Birds of a feather eat plastic together: high levels of plastic ingestion in Great Shearwater adults and juveniles across their annual migratory cycle

Abstract

Limited work to date has examined plastic ingestion in highly migratory seabirds like Great Shearwaters (Ardenna gravis) across the their entire migratory range, although this species is prone to ingest plastic as a wide-ranging procellariiform. We examined 217 Great Shearwaters obtained from 2008–2019 at multiple locations spanning their yearly migration cycle across the Northwest and South Atlantic to assess accumulation of ingested plastic as well as trends over time and between locations. A total of 2,328 plastic fragments were documented in the ventriculus portion of the gastrointestinal tract, with an average of 9 plastic fragments per bird. The mass, count, and frequency of plastic occurrence (FO) varied by location, with higher plastic burdens but lower FO in South Atlantic individuals from the breeding colonies. No fragments of the same size or morphology were found in the primary forage fish prey, the sand lance, (Ammodytes spp., n = 202) that supports Great Shearwaters in Massachusetts Bay, USA, suggesting the birds directly ingest the bulk of their plastic loads rather than accumulating via trophic transfer. Fourier-transform infrared spectroscopy indicated that low- and high-density polyethylene were the most common polymers ingested, within all years and locations. Individuals from the South Atlantic contained a higher proportion of larger plastic items and fragments compared to juveniles and non-breeding adults from the NW Atlantic, possibly due to increased use of remote, pelagic areas subject to reduced inputs of smaller, more diverse, and potentially less buoyant plastics found adjacent to coastal margins. Different signatures of polymer type, size, and category between similar life stages at different locations suggests rapid turnover of ingested plastics commensurate with migratory stage and location, though more empirical evidence is needed to ground-truth this hypothesis. This work is the first to comprehensively measure the accumulation of ingested plastics by Great Shearwaters over the last decade and across multiple locations spanning their yearly trans-equatorial migration cycle, and underscores their utility as sentinels of plastic pollution in Atlantic ecosystems.

1. Introduction

An estimated 5 – 125 trillion pieces of plastic greater than 100 μm in size are buoyant within the ocean surface layer, where these items are subject to oceanic scale circulation and varied biophysical interactions (Eriksen et al., 2014; Lindeque et al., 2020; Sherman et al., 2015). The abundance and ready transport of small plastic pieces within ocean surface environments results in an increased probability of encounter between epipelagic marine fauna and plastic items, with subsequent opportunity for ingestion of plastic items. Ingestion of plastic may occur via direct ingestion of plastic pieces (Savoca et al., 2016), or ingestion of prey items containing plastic (Nelms et al., 2018). In vertebrate species, ingested plastics can result in impaction or perforations within the digestive tract (Pierce et al., 2004), reduced feeding due to false satiation (Santos et al., 2020), potential uptake of adsorbed chemical pollutants or innate chemical additives (Koelmans et al., 2016; Tanaka et al., 2015), and as-of-yet poorly characterized sublethal impacts on individual health and fitness (Roman et al., 2019c; Senko et al., 2020).

The accumulation of ingested plastics in seabirds has been documented since the 1960s (Kenyon and Kridler, 1969). Meta-analysis efforts conducted by Wilcox et al. (2015) found that 59% of seabird species examined between 1962 and 2012 had ingested plastic, and, within those studies, on average 29% of individuals had plastic in their gastrointestinal tract (Ryan et al., 2009; Wilcox et al., 2015). Seabirds in the order Procellariiformes (e.g. albatrosses, shearwaters, petrels, and storm-petrels) are thought to be most vulnerable to plastic ingestion (Moser and Lee, 1992; Provencher et al., 2014) related to their wide-ranging pelagic habits (Davies et al., 2010) and substantial use of olfactory and visual cues during foraging (Nevitt, 2008; Nishizawa et al., 2021), yet the frequency of plastic ingestion varies widely between members of this order. Some albatross species have been shown to infrequently ingest plastics or retain ingested plastics on shorter time-scales due to particular gut morphology that facilitates ready regurgitation of indigestible plastics (Furness, 1985; Ryan et al., 2016). In contrast, numerous research efforts spanning multiple ocean basins suggest shearwaters in the genera Ardenna and Calonectris frequently ingest variable loads of plastic. For example, ingested plastics were found in 65–90% of adult or juvenile short-tailed Shearwaters Ardenna tenuirostris collected in Eastern Australia in 2010 – 2013 (Acampora et al., 2014; Roman et al., 2016). 90% of flesh-footed shearwater fledglings sampled in 2011 on Lord Howe Island in Eastern Australia contained ingested plastics, with a mean plastic count of 18 items and a maximum count of 276 items in one individual (Lavers et al., 2014). In the Atlantic basin, Great Shearwaters (Ardenna gravis) have been found to ingest plastic at high rates for decades, with multiple studies documenting 70–95% ingestion rates in Great Shearwaters sampled within their breeding or non-breeding ranges (Bond et al., 2014b; Furness, 1983, 1985; Ryan, 2008, 1987a, 1987b). However, several studies have noted a lower frequency of plastic ingestion in this species, ranging from 0–39% frequency of ingestion (Bond et al., 2014b; Brown, 1981; Haman et al., 2013a; Ryan, 2008).

Rates of plastic ingestion and subsequent accumulation are thought to vary based a suite of interacting factors, including gut morphology, life stage, foraging strategy, and geographic location. In procellariiform seabirds, the isthmus connecting the proventriculus and ventriculus is angled and narrow, unlike the wider, more linear junctions seen in other seabirds (Furness, 1985; Jackson, 1990; Ryan and Jackson, 1987). This unique configuration is thought to restrict the passage of materials from the ventriculus back into the proventriculus, leading to an increased proclivity to accumulate ingested plastics. Foraging strategy is also a primary driver of plastic ingestion, with trophic generalists and surface seizing birds reliant on crustacean diets considered the most vulnerable (Day et al., 1985; Roman et al., 2019a; Ryan, 2019, 1987a). The incidence of plastic ingestion is also thought to increase with abundance of plastics in the environment and use of regions considered plastic hotspots such as subtropical gyres and the southern boundary of the Atlantic, Pacific, and Indian Oceans (Ryan et al., 2009; Van Franeker et al., 2015; Wilcox et al., 2015). For example, Northern Fulmars in the North Sea have been monitored for plastic ingestion at their breeding colonies since the 1970s. The ingestion of industrial pellets by fulmars has roughly tracked the measured abundance of industrial pellets in ocean surface water over time (Van Franeker et al., 2015).

