Long-term changes in the incidence and characteristics of plastic ingested by White-chinned Petrels

Plastic floating at sea is difficult to measure due to its high spatial and temporal variation. White-chinned Petrels Procellaria aequinoctialis are surface-foraging seabirds found in the Southern Ocean that often ingest plastic. They are susceptible to being caught accidentally on long-line fishing gear, providing carcasses that can be used to monitor changes in the incidence and characteristics of plastic floating at sea. Of the 2486 White-chinned Petrels sampled off South Africa between 1979 and 2024, 57 % contained plastic. Data were grouped into time periods to determine temporal variation while accounting for unequal yearly sample sizes. The proportion of birds containing plastic has not changed since 1979. The number of plastic items ingested increased from an average of 2 items per bird in the early 1980s to 7 in 2017-24, mainly due to an increase in the last 5 years, but there has been no change in the total mass of ingested plastic. The proportion of pellets declined from 25 % to 17 %, with the average number of pellets per bird following a similar trend until two highly impacted birds were found in 2022 and 2023, possibly reflecting recent large pellet spills at sea off South Africa. White-chinned Petrels ingest more flexible plastics (threads and films) than other petrels, potentially linked to their behaviour of scavenging behind ships. Some birds contained fibrous gastroliths, up to 20 mm in diameter. Recording plastic loads in White-chinned Petrels offers a useful method to monitor long-term changes in floating plastic at sea.