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Depth-dependent indicators of algal turf herbivory throughout the Main Hawaiian Islands

Coral Reefs volume 40, pages 1397–1408 (2021)Cite this article

Abstract

Herbivorous fish are key to maintaining a balance between coral and algae on reefs, where reefs with greater herbivore biomass often show lower algal cover. For reefs worldwide, algal turf cover is expanding and is increasingly used as an indicator of disturbance. Water depth affects reef fish composition; thus, it may be expected that herbivory could also differ by depth. We examined relationships between algal turf cover and biomass (g m−2), density (# m−2) and size (cm) of herbivore groups (grazers, browsers and scrapers) across shallow (< 6 m), mid (6–18 m) and deep (18–30 m) coral reefs in the Main Hawaiian Islands. We find that across all depth classes, algal turf cover decreased with increasing grazer and scraper density, with steeper relationships observed at mid and deep reefs than in shallow reefs. In contrast, algal turf cover slightly increased with increasing grazer and browser biomass at deep reefs. Considering fish size, algal turf cover increased with larger grazer and scrapers at mid and deep reefs. The results indicate that herbivorous fish density, rather than biomass, is a better indicator of reductions in algal turf cover and resulting coral-algal balance on Hawaiian reefs, where smaller fish exert greater top-down control on cover than larger fish. Despite significant differences in herbivorous fish compositions, length-frequency distributions and fishing intensities across depth, algal turf cover remains similar across depths. Increases in fishing would have a disproportionately negative impact in deep than shallow reefs due to a lower overall fish density, where grazing functions in deep reefs are maintained by significantly fewer and smaller grazers and browsers, and larger scrapers, than in shallow reefs. Developing an understanding of patterns of algal turf herbivory by depth is important to understanding the spatial scale at which herbivory and regime shifts operate.

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Data availability

Data are publicly available at https://doi.org/10.7289/v59c6vr5.

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Acknowledgements

We thank the National Oceanic and Atmospheric Administration’s (NOAA) Coral Reef Ecosystem Program (CREP) for compiling and sharing fish data for the Main Hawaiian Islands. We also thank Rachel Carlson for helpful discussions and comments on the results and manuscript. This study was supported by the Dorrance Family Foundation. On behalf of all authors, the corresponding author states that there is no conflict of interest.

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  1. Center for Global Discovery and Conservation Science, Arizona State University,, Hilo, HI, USA

    Shawna A. Foo & Gregory P. Asner

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  1. Shawna A. Foo
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Correspondence to Shawna A. Foo.

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Foo, S.A., Asner, G.P. Depth-dependent indicators of algal turf herbivory throughout the Main Hawaiian Islands. Coral Reefs 40, 1397–1408 (2021). https://doi.org/10.1007/s00338-021-02162-2

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Keywords

  • Herbivore
  • Coral reefs
  • Hawaii
  • Phase shifts
  • Algal turf
  • Size spectra