Despite a rapid increase in the number of published studies focused on plastic pollution in shearwaters and other seabirds over recent years, data gaps remain surrounding the dynamics of seabird plastic ingestion as influenced by migration, life stage, and polymer makeup. Existing studies often focus on samples collected from species of convenience or highly monitored species during the breeding season when chicks are stationary and adults are central place foragers (Rayner et al., 2010), though sample collection mechanisms employed in breeding colony studies varies widely (e.g. birds killed by fisheries, beached birds, unsuccessful fledglings, remains in predator regurgitations) (Provencher et al., 2019; Ryan, 2019).

The Great Shearwater is a procellariiform seabird endemic to the Atlantic Ocean. Monitoring of this species provides an opportunity to monitor and interrogate the ecological occurrence and impacts of plastic in the Atlantic; this stands as a continued need, given the projected increases in plastic leakage into the environment. Plastic assessment in this species is readily achievable and valuable due to their abundance, well-described habitat use, co-location with key marine megafauna, and status as an ecological indicator related to their compact representation of temperate and subtropical Atlantic regions (Cuthbert, 2005; Powers et al., 2020, 2017; Silva et al., 2020). They are also frequently bycaught across their migratory range, resulting in abundant sample collection opportunities.

The accumulation of ingested plastics has been previously assessed in this seabird at discrete locations across its breeding and non-breeding range over the past sixty years, via studies covering a single year, or a span up to 14 years including variable numbers of individuals (n = 1 to 86) (Bond et al., 2014a; Furness, 1985; Haman et al., 2013b; Moser and Lee, 1992; Ryan, 1987a, 1987b). The most recent monitoring effort focused on this species examined 84 individuals collected between 2000 – 2011 from Sable Island, Nova Scotia, Canada in the NW Atlantic, but did not include any formal time trend analysis due to highly variable yearly sample sizes (n = 0 – 25/year) (Bond et al., 2014a). Alternatively, Ryan (2008) examined Great Shearwaters from 1983 – 1985 (n = 33) and 2005 – 2006 (n = 53) and found major shifts in the composition but not the total amount of ingested plastics in this species, with decreased abundance of virgin industrial pellets over time (Ryan, 2008), similar to results observed in short-tailed shearwaters over time (Vlietstra and Parga, 2002). No efforts focused on this species have yet described the polymer signature of ingested plastics, the influence of demographics on plastic ingestion, trends of ingested plastics over the annual migratory cycle of this highly mobile marine species, or time trend analysis over the last decade.

Here we seek to remedy these data gaps, and comprehensively describe plastic ingestion in Great Shearwaters, using bycatch and beach-cast adult and juvenile samples obtained over 2008 – 2019 from multiple regions frequented in the annual cycle of this trans-equatorial migrant. The goals of this study were to a) deduce temporal trends of ingested plastics, b) compare plastic categories and polymer types found between different locations, age classes, and over time, and c) explore the trophic or environmental origin of ingested plastics. We hypothesized that (i) plastic ingestion would show no temporal trends over time, (ii) adult birds would contain less ingested plastics compared to juvenile birds, and (iii) fragments of polyethylene and polypropylene would make up the majority of ingested plastics over time and space.

2. Materials and Methods

Study species

The Great Shearwater is a long-lived, pelagic seabird that feeds on squid, crustaceans, and small forage fish including sand lance (Ammodytes spp.), and has regularly been observed attending fishing vessels seeking fisheries offal (Bugoni et al., 2010, 2008; Powers et al., 2020). Most foraging activities occur within the top 10 m of the water column via surface plunging and seizing behaviors, with most dives less than 2 m in depth (Ronconi et al., 2010). This species breeds during the boreal winter in the South Atlantic, on Gough Island, Inaccessible Island, and Nightingale Island in the Tristan da Cunha island group and Kidney Island in the Falkland Islands (Cuthbert, 2005). Tracking data suggests use of large pelagic regions of the temperate South Atlantic between South Africa and South America during the breeding season (Ronconi et al., 2018; Schoombie et al., 2018). Great Shearwaters inhabit the NW Atlantic during their non-breeding period over the boreal summer; tracking and necropsy data suggest some degree of age-specific segregation in the Gulf of Maine and along NE Canada, with preferential use of the SW waters and Massachusetts Bay by juveniles (Powers et al., 2020, 2017).

Specimen Collection, Necropsy, and Sample Condition

A total of 217 Great Shearwaters were analyzed for plastics, from six locations including sites within breeding/wintering areas as well as places along migratory pathways (Fig. 1). Samples obtained included both bycatch and beach-cast individuals, procured through a variety of collection and monitoring programs. Most individuals were frozen individually as a whole specimen upon collection and stored at −20°C until necropsy, while the chicks from Nightingale Island were killed, processed for edible parts, and stomach contents retained frozen for further plastic analysis. Table S1 summarizes the sample set and collection locations included in this study.

Figure 1.

Map of Great Shearwater migratory routes (black and dashed lines) and sample collection locations (marked by colored circles). Solid lines are considered the dominant migratory route based on tracking data. No samples were collected off the Patagonian Shelf (marked by the yellow square), but this location is heavily used by both adult and juvenile Great Shearwaters as a staging and feather molt location⁵² surrounding trans-equatorial migration. Purple dashed lines indicate areas where fishing activity may contribute plastics to the environment. Surface plastic density data is derived from a compilation of literature sources curated by the NOAA Marine Debris Program, freely available online.

Most individuals were necropsied in a standardized manner, including measurements of morphometric characteristics, body condition, and organ mass (van Franeker, 2004). Clinical necropsies were undertaken using pre-cleaned metal tools on stainless steel tables to minimize risk of environmental contamination. Age was determined following the age characterization of Great Shearwaters as defined previously considering gonad size and color, molt status, and presence of Bursa of fabricius (Table S4), while sex determination was solely based upon gonad presentation (Powers et al., 2020). The entire gastrointestinal tract (GIT) was collected, wired shut using metal wire, and frozen at −15°C until analysis. The stomach contents of chicks from Nightingale Island were obtained without clinical necropsy of associated individuals as part of harvesting activities. Table S2 summarizes data collection efforts associated with each set of birds.

