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In the South Pacific Ocean, anthropogenic marine debris (AMD) originates from the surrounding landmasses and oceanic sources (Thiel et al., 2003, 2013; Kiessling et al., 2017). Especially in the eastern part of the S Pacific, litter pollution comes from land sources (e.g., Hidalgo-Ruz et al., 2018), beach-goers (Bravo et al., 2009), and marine activities including aquaculture (Astudillo et al., 2009; Hinojosa and Thiel, 2009), coastal (Perez-Venegas et al., 2017) and high seas fisheries (Kiessling et al., 2017). Rivers also contribute large amounts of macro- and microplastics (Rech et al., 2014, 2015).

The majority of the litter from land sources is probably trapped in coastal waters or on shores very close to its sources (Hinojosa and Thiel, 2009; Thiel et al., 2011; Rech et al., 2014). The low incidence of marine organisms growing on marine litter stranding on the continental coasts of the SE Pacific suggests that this fouling-free litter has likely passed very little time (if at all) at sea. On the other hand, strong offshore currents within the HCS are thought to move floating plastics quickly to the open ocean, where they become trapped in the SPSG (Martinez et al., 2009).

Given the multiple sources and transport dynamics of floating AMD in the SE Pacific, the density of macroplastics is high in immediate coastal waters, but rapidly decreases further away from the continental coast (Thiel et al., 2003; Miranda-Urbina et al., 2015). When approaching the center of the SPSG, densities of floating AMD reach very high abundances, as also reflected in recent data taken in two surveys conducted in 2015 and 2016 (Figure 1). A total of 477 items of floating AMD were observed across both surveys, the majority corresponding to 2015 when a major area including the Easter Island ecoregion was surveyed (Figure 1). Of the total floating AMD, 77% were macroplastics, most of which were large plastic fragments (95.4%). Other items included lines (17.7%), buoys (7.6%), plastic trays (4.9%), and plastic bags and nets (2.5 and 2.2%, respectively) (Figure 1). Accounting for the total number of AMD in both years, it seems that the distribution of floating AMD is comparable to patterns previously determined for the SE Pacific (see Miranda-Urbina et al., 2015). Out of the total (477 items), 8% were found in the HCS, only 3% occurred around Juan Fernandez and Desventuradas Islands, while 78 and 11% of all floating AMD were concentrated in the Oceanic and Polynesian sector, respectively. Large numbers of marine litter accumulate on beaches of Rapa Nui (Easter Island) and the uninhabited island Salas and Gómez in the Polynesian sector (Kiessling et al., 2017). Furthermore, marine litter on Salas and Gómez imposes a severe risk of entanglement for several seabird species breeding on the island (Miranda-Urbina et al., 2015).

The distribution of floating microplastics in the SE Pacific shows the typical distribution documented for other ocean basins (Eriksen et al., 2014; Law, 2017), with highest concentrations in the subtropical gyre (Eriksen et al., 2013, 2018). Microplastic densities in the HCS generally remain far below the densities reported from the gyre (Figure 2; NO, unpublished data). This distribution pattern is also well reflected in concentrations of small plastics from sandy beaches in the SE Pacific, where beaches along the HCS feature moderate densities, while sandy beaches on Rapa Nui have very high densities (Hidalgo-Ruz and Thiel, 2013). This pattern, with the abundances of microplastics increasing with distance from the continental coast (Figure 2), is also suggestive of progressive fragmentation of large plastics during their journey toward the SPSG.

A large number of marine vertebrate species from the SE Pacific were documented to have interacted with marine litter, including fishes, seabirds, sea turtles, and marine mammals (Thiel et al., 2011; Miranda-Urbina et al., 2015; Ory et al., 2017). In revising and compiling information from diverse sources we found that marine litter affects at least 97 different species (see Table 1 and text below for details). These reports comprise those of entanglement (including incorporation of plastics in seabird nests) and the ingestion of plastics.

This first review of interactions with AMD of marine vertebrates from the SE Pacific reported a total of 97 species (Table 1). Seabirds represent nearly 55% of the total, followed by fishes with 21%, marine mammals with 19%, and sea turtles with 5%. Considering the number of species per taxonomic group, the type of interaction (ingestion or entanglement), and environment (continental or oceanic), an overall pattern is emerging. For fishes, more species with plastic ingestion were documented along the continental coast than in the open ocean, and few oceanic species become entangled near oceanic islands (Rapa Nui and Salas and Gómez Island). However, it needs to be taken into account that the simple species list presented in Table 1 only shows whether a species had ingested microplastics or not, which does not allow for inferences on the individual risk of ingestion. For example, in the case of the Peruvian anchoveta Engraulis ringens from the HCS, of 116 studied individuals only one (<1%) had one microplastic in its stomach (Ory et al., 2018), whereas of 20 amberstrip scads Decapterus muroadsi from Rapa Nui 16 individuals (80%) had ingested on average 2.5 microplastics per individual (Ory et al., 2017), underscoring that the risk of microplastic ingestion is much higher for oceanic planktivorous species than for species from the HCS. In contrast to the general pattern documented for fishes, the incidence of microplastic ingestion in seabirds is much higher for oceanic species than for those from the HCS, but the latter are suffering intense entanglement (Table 2). Marine mammals are scarce in oceanic waters, so marine mammals are principally affected by entanglement with AMD floating in the productive waters of the HCS (Table 2). Regarding sea turtles, none of the species from the SE Pacific are safe from interactions with AMD, ingestion or entanglement, and thus are equally threatened in coastal and oceanic waters. This overall pattern agrees well with the spatial distribution of micro- and macroplastics in the SE Pacific (see Figures 1, 2). Future systematic studies in the SE Pacific should provide more information about the actual number of species affected, which will also help authorities to improve efforts for efficient solutions.

