Behaviour of mesopredatory coral reef fishes in response to threats from sharks and humans

Abstract

Both sharks and humans present a potentially lethal threat to mesopredatory fishes in coral reef systems, with implications for both population dynamics and the role of mesopredatory fishes in reef ecosystems. This study quantifies the antipredator behaviours mesopredatory fishes exhibit towards the presence of large coral reef carnivores and compares these behavioural responses to those elicited by the presence of snorkelers. Here, we used snorkelers and animated life-size models of the blacktip reef shark (Carcharhinus melanopterus) to simulate potential predatory threats to mesopredatory reef fishes (lethrinids, lutjanids, haemulids and serranids). The responses of these reef fishes to the models and the snorkelers were compared to those generated by three non-threatening controls (life-size models of a green turtle [Chelonia mydas], a PVC-pipe [an object control] and a Perspex shape [a second object control]). A Remote Underwater Stereo-Video System (Stereo-RUV) recorded the approach of the different treatments and controls and allowed accurate measurement of Flight Initiation Distance (FID) and categorization of the type of flight response by fishes. We found that mesopredatory reef fishes had greater FIDs in response to the approach of threatening models (1402 ± 402-1533 ± 171 mm; mean ± SE) compared to the controls (706 ± 151-896 ± 8963 mm). There was no significant difference in FID of mesopredatory fishes between the shark model and the snorkeler, suggesting that these treatments provoked similar levels of predator avoidance behaviour. This has implications for researchers monitoring behaviour in situ or using underwater census as a technique to estimate the abundance of reef fishes. Our study suggests that, irrespective of the degree to which sharks actually consume these mesopredatory reef fishes, they still elicit a predictable and consistent antipredator response that has the potential to create risk effects.

Figure 1

(a) Sample sites () located on the fringing reefs surrounding Lizard Island, located in the northern section of the Great Barrier Reef, Australia (map courtesy of Ooid Scientific). Photograph of a (b) blacktip reef shark model, Carcharhinus melanopterus (Simpson S.D.), (c) photograph of a snorkeler (180 cm TL, Asunsolor-Rivera A.) (d) photograph of a green sea turtle model (non-threatening model, Lester E.), (e) PVC pipe (control object, courtesy of Ooid Scientific) and (f) transparent Perspex (negative control object, courtesy of Ooid Scientific). (g) Shark model attached to pulley system and remote underwater video system (RUVS) placed near the edge of the coral bommie. Large arrow represents the shark model moving towards the coral bommie. Small arrows represent the traveling direction of the line in the pulley system (g courtesy of Ooid Scientific).

Figure 2

Type of escape response in proportion to treatment. Three different types of escape responses were identified: Flight, C-turn and Hide. The No response reaction indicates the fish did not change position as the treatment approached.

Figure 3

Variable importance scores from full subset GAMM analyses predicting flight initiation distance (FID) and speed of flight according to prey body length, treatment, treatment size, vertical height from benthos (VHB), genus of mesopredator (Genus) and number of fish (individual or school). X indicates the variables within the most parsimonious model. Grey indicates the variable was not included in the model.

Figure 4

(a) Average (± SE) flight initiation distance (FID) for each treatment. Points indicate distribution of raw data. The results of pairwise comparisons are indicated by an alphabetic character. (b) Flight initiation distance (FID) relative to the body length of mesopredatory fishes. The solid line indicates the estimated smoothing curve and the shaded area indicates the ± SE of the estimate. Points show distribution of raw data.

Figure 5

Average speed of flight (± SE) for each treatment. Points show distribution of raw data. The results of pairwise comparisons are indicated by an alphabetic character.