Offspring size resolves a latitudinal population growth rate paradox in rays and skates

Abstract

As animal populations continue to decline, understanding global patterns of life histories will improve our predictions of species vulnerability and extinction risk to prioritise management. Sharks and rays are threatened with extinction due to overfishing, particularly in the tropics and sub-tropics. Metabolic theory suggests that warm-water species and populations will have a higher maximum intrinsic rate of population increase (rmax) and therefore will be less intrinsically sensitive to exploitation. However, recent empirical work has highlighted a paradox and shown that warm, shallow-water tropical rays have lowerrmaxthan cold, deep-water temperate skates. We test whether the different reproductive strategies of live-bearing rays and egg-laying skates explain this observed paradox by comparing variation inrmaxwith adult body mass, offspring size, temperature, and depth across 85 species of rays and skates. Our results show that the large relative offspring size of warm, shallow-water tropical rays better explains their greater intrinsic sensitivity (lowerrmax) compared to cold, deep-water temperate skates. This is consistent with life history theory that suggestsrmaxis greater in species with smaller relative offspring size. We hypothesise that the larger relative offspring size may be driven by greater predation risk in the tropics and may help explain global patterns of intrinsic sensitivity to overexploitation.