Similar to other oceans (Carson, 2013), in the SE Pacific litter is frequently bitten by large marine organisms, which are thought to be vertebrates (Eriksen et al., 2017), but might also originate from large invertebrates with powerful jaws, in particular squids and/or cuttlefish. On Rapa Nui, up to 10% of stranded AMD can have bitemarks (MT, unpublished data).

The results from this review indicate that microplastic ingestion is uncommon along the Pacific coast of South America. Neither fishes nor seabirds from the continental coast had high frequencies of microplastic ingestion, and information on the diet of marine mammals in the SE Pacific is very limited and thus does not allow inference on the risk of microplastic ingestion. An exception from this pattern seems to be the relatively high incidences of plastic ingestion in sea turtles in the HCS, but most of those plastics are of larger sizes and can be characterized as meso- and macroplastics. Also, intertidal habitats, such as beaches, tidepools, estuarine saltmarshes, and especially the seashores in the fjords of southern Chile require future research attention, because intense microplastic pollution may cause localized impacts in species from shore habitats (see e.g., Mizraji et al., 2017).

The low incidences of microplastic ingestion in most marine vertebrates from the HCS could be resulting from low concentrations of microplastics in coastal waters or due to specific foraging behaviors, or a combination of both. A data comparison from the SPSG and from other parts of the world can shed some light on these questions. For example, some planktivorous fish species from the SPSG had a very high frequency of microplastic ingestion (Ory et al., 2017), and other species had ingested microplastics more frequently than any of the planktivorous species from the HCS (Chagnon et al., unpublished data; Ory et al., 2018). For seabirds, the pattern was similar: microplastic ingestion was much more common in oceanic species than in species from the HCS (Tables 1, 2), regardless of feeding types (planktivorous, invertivorous, and piscivorous). This suggests that the observed pattern might not be due to differences in foraging and feeding behaviors, but rather to differences in microplastic abundances. Nevertheless, plastic ingestion was also documented in a number of species from the HCS, which might be consequence of their biology. For example, incidence of plastics in the digestive system of different seabird species is related to their foraging behavior (e.g., surface feeders, pursuit feeders, among others) and the digestive system morphology (i.e., muscular gizzard in petrels) (Ainley et al., 1990; Spear et al., 1995; Roman et al., 2016). This latter aspect causes indigestible items to become trapped in the bird's gut system (Ainley et al., 1990).

Other factors such as the geographic distribution may impact the amount of plastic ingested, as found by Spear et al. (1995), who reported that seabirds foraging predominantly in the North Pacific had higher incidence of ingestion than species from the South Pacific. However, the paper suggested that this pattern may be biased by the lack of studies from the SE Pacific. This is very similar for marine mammals, in particular dolphins and whales. Since the end of whaling in the region during the early 80's, the scientific examination of body parts (including stomachs) of large baleen and sperm whale carcasses have considerably ceased in the region. Scientific treatment of stranded animals is limited due to the difficult logistics needed to examine cases of cetacean strandings in remote areas (e.g., Haro et al., 2015; Häussermann et al., 2017). In this regard, there is an urgent need for the implementation of effective action plans for the scientific treatment of stranded whales and dolphins, if we aim to understand the impacts of the ingestion of marine litter in megafauna. This is particularly important since researchers are adopting non-invasive methods (e.g., stable isotopes, DNA analysis) for both live (Haro et al., 2016) and stranded charismatic animals. At present, it is difficult to understand the magnitude of the plastic ingestion problem for marine mammals in the SE Pacific. However, plastic ingestion by diverse cetacean species has been reported in several coastal areas elsewhere (Baulch and Perry, 2014; Lusher et al., 2018), often with lethal consequences (see Jacobsen et al., 2010; de Stephanis et al., 2013; for cases on sperm whales). Microplastics of several polymer types have been documented for the first time in a humpback whale stranded on the coast of the Netherlands (Besseling et al., 2015). In the SE Pacific, the problem is likely being underestimated.

The relatively high frequency of bitemarks in plastics stranded on Rapa Nui shores indicates that some species directly bite into floating plastics. It is currently not well known which species engage in this behavior and why (Carson, 2013), but most of the bitemarks found on plastics from Rapa Nui resemble those of the green turtle Chelonia mydas (Eriksen et al., 2017), a species commonly reported to ingest larger plastic pieces (see above). The potential risk of plastic ingestion as a result of biting into floating plastic litter is reason for concern.

