Contribution of mesopelagic fish and cephalopods to the diet of rorquals (Balaenoptera spp) and sperm whales (Physeter macrocephalus) beyond their feeding grounds

Cetacean conservation requires ecosystem-scale management with special focus on food webs. Rorquals and sperm whales are top predators of complex open ocean food webs and, although mesopelagic fish and cephalopods are predated by these cetaceans, their contribution to their diets is not fully understood. Here, we aimed to better describe the consumption of mesopelagic fish and cephalopods by identifying preferred species consumed by rorquals and sperm whales at mid-latitudes. To do so, we combined the fish and cephalopod community composition inferred from whale faecal and marine environmental DNA samples. We analysed 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 faecal samples of rorquals and sperm whales. We found that rorqual consumed mesopelagic fish that perform diel vertical migrations (DVM) such as myctophids. These species were found in depths that matched the deep foraging behaviours during daytime and shallow foraging behaviours during night, confirming that rorquals rely on the DVM to feed at these latitudes. Also, although a high diversity of cephalopods was found across the water column, the faecal content of sperm whales was mainly composed by Histioteuthis bonellii, which was abundant between 600 and 1200 meters and matches the diving patterns described for this species in the area. In this study, we present the first comprehensive genetic analysis of the diets of rorquals and sperm whales, expanding our understanding of open ocean trophic ecology to promote effective cetacean conservation.