Marine plastic debris poses significant threats to marine wildlife, including  sea turtles. Plastic debris found in digestive tract of sea turtles have been reported in Japan . Many previous studies focused on colour and texture suggested that visual similarity between plastic and jellyfish (their common prey) may lead to mis-ingestion. However, videos from animal-borne cameras on green turtles (Chelonia mydas ) have shown that not all encountered plastic debris was ingested under natural environments with encounter-ingestion ratio of 61.8%. Therefore, the mechanisms behind mis-ingestion remain ambiguous. Biofouling is known to occur almost everywhere in the ocean, thus long-persistent marine plastic debris is a target for attaching microorganisms. Such biofouling may alter properties of plastic debris in ways that could influence ingestion behaviour in sea turtles. We hypothesized that water-borne chemicals from biofouling may make plastic debris more attractive to green turtles and aimed to further understand the key sensory behind the mis-ingestion.
We conducted 2 experiments (I & II) using 8 wild by-catch juvenile green turtles  by focusing on visual and olfactory cues to test this hypothesis. All bio-fouled plastic sheets (25 x 30 cm) were artificially cultivated for 3 weeks before experiments. In experiment I, sea turtles were exposed to bio-fouled sheets  sealed in a zip bag with no holes on (no chemicals releasing, denoted as [FI]) and with holes (chemical cues presence, denoted as FI) to test the influences of olfactory factors. Same experiment protocol was used for experiment II to test the influences of visual cues by displaying brownish bio-fouled plastic ([FII]) and translucent clean plastic ([CII]) to sea turtles. Both [FII] and [CII] were sealed in zip bags to prevent the releasing of chemicals into the water. During these experiments, several behaviours of turtles were observed including noticing, close observing or touching, attempting to bite and actual bite. The latter two were counted as positive reactions for further analysis.
The results of experiment I showed that sea turtles reacted differently under chemicals presence (chi test, p <= 0.002) for a total of 39 positive interactions were seen towards FI (bio-fouled sheets with chemicals) but only 20 towards [FI] (no chemicals released from sheets). In contrast, no differences can be seen in experiment II (22 for [FII] vs. 19 for [CII], chi test, p = 0.062). Our findings suggest that chemicals emit from biofouling had a stronger effect than visual cues in plastic debris mis-ingestion by sea turtles.