Acute effects of removing large fish from a near-pristine coral reef

Abstract

Large animals are severely depleted in many ecosystems, yet we are only beginning to understand the ecological implications of their loss. To empirically measure the short-term effects of removing large animals from an ocean ecosystem, we used exclosures to remove large fish from a near-pristine coral reef at Palmyra Atoll, Central Pacific Ocean. We identified a range of effects that followed from the removal of these large fish. These effects were revealed within weeks of their removal. Removing large fish (1) altered the behavior of prey fish; (2) reduced rates of herbivory on certain species of reef algae; (3) had both direct positive (reduced mortality of coral recruits) and indirect negative (through reduced grazing pressure on competitive algae) impacts on recruiting corals; and (4) tended to decrease abundances of small mobile benthic invertebrates. Results of this kind help advance our understanding of the ecological importance of large animals in ecosystems.

Fig. 1

An example (a) partial exclosure. Responses of b fish biomass and c density to exclosure treatment (mean ± SE). Effects of treatment on d fish recruit density from the three families with the highest rates of recruitment (mean ± SE). All data is from SCUBA surveys. Asterisks mark the treatments that significantly differ from open treatments. Large fish were much reduced in full plots indicating that total exclosures functioned properly. Removal of large fish from full exclosures did not affect the biomass or density of fish ≤10 cm total length (TL), but caused marginal, although non-significant, increases in the recruitment of fish from families Scaridae and Acanthuridae

Fig. 2

Distance that the small planktivorous fish Chromis margaritifer hovered above refuge points (mean ± SE). Asterisks mark the treatments that significantly differ from open treatments. These prey fish foraged the farthest from safety in full exclosures where large predators had been removed

Fig. 3

Percent cover of benthic algae on recruitment tiles at the end of the experiment (mean ± SE). CCA: live/dead crustose coralline algae; non-CCA: all other benthic growth (excluding corals). Asterisks mark the treatments that significantly differ from open treatments. The percent cover of non-CCA was significantly higher in full exclosures than in open plots, but no differences were evident between treatments for better defended CCA

Fig. 4

Differences in herbivory on macroalgae (a) Avrainvillea amadelpha, (b) Dictyosphaeria cavernosa, (c) Caulerpa serrulata, (d) Halimeda taenicola, and (e) Halimeda opuntia (mean grams consumed day⁻¹ ± SE). Asterisks mark treatments that significantly differ from open treatments. Herbivory on A. amadelpha and D. cavernosa was significantly and near-significantly, respectively, reduced in full exclosures where large herbivorous fish were excluded (relative to open treatments)

Fig. 5

Total abundance of metamorphosed coral recruits (new settlers + surviving recruits) on tiles (mean/tile ± SE). Asterisks mark the treatments that significantly differ from open treatments. In month one and two, rates of recruitment were significantly higher inside full exclosures where recruits were protected from large herbivorous fish, but this difference diminished by month four