parrotfish (5 species), and increased population densities of invertebrate-eating fishes including angelfish (2 species), butterflyfish, emperors and snappers (not identified to species), wrasses (2 species), and a triggerfish. Negative effects were restricted to three damselfishes and one wrasse. Parrotfish, snappers and the total fish abundance showed a significant increase in size and biomass in the algal reduction plots over the year. No differences were found for macrophyte-feeding parrotfish (Calotomus carolinus). Increased herbivory was the likely cause of the slowness of the recovery of leathery macrophytes and the switch in dominance towards Halimeda.