Diet and condition of mesopredators on coral reefs in relation to shark abundance.

Reef sharks may influence the foraging behaviour of mesopredatory teleosts on coral reefs via both risk effects and competitive exclusion. We used a "natural experiment" to test the hypothesis that the loss of sharks on coral reefs can influence the diet and body condition of mesopredatory fishes by comparing two remote, atoll-like reef systems, the Rowley Shoals and the Scott Reefs, in northwestern Australia. The Rowley Shoals are a marine reserve where sharks are abundant, whereas at the Scott Reefs numbers of sharks have been reduced by centuries of targeted fishing. On reefs where sharks were rare, the gut contents of five species of mesopredatory teleosts largely contained fish while on reefs with abundant sharks, the same mesopredatory species consumed a larger proportion of benthic invertebrates. These measures of diet were correlated with changes in body condition, such that the condition of mesopredatory teleosts was significantly poorer on reefs with higher shark abundance. Condition was defined as body weight, height and width for a given length and also estimated via several indices of condition. Due to the nature of natural experiments, alternative explanations cannot be discounted. However, the results were consistent with the hypothesis that loss of sharks may influence the diet and condition of mesopredators and by association, their fecundity and trophic role. Regardless of the mechanism (risk effects, competitive release, or other), our findings suggest that overfishing of sharks has the potential to trigger trophic cascades on coral reefs and that further declines in shark populations globally should be prevented to protect ecosystem health

Regressions of Log length (Log L) on Log weight (Log W), Length (L) on height (H) and Length (L) on width (Wi) for <i>L</i>. <i>gibbus</i>, <i>L</i>. <i>decussatus</i>, <i>L</i>. <i>kasmira</i>, <i>M</i>. <i>grandoculis</i> and <i>L</i>. <i>bohar</i> at the Scott Reefs (grey) and the Rowley Shoals (black).

<p>Differences in the regression lines between species might be obscured due to variation in the range of the axes. See <b>Table C in</b> <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0165113#pone.0165113.s001" target="_blank">S1 File</a> for statistics.</p

Sample size (n) with % females in parentheses.

<p>Also shown are mean (range) values for length of five mesopredatory fishes at the Scott Reefs and the Rowley Shoals. The p-values for results of a 2 x 2 contingency table with no fixed margins (% females) and a two-sample t-test assuming unequal (<i>L</i>. <i>decussatus</i>, <i>L</i>. <i>kasmira</i> and <i>L</i>. <i>bohar</i>) and equal (<i>L</i>. <i>gibbus</i>, <i>M</i>. <i>grandoculis</i>) variances (total length) are shown.</p

Number of prey items and their percentage contribution (in parentheses) in the guts of <i>L</i>. <i>gibbus</i>, <i>L</i>. <i>decussatus</i> and <i>L</i>. <i>bohar</i> at the Scott Reefs and the Rowley Shoals.

<p>Sample sizes for each species and reef system are shown in square brackets.</p

Plot of regression of mean number (n) of fish per site-location (abundance, A) at the Rowley Shoals (R) against the Scott Reefs (S) for 16 species of mesopredatory teleost where A_S = 1.79×A_R+0.38 (n = 16, <i>p</i> = 1.7×10–8, R² = 0.90, α = 0.05).

<p>Abundance data were sourced from the Australian Institute of Marine Science Long-Term Monitoring Program. The plot includes the five study species (black) and eleven non-focal species (white). The dashed line shows a 1:1 relationship between abundances at each location. Abundance was averaged across 3 sites at each location (Clerke, Imperieuse and Mermaid at the Rowley Shoals and North Scott, South Scott and Seringapatam at the Scott Reefs). The inset figure shows logged values for mean fish abundance per site-location. See <b>Table A in</b> <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0165113#pone.0165113.s001" target="_blank">S1 File</a> and the AIMS website for details:<a href="http://www.aims.gov.au/docs/research/monitoring/monitoring.htm" target="_blank">http://www.aims.gov.au/docs/research/monitoring/monitoring.htm</a>.</p

Map of study sites in northwestern Australia.

<p>The map depicts the Scott Reefs, where sharks are targeted by Indonesian fishermen, and the Rowley Shoals, where sharks are protected.</p

Prey items in guts of three mesopredatory teleost species by origin.

<p>Plot of the mean percentage of prey items from the water column (±SE) in the guts at the Scott Reefs (pale grey) and the Rowley Shoals (dark grey) for three species of mesopredator (2x2 Chi-Square contingency table with no fixed margin: <i>L</i>. <i>bohar</i>, χ² = 30.9, <i>p</i><0.001, <i>L</i>. <i>decussatus</i>, χ² = 11.9, <i>p</i><0.001 and <i>L</i>. <i>gibbus</i>, χ² = 10.7, 0.001<<i>p</i><0.005).</p