Analysis of accumulated, ingested plastics

Food debris and/or seawater were commonly found in the esophagus and mouth of sampled bycatch individuals, due to regurgitation during entanglement and drowning. Given the heavy reliance on bycatch samples in this study (157 of 217), the esophagus and proventriculus were not assessed for plastic content, assuming any assessment of plastic occurrence in these compartments was compromised by regurgitation. Plastics were instead only assessed in the ventriculus, or “gizzard”, a muscular organ used to grind up indigestible, hard diet materials. The angled and narrow isthmus between the ventriculus and proventriculus likely insulates measurements obtained from this organ from bias related to regurgitation. Plastics were assessed in both the ventriculus and proventriculus of 10 individuals with visually full stomachs to test the validity of this assumption. Results suggested our approach was appropriate, as >95% of accumulated plastics occurred in the ventriculus of bycatch birds; this is further supported in the literature (Ryan, 2015a). However, we note that by entirely omitting assessment of plastics in the proventriculus, we are potentially truncating the maximum count and mass of plastics found in the most heavily polluted individuals, given plastic is usually only in the forestomach when the ventriculus is full, or the item is too large to pass into the ventriculus. Therefore, the maximum number of plastics observed in the most polluted individuals herein may be underestimated. We also note that migration of plastics from the gizzard back into the proventriculus may be possible to some limited degree, as Northern fulmars in rehabilitation have been documented to spontaneously regurgitate plastic, presumably from the gizzard, though lavage and reflexive regurgitation as a result of handling have not caused plastic or natural item offloading from the gizzard in Great Shearwaters and fulmars (Ryan, 1988; Terepocki et al., 2017a) (Furness 1985).

Plastics were obtained from the ventriculus as previously described (Van Franeker and Meijboom, 2002; van Franeker, 2004; Van Franeker et al., 2015). Briefly, the organ was cut open using a solvent-cleaned metal scalpel, and flushed thoroughly using deionized water over a metal sieve with 1 mm mesh. Plastic items were visually separated from natural items with the aid of a dissection microscope, magnifying lenses, and/or density separation techniques as required (Provencher et al., 2017; Van Franeker and Meijboom, 2002), and collected in solvent-cleaned glassware. Samples were dried in a clean and plastic-free fume hood for ~ 2 days. Personnel wore cotton lab coats during sample preparation and analysis to avoid fiber contamination. Plastic items from chicks harvested for human consumption were obtained by Tristan Conservation staff with appropriate training. Items were cleaned with ethanol and shipped to the University of Rhode Island for measurements and FTIR analysis with the rest of collected items. 26 of 29 chicks that were identified to contain plastic were included in plastic characterization and FTIR analysis.

Plastic material was visually sorted into qualitative categories by two trained individuals, as outlined elsewhere (Provencher et al., 2017; Van Franeker and Meijboom, 2002). Each suspected plastic item was first classified as an industrial (nurdle or pellet) or user plastic (non-industrial items likely sourced from consumer use). User plastics were further differentiated into five subcategories including soft sheet-like plastics (e.g., bag and film), thread-like plastics (e.g., microfibers, rope), foam (e.g., polystyrene), fragments (unknown hard plastics), and other (e.g., balloon, rubber, melted plastic). Each item was weighed to the nearest 0.0001 g using an analytical balance, and length, width, and thickness measured to the nearest 0.01 mm using a calibrated electronic caliper at the largest extent of each dimension. Soft amorphous plastics (e.g. balloons, melted/liquified rubbers, sheets) were weighed but dimensions were not measured. Each item was also classified into a size category, with microplastics defined as pieces 1 – 5 mm in their longest dimension, mesoplastics including pieces 5 – 25 mm, and macroplastics including items >25 mm (GESAMP, 2015). Plastic mass was compared to ecological quality objectives set out by the Convention for the Protection of the Marine Environment of the North-East Atlantic. This threshold suggests ecological integrity is sufficiently met when no more than 10% of a given seabird sample set containing >50 individuals contains a mass of plastic greater than 0.1 g or 100 mg (OSPAR Commission, 2008). Average mass and item size was calculated only based on those individuals containing plastic items to avoid left censorship of estimates. Length and width are reported instead of surface area given the irregular shape of most plastic items.

Plastics were visually assessed in the primary forage fish prey of Great Shearwaters as part of a larger diet analysis as described elsewhere (Suca et al., 2021); briefly, the stomach contents of 202 sand lance (Ammodytes spp.) were examined for prey species identification and visually apparent plastic items using a Leica 205C stereomicroscope.

Polymer analysis

91% of items visually identified as plastic were typed for polymer identity; 207 items were omitted due to cost and COVID-19 limitations. Prior to analysis, each item was cleaned with deionized water and/or ethanol to remove biological debris and filed with a metal file. Items were scanned through a Thermo Scientific Nicolet TM iS5 FTIR spectrometer with an ATR accessory, using the Thermo Scientific Omnic software (Thermo Fisher Scientific, Waltham, MA). A diamond crystal ATR plate was used for most analyses, however for analysis of black plastic a germanium crystal ATR plate was used due to the crystal’s high refractive index and its capacity to accurately scan high absorbing substances such as carbon-black colored objects.

A spectrum was obtained for each plastic fragment and compared to spectral libraries. The principal spectral libraries searched included the Center for Coastal Studies (Provincetown, MA) private library which includes the Center for Marine Debris Research Polymer Kit 1.0, the Aldrich Polymers library, and the Hummel Polymer and Additives library. Each result from the spectral library was verified by identifying the spectrum’s wavelength peaks and matching wavelength numbers to known wavelength ranges of the corresponding polymer. Only fragments with greater than a 70% confidence of assignment were reported (Lusher et al., 2013).

Each identified plastic fragment was categorized under one of 16 polymer groups; non-plastic items identified via FTIR were categorized as biological, vegetation, rock, glass, natural resin, wood, or natural fiber blend, and were not included in further count or polymer identity metrics. Polyethylene was further divided into low density polyethylene (LDPE), high density polyethylene (HDPE), and polyethylene (inconclusive on the type of polyethylene). Mixed polymers were labeled under the primary polymer, e.g. polyacrylate mix, polyethylene mix.

Only the first 50 suspected plastic items were analyzed in three outlier individuals that possessed over 50 suspected plastic items to maximize the number of individuals included in FTIR analyses. All items (n = 104 items) were analyzed for one adult individual from Gough Island found to contain an exceptional diversity of plastics that appeared to have died as a result of plastic ingestion; this individual was considered an outlier throughout statistical analysis. All plastic items found in all other individuals were assessed otherwise, and included in statistical analyses.

