Characterization of macroplastics ingested by 23% of stranded short-finned pilot whales (Globicephala macrorhynchus) in the Main Hawaiian Islands (2010−2023)

We examined the gastrointestinal tracts of 13 short-finned pilot whales that stranded between 2010 and 2023 in the Main Hawaiian Islands. Large, tangled masses of marine debris were observed at necropsy in stomachs of 3 of the 13 individuals examined, a frequency of occurrence of 23.1% (95% CI: ~5-54%). These findings increase the number of reported global cases of marine debris ingestion by short-finned pilot whales from 7 to 10. The total dry mass after 2.5 h at 60 °C of the ingested marine debris among the three individuals ranged from 561.4 to 1870.6 g with 6283-35,543 plastic pieces (>5 mm) found per stomach. Ingested marine debris were categorized as monofilament, multifilament, sheet plastic, hard fragment, or miscellaneous items. In all three whales, monofilament line and multifilament rope and netting, which we classify as abandoned, lost or otherwise discarded fishing gear (ALDFG) marine litter, comprised 70.2-84.2% of the macroplastics found by count, and 74.4-86.3% by mass. Sheet plastic (e.g. carrier or garbage bags) was also significant among the items recovered from the stomachs in all three individuals. By count, most of the plastic pieces were in the 5-20 mm or 20-100 mm size classes, but the greatest contribution by mass came from items in the >200 mm size class. Despite the substantial quantities of ingested plastic debris, there was no direct evidence that marine debris ingestion was the cause of death in any of the three cases. Our findings demonstrate the vulnerability of short-finned pilot whales to ALDFG ingestion, and highlight concerns regarding potential sub-lethal impacts, and contribute to greater understanding of the scale and composition of plastic ingestion in cetaceans.