Opportunistic Feeding and Parasite Assemblages in Sei Whales ( Balaenoptera borealis ) From Southern Chile Revealed by DNA Metabarcoding

ABSTRACT Balaenopterid whales are major consumers in pelagic ecosystems and play a fundamental role in marine food webs. The sei whale ( Balaenoptera borealis ) is a broadly generalist species that remains among the least studied baleen whales. In this study, we characterized diet composition and parasite assemblage in five stranded sei whales in southern Chile using DNA metabarcoding of stomach and intestinal contents. Four main prey categories were detected among the five individuals, with marked inter‐individual variations. Grimothea spp. occurred in all whales and dominated prey‐derived reads in the stomach contents (97.6%) with high values for the estimated occurrence indices (%FOO = 100, POO = 54.5 for 18S). Calanoid copepods (1.4%) represented the second most frequent prey group, occurring in three individuals. Myctophid fishes (0.6%), including Lampanyctus parvicauda and Triphoturus oculeus , previously unrecorded in the sei whale diet, were detected in two individuals. Euphausiids (0.4%) were detected in a single individual. Metabarcoding also detected DNA assigned to parasites, including trematodes, cestodes, and parasitic protozoans, in each whale examined. Homalometron spp. exhibited the highest read abundance across individuals, and the cestode genus Tetrabothrius was also consistently present. We additionally detected Adenocephalus spp., expanding the known host range of this genus to large baleen whales, and Cryptosporidium spp., which to our knowledge has not previously been reported in sei whales. Overall, our findings provide new insight into the trophic ecology of sei whales foraging in southern Chile and reinforce the ecological importance of Grimothea spp. as a key prey resource in austral marine ecosystems. This study also highlights the potential of DNA metabarcoding to recover both dietary and parasite‐associated signals from rare necropsy material in an endangered whale species.