Fig. 4: Evidence for the effect of reef sharks on the behaviour of teleost mesopredators at whole-reef scales.

Lester et al.¹⁷ surveyed fish and shark communities using stereo-baited underwater video systems (stereo-BRUVS⁴²) at the Rowley Shoals and the Scott Reefs, two adjacent sets of large, atoll-like reefs on the edge of the continental shelf off the coast of northern Western Australia. Sharks are abundant at the Rowley Shoals, which have been a no-take marine protected areas for over 30 years, whereas they are very rare at the Scott Reefs, which are open to shark fishing by Indonesian fishers using traditional harvest techniques (longlines). BRUVS were used to compare behaviours of teleost mesopredators (lutjanids, lethrinids serranids) at each of these reef systems. Lester et al.¹⁷ showed that common species spent more time in the water column at the Scott Reefs where sharks were rare (A, B), a result consistent with experimental studies of responses of these species to predation threat (see Fig. 3). Bargraphs show average percentage time spent in water column +/− S.E. for Lutjanus bohar (A) and L. gibbus (B). Points show raw data. For L. bohar, plot also shows relationship between mean relief and the percentage of time spent in midwater where the solid line indicates the estimated smoothing curve and the shaded area indicates the S.E. of the estimate. For L. gibbus, the plot also shows relationship between number of bites made by sharks of the bait bag of the BRUVS and the percentage of time spent in midwater by this species, where the solid line indicates the estimated smoothing curve and the shaded area indicates the S.E. of the estimate. The response to predation threat was mediated by both environmental context and behaviour of reef sharks; the time spent in the water column by Lutjanus bohar increased when the benthos was complex and there was a large amount of cover (A), whereas L. gibbus spent less time in midwater once sharks began actively biting on the bait bag of the camera systems (B). Data sets replotted from Lester et al.¹⁷. Image artwork credit: Dean Tysdale.