Statistical analysis

All data manipulation and statistical analyses were performed in R version 3.6.1 (R Core Team, 2020). Quantitative data were checked for normality and homoscedasticity using the Shapiro-Wilk test and Levene’s test. Plastic count and mass data were non-normal despite log transformation and therefore treated non-parametrically for statistical analyses; year groups displayed no significant differences in variance. Differences between years or groups were assessed using Kruskal-Wallis tests with post hoc application of Dunn’s test with Bonferroni correction for multiple testing as needed. Relationships between continuous variables (specifically morphometric measurements, plastic count, and plastic mass data) were assessed using Spearman rank correlation coefficients (R_s²). Chi-squared tests of independence or Fisher’s exact tests were used to compare ratios of plastic category and polymer type between age groups, sexes, locations and sampling years. Effects observed within all statistical tests were considered significant when p < 0.05.

3. Results

Accumulation and frequency of ingested plastics over time and between locations

Out of 2328 plastic-like items found in the ventriculus of 217 Great Shearwaters, 2121 items were assessed via FTIR, and a total of 2035 items were confirmed as plastic via FTIR; the mean confidence score of plastic identification was 96.8%. 86 items were of natural origin or FTIR results were inconclusive. The mass and number of plastics found in each individual varied by location and year; only those individuals found to contain plastic were included in summary calculations describing the count and mass of ingested plastic. Considering all years of data, Massachusetts Bay bycatch birds contained an average of 8 plastic items and 149 mg plastic (range = 1–42 items, 0.4 −797 mg, n = 139 birds), Gough Island adult individuals contained an average of 10 plastic items and 345 mg plastic (range = 1–104 items, 1–1867 mg, n = 15 birds), Nightingale chicks contained an average of 22 items and 1294 mg (range = 3–93 items, 40–8047 mg, 26 birds), individuals found off the coast of Brazil contained a mean of 8 pieces and 311 mg each (range = 1–40 items, 4–2330 mg, n = 13 birds), and individuals found off the coast of North Carolina contained an average of 12 items and 181 mg plastic (range = 3–28 items, 19–445 mg, n = 15 birds) (Table 1, Figs. S1, S2). Count and mass data were highly correlated across all locations (R_s² = 0.88, p <0.001).

Table 1.

Summary statistics describing accumulated ingested plastics by mass (in mg) and count (in parentheses) observed in individuals obtained from different regions and years.

Collection Location	Collection Type	Year	# birds	Mean mass & (count)	Median mass & (count)	Std. Dev. mass & (count)	Std. Err. mass & (count)	Min. mass & (count)	Max. mass & (count)
Gough	Found dead	2012	1	1867 (104)	na
Gough	Bycatch	2018	14	236 (6)	55 (1)	385 (12)	103 (3)	1 (1)	1432 (47)
Nightingale	Harvested	2019	26	1241 (22)	757 (14)	1625 (24)	302 (5)	40 (3)	8047 (93)
Brazil	Beached	2019	13	311 (9)	72 (4)	640 (12)	177 (3)	4(1)	2330 (40)
SE NC	Beached	2017	15	181 (12)	137(12)	140 (7)	36 (2)	19 (3)	445 (28)
Mass. Bay	Beached	2017	6	164 (9)	179 (8)	111 (8)	46 (3)	31 (1)	315 (23)
Mass. Bay	Bycatch	2008	1	74(5)	na
Mass. Bay	Bycatch	2010	16	126 (8)	50 (3)	161 (10)	40 (2)	0.4 (1)	463 (30)
Mass. Bay	Bycatch	2011	18	150 (10)	87 (5)	180 (10)	42 (3)	5 (1)	750 (34)
Mass. Bay	Bycatch	2012	23	154 (8)	69 (4)	189 (11)	39 (2)	0.6 (1)	641 (42)
Mass. Bay	Bycatch	2014	10	234 (9)	125 (7)	295 (78)	93 (3)	6 (1)	797 (22)
Mass. Bay	Bycatch	2015	11	105 (5)	91 (3)	96 (5)	29 (1)	2 (1)	264 (15)
Mass. Bay	Bycatch	2016	10	68 (4)	50 (3)	76 (4)	24 (1)	6 (1)	263 (14)
Mass. Bay	Bycatch	2017	23	163 (9)	160 (8)	136 (8)	28 (2)	5 (0)	429 (28)
Mass. Bay	Bycatch	2018	2	119 (5)	119 (5)	61 (1)	43 (1)	76(4)	162 (5)
Mass. Bay	Bycatch	2019	19	172 (10)	119 (7)	165 (10)	38 (2)	7 (1)	573 (41)

Frequency of plastic occurrence, or FO, varied by location, sampling year, and age of birds, with the lowest FO observed in breeding phase individuals from Gough and Nightingale Island (~60% in adults and 58% in chicks, respectively), ranging up to 100% in the NW Atlantic and off the coast of Brazil (Table 2).

Table 2.

Frequency of occurrence (FO) of plastic ingested by Great Shearwaters, by location, year, and age, along with percentage of individuals per collection location and year that contained greater than 100 mg of ingested plastic. FO by age class was only calculated using all Massachusetts Bay birds from all years to ensure a sufficiently large sample set. Hatch year birds are a subset of young birds that were confidently identified as first year birds based on bursa presence and molt patterns, see Table S4. SE NC = Southeast North Carolina, USA.

Location	Year	Age Class	# birds	FO (%)	% containing >100 mg of plastic
Gough	2012, 2018	Mature	25	60	44
Nightingale	2019	Chick	50	58	90
Brazil	2019	Juvenile, Mature	13	100	46
SE NC	2017	Juvenile	15	100	60
Mass. Bay	2008	Juvenile	1	100	100
Mass. Bay	2010	Juvenile, Mature	17	94	31
Mass. Bay	2011	Juvenile, Mature	12	92	44
Mass. Bay	2012	Juvenile, Mature	27	89	48
Mass. Bay	2014	Juvenile, Mature	8	100	50
Mass. Bay	2015	Juvenile, Mature	13	85	45
Mass. Bay	2016	Juvenile, Mature	10	100	20
Mass. Bay	2017	Juvenile, Mature	29	94	62
Mass. Bay	2018	Juvenile	2	100	50
Mass. Bay	2019	Juvenile	19	100	58
Mass. Bay	All	Juvenile	104	94	51
		Hatch Year	53	98
		Mature	23	87

Plastic item morphology

The largest ingested item by length measured 101.5 mm in its largest dimension, associated with a hard rigid fragment found in a harvested chick from Nightingale Island. The largest plastic item by length and width was a soft sheet-like plastic found in a bycatch juvenile from Massachusetts Bay, measuring 1595 mm² when extended fully for measurement. Consumer use plastics dominated, making up 91% of plastic items across the entire sample set (range 75 – 100%). Industrial plastics made up a small proportion of observed plastics from each year and location, with no significant changes in the percent of industrial plastics over time in Massachusetts Bay birds (mean = 10%, range = 5–25%). (Fig. 2).

