Ingestion of Marine Debris by Sea Turtles

Abstract

Ingestion of marine debris is an increasingly significant problem for marine wildlife, and is known to affect more than 170 marine species worldwide. Debris ingestion can have lethal outcomes either through the impaction or perforation of the alimentary system, but it can also have sublethal impacts. The world’s seven species of sea turtles, six of which are listed as threatened or endangered on the IUCN Red list, are all known to be affected by debris ingestion. Their long distance migrations coupled with a life history that incorporates both an oceanic and pelagic lifestyle, make them an ideal subject for studying the mechanisms of and threats from debris ingestion on a global scale. The aim of my doctoral research was to identify and characterise the risk factors affecting the ingestion of debris by sea turtles, including geographic distribution of debris, species and life history stage of turtles and visual and physical characteristics of debris ingested by turtles. Since sea turtles are primarily visual predators, the visual characteristics of the debris they ingest can help us to gain further insight into why they eat such harmful items. The impact of debris on sea turtles was first brought to attention by Balazs in 1985, in a paper summarising all known instances of both entanglement and ingestion. Subsequently a number of studies much smaller in scope were published. My analysis of all studies published since Balazs’ review revealed that the likelihood of debris ingestion has increased for both green and leatherback sea turtles, and shows an increasing trend for loggerhead turtles. Additionally, turtles that are omnivores, herbivores and gelatinovores (hawksbill, green and leatherback turtles) are more at risk than carnivorous species (loggerhead and Kemp’s ridley turtles). Plastics are by far the most commonly cited items of marine debris ingested by turtles. Understanding why turtles ingest debris first requires an understanding of what types of debris they ingest and their selectivity towards different items. A comparison of debris ingested by stranded sea turtles to debris found in the environment shows that benthic-feeding sea turtles select for soft, clear, plastics, while pelagic-feeding sea turtles prefer hard, white plastics, and are much less selective in their ingestion. To further investigate selectivity from the point of view of the turtle, I developed a visual model for the way turtles see the debris they ingest and compared it to environmental debris. Turtles preferentially select non-blue coloured items, and strongly prefer highly transparent and flexible items. This lends support for the hypothesis that they eat plastic because it resembles one of their natural prey items, jellyfish, and also suggests they may detect items that contrast against a blue ocean background more efficiently. Jellyfish and other transparent animals are partially birefringent, meaning that some of their tissues polarise light. Predators with developed polarisation vision may use this visual capability in locating jellyfish and other transparent prey. Plastic bags also alter the polarisation of light. If sea turtles are able to see polarised light, this could be an additional source of confusion between turtles’ natural prey of jellyfish and the bags that they often ingest. I used the innate phototaxis of sea turtle hatchlings to investigate their ability to see polarised light. Results suggested that sea turtles might possibly detect polarised light, but further data collection is required to confirm or reject the hypothesis. Finally, I synthesised the factors influencing debris ingestion by turtles into a global risk model, taking into account the area where turtles are likely to live, their life history stage, the distribution of debris, the time scale, and the distance from stranding location. The model was ground-truthed using data from stranded sea turtles. I showed that life history stage, species, proximity to stranding location, and the date of stranding are all critical model parameters. Life history stage is the strongest predictor for the likelihood of ingesting debris, with young, oceanic-feeding turtles most likely to ingest debris. The best-fit model takes into account plastic concentrations within 350 km or less from the stranding location, rather than across the entire habitat range, and uses the debris concentrations that would have been present when the turtles stranded. My doctoral research concluded that sea turtles, particularly green, loggerhead, and leatherback turtles, are at high and increasing risks from plastic ingestion. Importantly, comparative studies show that the problem is not unique to turtles. If we are to address this growing negative impact on marine and other life, we must take a comprehensive approach, addressing the issue at all scales including manufacturing, consumer choice, waste reduction, disposal, and removal.