Entanglement reports of fishes are very rare, while they are common for seabirds, marine mammals, and sea turtles. The lack of reports from fishes might be due to the fact that mortality at sea would immediately cause sinking, whereas seabird, mammal, and sea turtle carcasses float at the sea surface; not surprisingly many reports of entanglement come from dead animals (see above).

From all the records of top fish predators interacting with plastic debris around the world, carcharhinid sharks seem to be most at risk of entanglement (Laist, 1997; Sazima et al., 2002; Ceccarelli, 2009), probably due the high abundance and species diversity in this group (Compagno, 1984). Cliff et al. (2002) reported an increase over time in entanglement of a carcharhinid species from South Africa, which furthermore underscores that species from this group are at highest risk of negative interactions with floating litter. Oceanic shark species are also likely to be impacted by plastic debris, but the limited number of studies on oceanic sharks underscores that more research is required to determine the full extent of this problem.

Seabird entanglement is common in the world's oceans (Kühn et al., 2015). Herein entanglement was reported mostly for species from the HCS, and observations included fisheries litter (nets, lines) and consumer plastics (mostly plastic bags). While interactions with fisheries items are likely to happen at sea (e.g., Moore et al., 2009), entanglement with consumer plastics may occur on the shore or at waste disposal facilities, as highlighted by the frequent observations of kelp gulls L. dominicanus with plastic bag entanglement (Table 1). Our data suggest that entanglement is more common in species from the continental coast than in oceanic species. The fact that Procellariiformes (which are mostly oceanic species) have the lowest frequencies of entanglement (Kühn et al., 2015) seems to support this pattern. However, herein we observed plastic litter in the nests of several oceanic species (Table 1), which underscores the imminent risk of entanglement for these species.

Although sea turtle entanglement in AMD has been recognized as a cause of mortality globally, there are quantitative knowledge gaps on rates and population implications (Duncan et al., 2017). Recent studies report entanglements across all species, life stages, and ocean basins, with higher vulnerability in pelagic juveniles (Nelms et al., 2016; Duncan et al., 2017). However, reports of entanglement incidents in the scientific literature are scarce and it is likely that many individual cases are never published, and therefore these data may be highly underestimated (Nelms et al., 2016). Duncan et al. (2017) reported that derelict fishing gear contributed globally to the majority of entanglements, while debris from land-based sources contributed to a lesser extent. Entanglements are a greater threat to sea turtles than climate change and direct exploitation, but less of a threat than plastic ingestion and bycatch (active gear) (Duncan et al., 2017).

Incidences of entanglement were reported for many species of marine mammals from the SE Pacific (Table 1), mostly with items of fisheries origin. Similar to other areas of the southern hemisphere (e.g., Page et al., 2004), sea lions seem to be most at risk of entanglement with derelict fishing gear as indicated by several independent observations recorded herein (Table 1). However, our review also reveals that many whale species from the HCS become entangled in (active?) fishing gear. Some efforts are being conducted to mitigate the entanglement problem. In Peru, the implementation of acoustic alarms (pingers) has proven to have dissuasive effects, thus reducing dolphin entanglements (Mangel et al., 2013). The use of modified long lines in the Patagonian toothfish (Dissostichus eleginoides) fishery in southern Chile reduces the interaction of killer and sperm whales with active fishing gear (Moreno et al., 2008). Yet, these efforts are only localized and eventually should be implemented at the whole scale of the respective fisheries.

Several studies highlight that risk of both ingestion and entanglement is highest where main foraging grounds overlap with accumulation areas of floating AMD (Wilcox et al., 2013; Fossi et al., 2017). Our study showed that some species foraging in the highly productive HCS frequently interact with marine plastics, which at first glance might be surprising given that litter densities are substantially lower than in the open ocean. However, many of these species feed in areas where hydrographic features, e.g., frontal systems or meso-scale gyres, concentrate food and also floating plastics (Pichel et al., 2007). Interestingly, one of the first direct observations of this phenomenon comes from the HCS off the central coast of Chile, where Bourne and Clark (1984) observed planktivorous seabirds feeding in a coastal front that also had concentrated large amounts of floating plastics. The high incidences of entanglement and also plastic ingestion, especially by sea turtles and some seabird species from the HCS, likely occurred in these temporary hotspots. These interactions are common in the productive upwelling systems of the eastern boundary currents (for overview see Scales et al., 2014), and cause high risk for marine vertebrates despite the fact that densities of floating litter are lower than in the subtropical gyres.

In the open ocean, especially in the oligotrophic subtropical gyres, marine productivity is low, and often concentrated above seamounts or near oceanic islands. If these islands are located within the range of the litter accumulation zones of the subtropical gyres, some species are at high risk of negative interactions with floating plastics (Figure 5). Our review showed that planktivorous fish and seabirds living on the oceanic islands in the vicinity of the SPSG have high incidences of microplastic ingestion, possibly due to the extraordinarily high densities of floating microplastics in this region (Figure 5). The limited number of entanglement reports from this area is likely a combination of lower densities of marine vertebrates in the subtropical gyres (see also Titmus and Hyrenbach, 2011) and the limited number of observers, compared to the continental coasts.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.