Figure 2.

Proportions of user and industrial plastic observed in gastrointestinal samples of Great Shearwaters Ardenna gravis from each location, and across multiple years in Massachusetts Bay. Night. = Nightingale Island and SE NC = Southeast North Carolina, USA.

Proportions of each user plastic morphological category were assessed as a percentage of the total count of user plastic items measured within a given year and location. Category ratios differed between years in Massachusetts Bay (Fisher’s exact test, two-sided, p < 0.001), but were consistently dominated by fragments (mean = 85% fragments, range = 70 – 100% fragments) (Fig. 3). Fragments likewise made up the largest proportion of items across all locations (mean length = 7mm, mean width = 5mm). Category ratios marginally non-differed between all collection locations (X² _{16, 1954} = 25.17, p = 0.067) when considering all plastic items identified from each location, minus items found in one clear outlier from Gough Island (Fig. 3). Removing fragments from the comparison, the remaining category ratios were significantly different between collection locations (X² _{12, 231} = 24.42, p = 0.018), likely driven by a higher incidence of sheet-like plastics in Massachusetts Bay and Brazil, as well as a higher incidence of thread-like and “other” plastics in Massachusetts Bay.

Figure 3.

Proportions of each plastic morphology, as a percentage of the total count of plastic items measured within a given year and location. The summary of the Gough Island dataset presented herein omits one clear outlier that possessed 104 plastic pieces of greater category diversity than observed across other mature birds from Gough Island. SE NC = Southeast North Carolina, USA.

Spatial differences in plastic item size

Size of fragments differed between locations, as birds from the South Atlantic contained larger (by length and length x width) fragments compared to items found in individuals from the NW Atlantic (S. Atlantic mean length = 9.0 mm, NW Atlantic mean length = 6.8 mm, Kruskal-Wallis chi-squared = 105.29, df = 1, p < 0.001 – S. Atlantic mean length x width = 67 mm², NW Atlantic mean length x width = 30 mm², Kruskal-Wallis chi-squared = 171.5, df = 1, p < 0.001) (Fig. 4). These differences were likewise apparent when the comparison was restricted to plastic items found in mature birds from Massachusetts Bay compared to items found in mature birds from Gough Island, with mature birds from Gough Island containing larger items than mature birds from Massachusetts bay (Gough mature mean length = 8.0 mm, Mass. Bay mature mean length = 6.3 mm, Kruskal-Wallis chi-squared = 4.1912, df = 1, p = 0.040 – Gough mature mean length x width = 40 mm², Mass. Bay mature mean length x width = 32.1 mm², Kruskal-Wallis chi-squared = 4.328, df = 1, p = 0.037). Likewise, items found in hatch year birds from Massachusetts Bay were significantly shorter in at least one dimension and possessed decreased length x width compared to items found in chicks from Nightingale Island (Mass. Bay hatch year mean length = 7.0 mm, Nightingale chick mean length = 9.2 mm², Kruskal-Wallis chi-squared = 79.27, df = 1, p < 0.001– Mass. Bay hatch year mean length x width = 32.5 mm², Nightingale chicks mean length x width = 70.2mm², Kruskal-Wallis chi-squared = 138.44, df = 1, p < 0.001).

Figure 4.

A) Length and width and B) length × width and length × width × thickness of plastic fragments found in digestive tracts of Great Shearwaters Ardenna gravis. Colors and shapes indicate collection region. 95% of the data is displayed, with the upper 5% omitted for data and axis legibility. Bold circles in each plot present the mean, and associated error bars present 95% confidence intervals. SE NC = Southeast North Carolina, USA.

Patterns in polymer identity between regions and over time

Ingested plastic items from all locations were primarily composed of polyethylene (60 – 89% of items per location) (Fig. 5). Polypropylene and polypropylene mixes were the second and third most abundant polymer types, respectively. Polymer signature of ingested plastic items varied between collection locations and age classes, with Mass. Bay birds generally containing more diverse plastic types (Fig. 5, Table S3). The polymer make-up of plastic items found in mature individuals from Gough Island was significantly different compared to the polymer composition of plastic items found in mature individuals from Massachusetts Bay, likely driven by higher proportions of polypropylene and polypropylene mixes in Gough Island adults. Similarly, the composition of plastic items found in chicks from Nightingale Island was significantly different compared to hatch year individuals from Massachusetts Bay, likely driven by increased ingestion of polypropylene in Nightingale chicks coupled to increased abundance of rubber and resin polymers in Massachusetts Bay hatch year birds. Hypothesis tests and related statistics exploring differences in polymer data are reported in detail in Table S3.

Figure 5.

Polymer composition of plastic items ingested by Great Shearwater Ardenna gravis grouped by year and sampling location. Night. = Nightingale Island and SE NC = Southeast North Carolina.

Demographic, spatial, and temporal trends

Plastic count and mass were not significantly different between male and females, in any sampled location. In Massachusetts Bay juveniles contained more plastic pieces compared to adults from the same region (Juvenile mean = 9.3 items, adult mean = 6.3 items, Dunn’s test, p = 0.034); juveniles also contained marginally higher masses of plastic compared to regional adults (Juvenile mean = 162 mg, adult mean = 149 mg, Dunn’s test, p = 0.073). Breeding phase adults in the South Atlantic contained a significantly higher mass of plastic compared to their North Atlantic adult counterparts (S. Atlantic mean = 236 mg, NW Atlantic mean = 149 mg, Kruskal-Wallis chi-squared = 6.0179, df = 1, p= 0.049), but not a higher count of plastics. Chicks from Nightingale Island contained a higher number of plastic pieces compared to hatch year birds from Massachusetts Bay (Nightingale chick mean count = 22, HY Mass. Bay count = 10, Kruskal-Wallis chi-squared = 7.8502, df = 1, p = 0.005 – Nightingale chick mean mass = 1294 mg, HY Mass. Bay mean mass = 171, Kruskal-Wallis chi-squared = 24.816, df = 1, p > 0.001). Considering the entire sample set, juvenile birds contained higher masses and counts of plastic compared to adults (Juvenile mean count = 11, juvenile mean mass = 354 mg, adult mean count = 8, adult mean mass = 214 mg, Dunn’s tests, both p < 0.001). Individuals from all places contained more than 100 mg of accumulated plastic, with an overall 51% of birds containing more than 100 mg of ingested plastic (Table 2).

