Mesopelagic Fish and Cephalopods in the Diet of Rorquals ( <i>Balaenoptera</i> spp.) and Sperm Whales ( <i>Physeter macrocephalus</i> ) Around the Azores Using Fecal and Environmental <scp>DNA</scp>

ABSTRACT Rorquals and sperm whales are top predators of complex open ocean food webs and, although mesopelagic fish and cephalopods are predated upon by these cetaceans, the contribution of these prey groups to the whales' diet is not fully understood. Here, we aimed to better describe the consumption of mesopelagic prey by identifying targeted species at mid‐latitudes. We analyzed the prey availability and predator preferences by comparing the vertical distribution and abundance of fish and cephalopod species in the water column with the prey items found in fecal samples through DNA metabarcoding. We found that rorquals consumed mesopelagic fish that perform diel vertical migrations (DVMs), such as myctophids. These species were found at depths that matched the deep foraging behavior during daytime (0–200 m) and shallow foraging behavior during night (0–50 m), confirming that rorquals rely on the DVM to feed. Also, although a high diversity of cephalopods was found across the water column, the fecal content of sperm whales was mainly composed of Histioteuthis bonnellii , which was abundant between 600 and 1200 m and matches the diving patterns described for this species. Here, we present a comprehensive analysis of the diets of rorquals and sperm whales, expanding our understanding of open ocean trophic ecology to promote effective cetacean conservation.