Assessing mechanisms of intrinsic vulnerability in sharks occupying different marine environments

Abstract The status of many shark species worldwide is concerning due to a combination of their life-history traits and widespread exposure to threats, especially fishing. However, the intrinsic vulnerability of species differ given that life-history traits vary with environmental conditions. Here, we aim to assess the links between sharks’ reproductive strategy, maximum depth recorded, and environment occupied (shelf, deepwater and pelagic) to investigate the varying levels of intrinsic vulnerability of different shark species. We find that shelf habitats are dominated by placental viviparous species that rarely occupy other environments. In contrast, oviparous species are found in both shelf and deep-sea habitats but are absent from pelagic environments due to a lack of substrate for egg deposition. Importantly, oviparous species reveal a significantly lower maximum size, size at birth, size at maturity as well as continuous reproduction, all of which are indicators of relatively high productivity and thus lower intrinsic vulnerability. In comparison, placental viviparous species reveal a larger maximum size, size at birth, size at maturity as well as annual or biennial reproduction. These life-history traits are indicative of lower productivity and thus higher intrinsic vulnerability of this dominant species group in shelf habitats where fishing mortality is known to be high. In consequence, placental viviparous species reveal higher proportion of species listed as threatened (VU, EN, CR) in the International Union for Conservation of Nature (IUCN) Red List compared to oviparous species. However, threats to deep-sea oviparous as well as to aplacental and lecithotrophic viviparous species (which are also abundant in deeper waters) are likely to increase in future due to the expansion of fisheries into open ocean and deeper waters. Although key life-history traits of deep-sea oviparous shark species are indicative of relatively higher productivity, these species have evolved under lower levels of primary productivity and predation risk characteristic of deeper waters. Slower growth and low natural mortality in deep sea oviparous species mean that the risk of population depletion is still high when mortality levels are elevated. Therefore, these findings underscore the importance of adopting precautionary and proactive strategies for sharks inhabiting deep-water environments.