Results

We opportunistically necropsied 362 specimens of 16 species, and documented plastic ingestion in 11 species, belonging to 7 families and 4 orders. Every sampled Tristram’s Storm-petrel and Bonin petrel contained plastic, with albatrosses (Laysan, Black-footed) chicks / adults having high incidence rates (>50%). We also documented plastic ingestion in wedge-tailed shearwaters, boobies (red-footed / masked / brown), red-tailed tropicbirds, greater frigatebirds and brown noddies.  These findings highlight that plastic pollution is widespread and pervasive in Hawaiian seabirds.

Plastic Incidence: 11 of the 16 species we sampled had ingested plastic. We documented plastic ingestion in five foraging guilds, which were characterized by high levels of species-specific rates: plunge divers (100% of 4 species), albatrosses (100% of 2 species), frigates (100% of 1 species), nocturnal petrels (67% of 3 species), and tuna birds (40% of 5 species). While we did not document plastic ingestion in the neuston feeding tern foraging guild, only one specimen was examined.

Overall, 10 of the 35 species * age groups we sampled had sample sizes large enough to characterize their plastic incidence and loads. These species * age classes showed wide variability in their incidence rates of plastic ingestion, ranging from 0% (Sooty Tern adults / immatures) to 100% (Tristram’s Storm-petrel chicks)

The petrels were characterized by high ingestion rates, for both chicks and adults. In particular five species * age groups had 100% incidence rates: Tristram’s Storm-petrel chicks (60 specimens), Tristram’s Storm-petrel adults (1 specimen), Wedge-tailed Shearwater adults (2 specimens), and Bonin Petrel chicks (5 specimens), and Bonin Petrel adults / immatures (1 specimen).

Conversely, the terns and noddies did not show signs of plastic ingestion, despite the larger sample sizes and multiple age classes sampled: Sooty Terns (14 specimens from three age classes), White Terns (11 specimens from three age classes) and Black Noddies (9 specimens from three age classes). Only 7.1% (1 of 14) of the immature Black Noddies we sampled had ingested plastic.          

Plastic Incidence Across Foraging Guilds: To compare plastic prevalence across foraging guilds, the overall occurrence of plastic ingestion (from both stomach chambers combined) was calculated for every species with at least two sampled specimens (n. 2), by combining the rates across age classes. For each species, the average occurrence rate was calculated by averaging across all the sampled age classes.An analysis of variance (ANOVA) test was used to compare species-specific prevalence rates across four foraging guilds with two or more sampled species: albatrosses (2 species), nocturnal-foraging petrels (3 species), plunge-divers (4 species), and tuna-birds (5 species). Thus, this analysis involved 322 seabirds, 14 species and 4 guilds.

In addition to comparing these four categorical groupings, this test considered two co-variates: the sample size (log of the number of birds sampled per species) and the proportion of chicks in the sample (arcsine of the square root of the proportion of chicks sampled per species).Because sample sizes varied widely across the 16 species, from 126 (LAAL) to 2 (BUPE, MABO), this potential bias was considered in the analysis. Additionally, because the proportion of chicks sampled varied from species to species, from 0% (BRBO, BUPE) to 98.3% (TRSP), this factor was considered to address age-specific disparities in plastic ingestion rates. To ensure normality, the occurrence data were arcsine transformed (yf = arcsin (y)1/2), prior to performing the ANOVA.

The comparison of the species-specific occurrence rates did not reveal significant differences across foraging guilds, (F3,8= 1.088; p =0.408), and no bias due to the varying sample sizes (F1,8= 0.166; p =0.694). Yet, the proportion of chicks/juveniles in the sample did have a significant influence on the plastic occurrence rate (F3,8 =6.146; p = 0.038). Finally, the ANOVA residuals were normally distributed (One-sample Kolmogorov-Smirnov test, n= 14, max_diff = 0.198, p = 0.574).

A follow-up linear regression confirmed that species-specific occurrence rates increased significantly (F1,12 = 15.410; p = 0.002), with a higher proportion of chicks/juveniles in the sample (slope coefficient = 1.021 ± 0.260 S.E.). This model accounted for 56.2% of the observed variance, and the regression residuals were normally distributed (One-sample Kolmogorov-Smirnov test, n= 14, max_diff = 0.143, p =0.899).

Plastic mass, for species * age groups with large enough sample sizes (where ≥ 8 birds sampled). To achieve  normality,  the mass values are Log10 (X+1) transformed. The box plots show the distribution quartiles (25th, 50th, 75th %), the whiskers show the range (10th, 90th %), and the circles highlight outliers. Note that the mean is depicted by the blue dashed line.

Total Plastic Mass: When we compared the ingested plastic mass was across species * age groups, albatrosses had the largest loads, with maximums of 60.3305 g (Black-footed chick) and 55.4142 g (Laysan chick). Yet, adult albatrosses had substantially lower plastic loads, with maximums of 3.2639 g (Black-footed) and 2.8551 g (Laysan).  Nevertheless, we observed highly-skewed distributions of plastic loads in all of the species * groups that were examined. The large S.D.s and outliers suggest there is a high degree of individual variability in the plastic masses ingested by chicks and adults.

Plastic Mass by Organ: Contingency G tests comparing the incidence (presence / absence) of ingested plastic in the two stomach chambers of petrel species revealed significant differences between the proventriculus and the gizzard for the two albatrosses. NOTE: Tristram's Storm-petrels had 100% incidence in the proventricculus and the gizzard.

These patterns suggest that these seabirds are retaining ingested plastic in both the ventriculus and the proventriculus. This is especially apparent in chicks, which are characterized by equal incidence rates in both stomach organs. In the case of the Black-footed Albatross, both chicks and adults had higher plastic incidence in the proventriculus. We hypothesize that the line and foam ingested by this species may have a difficult time passing into the gizzard leading to its retention in the proventriculus. Comverselly, adult Laysan Albatross had higher plastic incidence in the ventriculues, suggesting thatthe fragments ingested by this species can pass through the proventriculus more easily.   

Examples of the plastic items ingested by seabirds, highlighting the variety of fragment colors / sizes.  The panels illustrate the size spectrum of ingested items, ranging from < 1 mm in the Tristram Storm-petrel (A) to a few cm in the Laysan Albatross (B). Note the 10 mm grid scale, used for reference

Conclusions

These results underscored a series of research gaps, relating to the incidence / loads of plastic ingestion in specific species * age class groups. Thus, these research gaps highlight 4 priorities:  unstudied species, poorly-studied species with documented plastic ingestion, poorly-studied species without documented ingestion, and species with high plastic ingestion rates.  Ideally, sample sizes of 8 - 20 specimens per age * species are needed to develop robust incidence rates, with small confidence intervals.  Additional samples (20 - 50) are required to quantify plastic loads by species * age class, especially given the low incidence rates and the non-normal distributions of the ingested plastic mass