No statistically significant trends in number of plastic pieces or mass were detected over 2010 – 2019 considering all Massachusetts Bay individuals, potentially related to low sample size and resulting variability apparent in some years (e.g. 2014, 2016, 2018). Only considering those years with including more than 15 birds from Massachusetts Bay, plastic mass increased over time between 2010 and 2019 (R_s² = 0.87, p = 0.044).

Plastic in sand lance

We found no visibly identifiable plastic fragments of the same size range or morphology as those found in Great Shearwater stomachs. A single thread-like item was found in one fish; this item was not confirmed as plastic via FTIR due to COVID-19 limitations.

4. Discussion

Ingested plastics compared to prior studies

Dating back to the early 1980s, multiple studies have documented a high frequency of plastic ingestion in Great Shearwaters across the entirety of their range. For example, Furness (1983) documented a 90% FO in the Benguela Current, while Ryan (1987a) documented a 95% FO in individuals from Gough Island. Overall, Moser and Lee (1992) reported FO = 64% (n = 55 individuals) from the NW Atlantic Ocean, collected between 1975 – 1989 (Moser and Lee, 1992). More recently, Bond et al. (2014) documented a FO = 88% (n = 84) on beach-cast shearwaters on Sable Island, Nova Scotia, Canada from 2000 – 2011(Bond et al., 2014a). On a year-by-year basis we report similar or slightly higher FO values here (range = 58–100%) compared to prior work (range = 0–95%) (Table S5), suggesting Great Shearwaters are frequently ingesting plastic and have been prone to do so for at least four decades. The range of FO values observed within this study also coincides with reported FO values for short-tailed shearwaters (63–90%) (Acampora et al., 2014; Roman et al., 2016) and flesh-footed shearwaters in the Pacific (0–100%) (Hutton et al., 2008; Lavers et al., 2014), emphasizing the prevalence of plastic ingestion in procellariiform seabirds across the globe. FO values reported in this study were higher than those observed for wedge-tailed shearwaters in Australia (0–43%) (Hutton et al., 2008; Lavers et al., 2018; Roman et al., 2016; Verlis et al., 2013) and sooty shearwaters in the Pacific and Atlantic (0–72%) (Barbieri, 2009; Bond et al., 2014b; Brown, 1981; Furness, 1983; Terepocki et al., 2017b). The rationale behind variable FO between similar large shearwaters remains unclear, but Hutton et al. (2008) hypothesized such differences may be related to variability in foraging strategy and/or foraging range, with higher environmental abundance of plastics leading to higher rates of ingestion (Hutton et al., 2008). However, we acknowledge that sample collection and plastic analysis procedures varied between this study and other work (Table S5), and therefore these comparisons of FO may be confounded to some degree.

The number of plastic pieces and the mass of ingested plastic varied with location, but the range observed generally overlapped with previously published metrics published for this species, suggesting consistent vulnerability to plastic ingestion (mean number of plastics range = 4 – 22 items, mean mass plastics range = 68–1294 mg, prior studies mean count range = 6–16 items, prior studies mass range = 236 – 2510 mg) (Bond et al., 2014a; Moser and Lee, 1992; Ryan, 1987a, 1987b) (Table S5). Additionally, juveniles ingested more plastic by mass and count compared to adults as seen in prior work in short-tailed shearwaters in eastern Australia (Acampora et al., 2014), underscoring the increased sensitivity of naïve, juvenile birds to plastic ingestion compared to adults.

Prior work in this species has demonstrated changes in composition and number of plastics over time (Ryan, 2008). No trends were apparent in count or mass over time within our complete time series data set or when compared to prior work, but we note that temporal trends warrant continued investigation, as the mass of plastic increased over time in Massachusetts Bay birds when only considering those years with large sample sizes.

Measurements of plastics in prey and surface water do not explain the composition of ingested plastics in Great Shearwaters

Limited data details the occurrence of different plastic categories in surface water within the Gulf of Maine and South Atlantic, as most large-scale datasets present abundance by size rather than plastic piece category or polymer type (Eriksen et al., 2014). However, 2013 surface water surveys conducted near Isles of Shoals in the SW Gulf of Maine found microfibers dominated (84%) in the surface layer (~1 m) compared to fragments (14%) (Lindeque et al., 2020). Although limited in spatial coverage, this survey dataset suggests the composition of plastics found in the surface ocean does not align with the composition of plastics ingested and accumulated by Great Shearwaters in the region, which contained ~85% hard fragments. This suggests Massachusetts Bay birds do not inherit their consistent plastic category signature from their environment and instead seek out plastic fragments selectively, as opposed to accumulating plastics based on likelihood of encounter in the environment. This selectively has likewise been observed in short-tailed shearwaters from Eastern Australia that were found to preferentially select fragments, balloons, and rubber compared to an abundance of soft plastics and thread-like plastics in the environment (Acampora et al., 2014).

Additionally, stomach content analyses performed on 202 Ammodytes spp. found only one thread-like item; this species is the primary forage item of Great Shearwaters during their non-breeding period in the Gulf of Maine. This suggests that Great Shearwaters in Massachusetts Bay are not deriving their accumulated plastic loads, particularly mesoplastic fragments, from trophic transfer. This finding corroborates evidence from other seabirds suggesting transfer of plastics between fish prey and seabird predators may be limited (Carbery et al., 2018; Nelms et al., 2018). We believe these data empirically indicate that Great Shearwaters seek out plastic items directly from their environment, ingesting a high proportion of rigid fragments despite their low or negligible abundance in the local environment or primary prey. This selection bias may be related to the interplay of visual and olfactory cues that inform seabird foraging. Polyethylene and polypropylene have been previously shown to assimilate dimethyl sulfide (DMS) in situ; DMS is a critical infochemical that serves as a proxy of productivity and subsequently a foraging cue for DMS-responsive marine predators like Procellariiform seabirds (Pfaller et al., 2020; Savoca et al., 2017, 2016). Great Shearwaters and other olfactory predators may therefore be generally lured by the DMS-laden scent of large plastic items or high densities of plastic in the environment (Savoca et al., 2016), and once in visual range, other characteristics of ocean-borne plastic (e.g. color, size, movement, similarity to prey aspect, fish aggregating ability) likely drive the final choice to consume the item (Nishizawa et al., 2021; Roman et al., 2020, 2019d, 2019a; Vlietstra and Parga, 2002).

