Offspring size resolves a population growth paradox in rays and skates

The maximum intrinsic population growth rate, rmax, is a key determinant of the limits for sustainable fishing and is increasingly used in risk assessments. Metabolic theory suggests that rmax scales with adult body size, temperature (and hence depth) such that smaller-bodied species and those in warmer, shallower waters have greater rmax and, therefore, will be less sensitive to overexploitation. However, warm shallow-water tropical rays have lower rmax than cold deep-water temperate skates contra to the metabolic expectation. To resolve this paradox, we build from recent advances that suggest that offspring size may be key to understanding rmax. Specifically, we examine how rmax is related to adult size, offspring size, temperature, and depth across 85 ray and skate species. Our results show that offspring size mediates relationships between rmax, adult body size, temperature, and depth. Indeed, tropical rays had, on average, larger offspring and lower rmax compared to the temperate skates, despite living in warmer, shallower waters. Thus, despite the expectation from theory that tropical species should have faster life histories compared to temperate species, our result explains why tropical rays are actually less resilient. It remains unclear as to why tropical rays have such large offspring but we speculate that this is due to greater predation risk in shallow tropical waters driving the additional maternal investment in offspring size via the evolution of viviparity and matrotrophy. Our work highlights the complex relationships among life histories and the environment and may help explain global biogeographic patterns of intrinsic sensitivity to overexploitation.