On the Washington and Oregon coast of the NE Pacific Ocean from 2008-2015, we found that juveniles represented 83% of Northern Fulmars that were beached. In comparison to older birds, juveniles averaged more mass and pieces of plastic in stomachs. This reflected relatively larger plastic loads in the cranial stomach section, the proventriculus, which we found was associated with relatively large accumulations in the caudal stomach section, the ventriculus. We estimate that the proventriculus could retain almost 10 times as much plastic as the ventriculus and that retention of proventricular plastic largely accounted for the difference in juveniles versus older birds. Our findings contrast with published Atlantic Ocean reports where the proportion of immatures was lower and plastic was retained mostly in ventriculi. The differences in demography and gastric distribution between NE Pacific and Atlantic fulmars may reflect the different sizes of plastic particles that were available. The preponderance of juveniles in NE Pacific Ocean samples and associated relatively large plastic loads overestimate the load for the species. Without accounting for age and gastric distribution, comparison to other regions is ill-advised. An unrecognized complication in stomach-based sampling is that differences in size of plastic at spatiotemporal scales could affect the utility of stomach samples as bioindicators of oceanic plastic pollution.

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