Size-mediated environmental distribution of plastics is reflected in Great Shearwater plastic ingestion

Chicks, juvenile, and adult Great Shearwaters collected in the South Atlantic contained larger plastic pieces compared to their North Atlantic counterparts, with chicks containing the largest plastics overall, the highest counts of plastic, and the highest masses of ingested plastic. The increased abundance of larger plastic items in South Atlantic birds and chicks in particular is problematic as larger plastic items pose an increased risk of physical impairment or injury (Roman et al., 2019b); potential harm may be particularly deleterious to migratory success of post-breeding adults in poor body condition or naïve chicks making their first northern migration.

This abundance of larger plastic fragments may be related to size-mediated distribution of plastic items in the surface ocean (Ryan, 2015b; Van Sebille et al., 2020), coupled to the distinct foraging ranges of Great Shearwaters in their breeding and non-breeding phases. While Great Shearwaters are highly mobile predators, tracking data reveals core foraging ranges within their respective breeding and non-breeding ranges. During the breeding season, adults utilize vast expanses of open ocean habitat, alternating between short and long foraging trips during incubation and chick-rearing (Schoombie et al., 2018). Utilization density estimates suggest these foraging activities center around Gough, Inaccessible, and Nightingale Islands in the remote South Atlantic Ocean and the Antarctic Polar Front, 1000s of km from major population centers and continental margins (Ronconi et al., 2018; Schoombie et al., 2018). Additionally, Great Shearwaters forage along the Patagonian Shelf during the breeding season and as a migratory staging area; the adjacent land area is sparsely populated, with an estimated population of 4 inhabitants/km² (Gil et al., 2019). Alternatively, during the non-breeding season Great Shearwaters forage in the Gulf of Maine, frequenting Massachusetts Bay, George’s Bank, and the coastal margins of NW Canada. Tracking data indicates core foraging areas exist proximate to highly urbanized coastal margins of the northeast USA, near the major metropolitan center of Boston, MA and the greater Washington – Boston megalopolis region (Powers et al., 2020, 2017). Circulation in these key foraging regions and across the wider Gulf of Maine is driven by local and regional processes (Li et al., 2014; Townsend et al., 2014). Therefore surface water environments in the Gulf of Maine that support Great Shearwaters likely reflect a higher abundance of regionally-sourced plastics from highly developed land margins (Brooks, 1992; Lindeque et al., 2020; Pershing et al., 2015). Additionally, coastal surface waters often host smaller, more polymerically diverse, and less buoyant plastic items compared to items found in open ocean environments (Erni-Cassola et al., 2019; Fazey and Ryan, 2016; Lusher, 2015; Ryan, 2015b). Smaller plastic items, often associated with coastal environments, typically possess higher surface area:volume ratios and settle out of the water column rapidly with increasing distance from coastal regions due to biofouling and other biophysical processes (Van Sebille et al., 2020); this size-mediated settling process has the potential to influence the distribution of plastic items in the size range ingested by Great Shearwaters (Kooi et al., 2017). Larger, more buoyant pieces preferentially remain in the surface ocean to circulate to more remote regions (Ryan, 2015b).

The starkly different environmental characteristics of breeding vs nonbreeding ranges have previously been demonstrated to expose Great Shearwaters to higher levels of human-derived pollutants compared to other seabirds from the same breeding location that solely rely on South Atlantic environments (Robuck et al., 2020; Roscales et al., 2019). We believe a similar dynamic is driving the physical characteristics of plastic items observed in this study. We hypothesize that Great Shearwaters are more likely to encounter a higher proportion of larger, rigid plastic items during their breeding and post-breeding season when they are primarily utilizing pelagic, remote environments in the South Atlantic; larger, low surface area: volume items are preferentially retained in the water column at great distance from coastal margins through size-mediated, oceanic-scale dispersal processes. Conversely, Great Shearwaters during their non-breeding phase forage adjacent to urbanized coastal margins in the NW Atlantic, and have the opportunity to ingest and accumulate smaller, more diverse, or less buoyant plastic items that have not yet settled out of the water column due to proximity to coastal sources. In our data, these hypotheses are supported by the distinctly large item size of plastics ingested by chicks and adults at the breeding colony and found beachcast in Brazil (Fig. 4), and to a lesser extent, the reduced FO seen in individuals from the breeding colony. Moreover, plastic item type and morphology has been observed to vary between seabirds collected inshore versus those collected offshore as bycatch, with sheet-like plastics more common in beachcast, inshore samples while rigid fragments and fibers dominated in individuals collected offshore (Roman et al. 2020). Ostensibly, such item distributions reflect size- and buoyancy mediated environmental distribution, with smaller items or items with high surface area:volume ratios less likely to migrate into offshore oceanic environments. Our data mirrors this, with a higher percentage of sheet-like, thread-like, balloons, and other non-fragment items in Massachusetts Bay individuals compared to individuals from Gough and Nightingale Islands (Mass. Bay mean FO of non-fragment items = 16%, Nightingale mean = 7%, Gough mean = 10%, - Dunn’s test, p < 0.01). We acknowledge that wear in the gizzard and intergenerational transfer could also play a role in the size and morphology of plastic items observed between locations. However, as described below, we believe our polymer and category observations strongly suggest turnover of the predominant plastic signature at rapid (4–6 month) time scales, suggesting these factors are not the dominant drivers of size dynamics discussed here. We also note that chick-provisioning behavior could influence the size of plastic items found in chicks and adults in the South Atlantic, with adults more likely to select larger items to provision chicks compared to non-breeding prey/item selection. More concerted research is required to rigorously explore this hypothesis and its ramifications for plastic ingestion and related risk assessment in remote and coastal marine fauna.

