Offspring Size Resolves a Population Growth Paradox in Rays and Skates

ABSTRACT The maximum intrinsic population growth rate, r max , is a key determinant of sustainable fishing limits and is increasingly used in risk assessments. We previously showed how the r max of rays and skates (subclass Batoidea) scales with adult body size, temperature (and hence depth) such that smaller‐bodied species and those in warmer, shallower waters have greater r max and, therefore, will be less sensitive to overexploitation. Paradoxically however, warm shallow‐water tropical rays have lower r max than cold deepwater temperate skates, contra to the expectation from metabolic scaling theory. To resolve this paradox, we examine how r max is related to adult and offspring size (while accounting for temperature and depth) across 85 ray and skate species. Offspring size mediates relationships between r max , adult body size, temperature and depth. Despite inhabiting warmer, shallower waters, tropical rays generally have larger offspring and lower r max compared to temperate skates. Our result explains why tropical rays are less resilient to overfishing despite expectations from metabolic theory that tropical species should have faster life histories and therefore higher r max and greater resilience than temperate species. Although the drivers of large offspring size in tropical rays remain uncertain, we use basic models of temperature‐ and size‐dependent predation to hypothesise that this is due to greater predation risk in shallow tropical waters selecting for increased maternal investment in offspring size via evolution in 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 exploitation.