Figure 4

Predation risk modelling.

(a) Relationship between upward detection distance and target depth for two black target widths. (b) Relationship between black target width and upward detection distance for three mesopelagic depths (log-log plot). (c) Log-log plot of PAP and interpectoral distance. For clarity, regression line (log (PAP) = −0.74 log (interpectoral distance) + 0.583, r² = 0.21, P = 0.0044, n = 37) is not shown. Axis on the right indicates occurence depth of sharks in oceanic waters calculated from PAP using the relationship from Fig. 3c. Blue dashed lines represent isometric predator detection distance lines (standardized by interpectoral distance) according to calculated occurrence depth and interpectoral distance. These lines provide a relative idea of the predation risk encountered by the sharks (the higher the line value, the higher the relative risk). The one interpectoral distance isometric line, which occurs at ~775 m (i.e. PAP ~10%) corresponds to the lower limit for efficient counterillumination. Note that the minimum convex polygon encompassing all species with lateral photophore markings (black symbols) only includes a single species with no markings (grey symbols), Isistius brasiliensis. This polygon is situated in a low-risk area (detection distance inferior to 1000 interpectoral distance) and contains species with a minimum PAP of ~5%. Square symbols indicate species for which mass captures by natural predators have been recorded. Unless determined otherwise, all dots represent species of the genus Etmopterus. Ac, Aculeola; Ce*, C. ritteri; Et* E. princeps; He, Heteroscymnoides. Other abbreviations same as in Fig. 3.