Macro-litter ingestion in deep-water habitats: is an underestimation occurring?

Marine litter affects marine organisms, posing threats to biodiversity conservation and to ecosystem structure and functioning. Providing a suitable assessment of marine litter effects on marine life through bioindicator species is crucial to drive an effective waste management policy. However, to date no standardized tool has been developed to describe and monitor the impact of marine macro-litter on marine life within deep-water habitats. Modifying the protocol proposed to monitor macro-litter ingestion by the loggerhead sea turtle Caretta caretta, we perform a preliminary investigation on the suitability of different elasmobranch species for monitoring macro-litter ingestion in deep-sea. A total of 122 specimens representing 7 elasmobranch species were collected and examined. External visual inspections documented no clear evidences of disease due to marine litter entanglement. A total of 7 ingested litter items were found in the stomach or in the esophagus of 6 specimens of 4 different species (frequency of occurrence = 4.9%), with a maximum number of 2 items per specimen. No litter items were found in the intestinal contents, as well as no evidence of gastrointestinal blockages due to litter items were detected. The low number of collected litter items suggests that the number of samples required is too large for a feasible monitoring program. Moreover, we observed that the anatomy of the intestinal spiral valve may represent an obstacle to the transit of macro-litter items, which could be spontaneously regurgitated as it happens in the selective elimination of undigested remains, such as bones and scales. Therefore, macro-litter retention time in the gastrointestinal tract of elasmobranchs could be brief and shorter than retention time of food. This may lead to an underestimation of macro-litter ingestion frequency by deep-water elasmobranchs. This study reports the first evidence of plastic ingestion by the kitefin shark Dalatias licha.