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

Campbell, Abigail M

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

Thesis / Dissertation

2024

Publisher

University of Cape Town

Department

Department of Biological Sciences

Faculty

Faculty of Science

Abstract

Environmental 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 on fishing gear, providing large numbers of carcasses that can be used to monitor changes in the incidence and characteristics of plastic floating at sea. Of the 2477 White-chinned Petrels caught off South Africa between 1979 and 2023, 56% 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 remained between 47% and 63% since 1979. Changes in the number of plastic items ingested were determined by fitting the data with a negative binomial regression model. The number of plastic items ingested has significantly increased since 1979, although not consistently, with the lowest average load in 1979–85 (1.86 ± 3.79 items per bird) and the highest in 2017–23 (6.81 ± 18.51). Although the average ingested plastic load was greatest in the last 7 years, the sample size is smallest for this period due to reduced bycatch rates, so more data are needed to confirm the recent increase in ingested plastic. The proportion of pellets declined from 25% to 14%, with the average number of pellets per bird following a similar trend until two highly impacted birds were found in 2022 and 2023 containing 48 and 22 pellets respectively, possibly reflecting recent large pellet spills at sea off South Africa. The proportion of flexible plastics (fibres and films) ingested is high compared to other seabirds and has increased over time, potentially due to their behaviour of scavenging behind ships. Some birds contain fibrous gastroliths up to 20 mm in diameter. Recording plastic loads in White-chinned Petrels killed accidentally on fishing gear offers a useful method to monitor long-term changes in floating plastic at sea.