Ecological effects of anthropogenic litter on marine mammals: A global review with a black-list of impacted taxa

In this work we would define an historical arrangement of the state of knowledge regarding the ecological impact of anthropogenic litter on marine mammals, assessing the role of different type of impacts (ingestion vs. entanglement) and pressures (three size-based categories). Analyzing 203 references (from 1976 to 2016), we obtained a black-list of 101 species impacted by marine litter (78.9% on 128 species totally known). At species level, four cetacean (Megaptera novaeangliae, Physeter macrocephalus, Tursiops truncatus, Eubalaena glacialis) showed the highest number of bibliographic citations. A significant higher number of species was impacted by entanglement when compared to ingestion. Macro-litter represents the main factor of pressure in all groups; micro-litter showed the highest frequency in Mysticeti, probably explained from their food filtration behaviour. Both intrinsic eco-biogeographic traits (e.g. trophic niche, food catching behaviour, species range) and extrinsic methodological biases could explain our patterns. Since the entanglement is easier to record because of imply only an external observation without further post-mortem examination, and that large litter is easier to detect in respect to meso-and micro-litter, we hypothesize that both this information could be largely biased. Moreover, we observed a direct correlation between the research effort on species (obtained from Scholar recurrences) and the number of citations related to marine litter events, although some exceptions are present: therefore our black list of impacted species is not complete and could be increased focusing research on poor-studied neglected species. After 2005 the number of studies on this topic showed a large increase: however, literature appeared extremely heterogeneous. In this sense, we suggest the use of a standardized nomenclature for pressures and impacts to reduce the loss of information.