Prey items were primarily copepods or chaetognaths. Stomachs also contained smaller numbers of amphipods and other crustaceans, polychaetes, and pteropods. Three yellowtail ingested small
fish. Synthetic items, presumably plastic, were visually identified in the stomachs of 2 of the 19 fish, representing 10.5% of the sample population, with 95% confidence limits on the probability of exposure of 1.30% to 33.0%. No plastic debris was detected in the intestines. Calculation of the confidence interval was based in part on the assumption of independently drawn random samples. In this case, the sampling units were fish, a group of organisms notorious for being non-randomly distributed in the ocean. Depending on the temporal and spatial scales of association, it is possible these fish had similar recent histories regarding their exposure to
plastic. We therefore acknowledge a possible sampling bias that might have affected the estimates. The synthetic debris consisted of a blue fragment 0.5 mm 1mm (Fig. 1) in Fish #15 and a filament 10 mm in length (Fig. 2) in Fish #16. Despite a small sample size, our results
were similar to the 11.6% incidence of plastic debris in stomachs of vertically migratory myctophid fish collected in surface tows from the NPCG (Davison and Asch, 2011). Based on the incidence reported here, we estimated the percentage of the yellowtail population ingesting plastic in a 30-day period. Using a range of scenarios of gut clearance rates for plastic between 6 h and 96 h, the estimated percentage of the ‘‘exposed’’ population, on average, ranged
from 100% to 55%, respectively (Table 1). Due to the lack of information on residence time of plastic debris in the gastrointestinal tract and occurrence and rates of regurgitation, reports of the incidence of plastic ingestion by fish may be conservative.