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Marine Ecology Progress Series

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MEPS 493:165-177 (2013)  -  DOI:

Fish and sea urchin grazing opens settlement space equally but urchins reduce survival of coral recruits

Jennifer K. O’Leary1,3,*, Donald Potts1, Kathryn M. Schoenrock1, Timothy R. McClahanan

1Department of Ecology and Evolutionary Biology, University of California, Santa Cruz, California 95064, USA
2Marine Programs, Wildlife Conservation Society, Bronx, New York 10460, USA
3Present address: Hopkins Marine Station, Stanford University, Pacific Grove, California 93950, USA

ABSTRACT: Grazing fishes and invertebrates are influential in marine ecosystems because they open space for benthic recruits, alter post-settlement recruit survival, and can often determine benthic community composition. On tropical reefs, grazing fishes and sea urchins can play key roles in limiting growth of fleshy macro-algae, thereby facilitating coral recruitment and maintaining coral-dominated communities. However, as grazer abundance increases, grazer influence on corals (or other settlers) may shift from being positive and indirect by reducing space competitors, to being negative and direct by damage or removal of coral recruits. Fishing can alter both the abundances and types of dominant grazers, with potential cascading effects on coral recruitment and subsequent community organization. In Kenya, sea urchins dominate grazing on heavily fished reefs, while herbivorous and omnivorous fishes dominate grazing within fisheries closures (marine protected areas). We used reefs under these 2 fishery management systems to investigate the effects of fish versus sea urchin grazing on the availability of settlement substrate for corals, subsequent coral settlement, and mortality of coral recruits. Fish and sea urchin grazers were equally effective at clearing benthic space for coral settlement. However, grazing associated with high densities of sea urchins on fished reefs removed many coral recruits after settlement. In contrast, fish grazing within fisheries closures enhanced coral survival compared to non-grazed treatments. We conclude that the effects of reduced abundance of grazing fishes (on available space for coral settlement) may be initially offset by increased sea urchin grazing, but that higher urchin abundances may ultimately reduce coral cover by their negative influence on post-settlement survival.

KEY WORDS: Coralline algae · Diadema · Herbivory · Predation · Phase shifts · Trophic cascades · Marine protected areas · Western Indian Ocean · Kenya

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Cite this article as: O’Leary JK, Potts D, Schoenrock KM, McClahanan TR (2013) Fish and sea urchin grazing opens settlement space equally but urchins reduce survival of coral recruits. Mar Ecol Prog Ser 493:165-177.

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