Differences in polymer signature between locations suggests plastic items likely turn over within 4–6 months in vivo

Despite an increasing amount of research documenting seabird plastic ingestion, the retention time of plastic items in seabirds remains largely unclear. Species- and item-specific factors confound general estimates of retention time, as this metric varies based on co-ingested diet items present in the GIT, species-specific size thresholds required to pass items through ventriculi to the lower GIT, initial item size, item composition, and rate of wear within the GIT (Ryan, 2015a). Fragments, the dominant morphology found in Great Shearwaters within this study, may also be prone to reduced retention times due to increased exposure to UV and other environmental stressors that may degrade structural integrity and resilience prior to ingestion (Ryan, 2015a). Observational evidence underscores the complex nature of this question. Van Franeker and Law (2015) suggested Northern Fulmars depurate approximately 75% of their plastic load within one month based on assessment of plastic loads in fulmars between their breeding and non-breeding locations (Van Franeker and Law, 2015). However, their inference did not account for possible transfer from adults to chicks during chick-rearing, which may impact the incidence of plastics in breeding phase adults via offloading to chicks. Alternatively, direct studies of plastic pellet breakdown in seabirds or in controlled environments suggest slow breakdown of plastics in vivo or in situ (Chamas et al., 2020; Ryan, 2015a); for example, virgin industrial pellets fed to White-chinned Petrel (Procellaria aequinoctialis) fledgings lost only ~1% of their mass over 12 days within the GIT, making break down within a month improbable and suggesting a half-life of over a year (Ryan and Jackson, 1987). However, no studies to date have empirically measured the retention time of plastics in Shearwaters or other procellariiform seabirds using weathered plastics, which are considerably more fragile compared to virgin plastic items of the same polymer type (Min et al., 2020).

While we are unable to account for all variables that may influence in vivo plastic retention time or directly measure this phenomena, our data further refines current hypotheses by suggesting that a significant proportion of plastic fragments retained in Great Shearwater ventriculi turnover within 4–6 months, as items collected in Great Shearwaters from disparate locations approximately 4–6 months apart reflected significantly different polymer signatures and morphological categories (Table S3, Fig. 5). In order for these signature differences to occur, a significant portion of accumulated ingested plastics must be replaced by new items of different morphology and polymer type, ostensibly via egestion and ingestion of new items. Changes in polymer and category signature between locations could not be explained by dilution of an existing plastic signature by accumulation of more plastic items (e.g. apparent lower proportion of PP mixes in North Atlantic birds due to accumulation of more plastics overall), because birds from most locations contained similar or lesser counts of plastic items (Table 2). Moreover, chicks contained some of the largest plastic items that would ostensibly take the longest to breakdown; however, the chick polymer signature is not retained in hatch year birds collected in the North Atlantic, which contained more polyethylene, rubber, and resin-based materials. Chicks also contained a higher number and mass of plastics that was not reflected in hatch year birds in Massachusetts Bay (Table 1). Moreover, if plastics were retained indefinitely, we would expect homogenous FO in comparable life stages between locations, which is not the case (Table 2). Crucially, this suggests mitigation of plastic sources within both the North and South Atlantic are key to reduce the high incidence of plastic ingestion in this species, which occurs at levels well above ecological quality objectives thresholds developed for other seabirds in the Northeast Atlantic. Considering all plastic items, 84% of all items were composed of polyethylene and polypropylene, and thus originated from plastic types widely recyclable in the US and around the globe, indicating infrastructure and knowledge already exist to mitigate loads of these polymers into aquatic environments.

Industrial plastics generally remain a minor component of ingested plastic loads

The percentage of industrial plastics within this sample set (mean = 9%) was equal to or lower than the percentage of industrial plastics reported in previous studies in seabirds and specifically Great Shearwaters (Fig. 2) (Avery-Gomm et al., 2017; Bond et al., 2014a; Kühn et al., 2020; Moser and Lee, 1992; Ryan, 2008; Van Franeker et al., 2015). The reduced incidence of industrial plastics is likely related to decrease of industrial pellets in marine environments due to source reduction efforts over time (Van Franeker et al., 2015). However, the ratio of industrial pellets ingested by Great Shearwaters found on the coast of Brazil in 2019 (25%) was higher than other locations and more comparable to historical occurrence of industrial pellets in Great Shearwaters examined in the South Atlantic in 1985/1986 (up to 64%) (Ryan, 2008)mm.

Recommendations for future monitoring

Accumulation of ingested plastics significantly increases risk of mortality in seabirds, while the sublethal impacts of plastic ingestion are more poorly understood due to the range of factors that might influence the magnitude of harm (Roman et al., 2019b). To date, it is unclear how, if it all, plastic ingestion may be impacting the health of Great Shearwater populations, due to an overall lack of information about chronic adverse impacts, and the many confounding factors preventing clear investigation of impacts (Roman et al., 2021; Senko et al., 2020). However, over 50% of Great Shearwaters sampled in this study exceed an ecological quality objective for plastic ingestion in seabirds, established as an ecologically-prudent goal to minimize impacts from plastic ingestion in seabirds within the Northeast Atlantic (van Franeker, 2004; Van Franeker et al., 2015), and juvenile Great Shearwaters are most vulnerable to plastic ingestion across the entirety of their range. Our work highlights that plastic ingestion is a continued stressor of pelagic seabirds in the Atlantic Ocean. We recommend increased standardization of methodology and approach in studies evaluating plastics in Great Shearwaters, to more fully leverage the utility of this species as a compact and affordable indicator of plastics in subtropical and temperate Atlantic regions. Mass, count, and FO should be presented for each compartment of the GIT as a function of location, age, and sex given the highly variable mass, count, and FO reported between different ages and locations across time, following guidelines outlined previously for assessment of plastics in marine birds (Provencher et al., 2019). We particularly highlight the need for reporting plastic ingestion within each GIT compartment separately to facilitate increased use of bycatch samples within monitoring studies, as Great Shearwaters are the most abundantly bycaught species in the Gulf of Maine (Hatch, 2018; Hatch et al., 2016) and are also bycaught in the South Atlantic along the Patagonian Shelf (Bugoni et al., 2008). Bycatch samples are also considered healthy prior to death and lack bias potentially associated with beach-cast samples which may have been sickly or compromised leading to death (Colabuono et al., 2009; Rodríguez et al., 2018).

Great Shearwaters offer an abundant, feasible method to monitor future plastic dynamics in the wider Atlantic basin, as the subtropical North and South Atlantic are projected to experience continued accumulation of plastic pieces in the coming decades (Wilcox et al., 2020). Monitoring efforts focused on this species also present an opportunity to assess the efficacy of any future interventions to address plastic pollution, given the breadth of data presented here and in previous studies. We encourage transboundary cooperation to more fully develop monitoring of this species, using the body of work surrounding the Northern Fulmar in the North Atlantic as a model.

10. Data Availability Statement

Data not yet available, but will be made available prior to publication via the NOAA Marine Debris Program