Environmental and Protection Effects of Shark-Companion Associations Across Three Ocean Basins

Ecological associations such as mutualism, commensalism and parasitism strongly influence animal fitness and ecosystem structure, yet they remain poorly understood in the open ocean. Ecological interdependence means that the loss of one species can trigger co-declines and accelerate biodiversity loss. Large, mobile animals often host smaller 'companion' species that gain benefits such as protection from predation, foraging opportunities, or host-mediated transport through close association. We quantified companion-host associations among seven shark species present across three ocean basins. This is the first large-scale assessment to investigate how environmental conditions and marine protected-area (MPA) status shape these associations. Data on species identity, abundance and length were obtained from a curated database derived from midwater Baited Remote Underwater Video Systems (BRUVS). Almost half of individual sharks hosted companions, with marked variation among species. Companions were most common on Australian blacktip sharks (Carcharhinus tilstoni) and least common on scalloped hammerheads (Sphyrna lewini). Companion presence was best predicted by sea surface temperature, salinity, wind speed and distance to shore, while companion abundance was predicted by primary productivity, wind speed and salinity. Several of these environmental factors are highly sensitive to climate change, potentially disrupting companion-host associations. Partially protected areas (IUCN IV-VI) had a higher probability of companion occurrence than areas outside partial protection, whereas highly protected areas (IUCN I-II) supported greater companion abundance when companions were present. Declining host populations and changing environmental conditions may disrupt companion associations, removing the interspecies linkages and increasing the risk of co-decline of dependent species. Effective protection of these associations will support the persistence of biodiversity and ecological processes in the open ocean.