Ecological interactions between 19 shark species in the Indian Ocean

Apex predators such as sharks are a critical component of ocean ecosystems. Yet the ecosystem consequences of shark declines remain poorly understood, primarily because of a lack of population and community baselines. The Indian Ocean is especially data-poor in ecological data, and even moreso in historical data. We utilized a longline survey dataset from 1966 through 1989 that spanned the majority of the Indian Ocean and recorded 19 shark species. This time period corresponds to the start of large-scale industrial fishing in the region. Trends across the species were highly variable; life history and fishing pressure metrics were not able to explain differences in responses between species, suggesting that changes in ecological interactions such as competition and predation had a prevalent role historically. To further explore ecological interactions between the species, we conducted a literature review of the study species’ diets with a focus on intra-guild predation. We constructed an interaction web to identify keystone species. Several species were neither predator nor prey of other sharks, suggesting that competition may be the more salient relationship to other sharks. Overall, species with broader habitat preferences and smaller individuals are now a larger part of the pelagic shark community, whereas open-ocean species have declined. These results suggest that industrial fishing restructured shark communities and diminished the top-down control of sharks in pelagic ecosystems.