Marine macrolitter ingestion by deep-diving cetaceans stranded in the northwestern Iberian Peninsula

Marine litter represents a significant global threat to marine ecosystems and fauna, and has been extensively documented within the gastrointestinal tracts of numerous cetacean species. Beaked whales and sperm whales are considered particularly vulnerable to marine litter ingestion, likely as a consequence of their deep-diving foraging strategies. In this study, we investigated the ingestion of marine macrolitter and associated mortality in deep-diving cetaceans of the families Ziphiidae, Physeteridae and Kogiidae stranded along the northwestern Iberian Peninsula. In total, 167 stranded individuals were registered by the strandings networks (Ziphiidae: 86; Physeteridae: 38; Kogiidae: 43), although full or partial necropsies were possible in only 57 cases (34.1%). Macrolitter items were found in the digestive tracts of 26 individuals (45.6%), mostly plastics and fishing gear items. Specifically, high frequencies of occurrence (F.O.%) of macrolitter ingestion were found in Cuvier's beaked whales (57.1%, n = 12), sperm whales (55.6%, n = 5) and pygmy sperm whales (27.8%, n = 5). Among the 26 individuals in which macrolitter ingestion was detected, lethal (26.9%), sublethal (57.7%) and minor consequences (15.4%) were associated to macrolitter ingestion. Lethal cases of marine litter ingestion were found in Cuvier's beaked whales and sperm whales (respectively 41.7% and 40.0% of each species' analysed digestive tracts). These results emphasise the importance of litter ingestion in the mortality of species that feed on or near the seafloor. Improving data collection from deep-diving stranded cetaceans is essential to assess and better understand the effects of marine litter on these cetacean populations.