Seabirds of the New Zealand region of the Southern Ocean known to ingest plastic particles (sources : Jehl et al., 1980; Reed, 1981; Bourne and Imber, 1982; Furness, 1983, 1985; Randall et al., 1983; Day et al„ 1985; Brown et al., 1986; Skira, 1986; Harper and Fowler, 1987; Ryan, 1987b, c; Ryan and Jackson, 1987).
Blue Penguin (?) Rockhopper Penguin Adelie Penguin (?) Wandering Albatross Southern Royal Albatross Salvin's Mollymawk Black-browed Mollymawk Grey-headed Mollymawk Yellow-nosed Mollymawk Sooty Albatross Northern Giant Petrel Southern Giant Petrel Cape Pigeon Kerguelen Petrel Soft-plumaged Petrel Atlantic Petrel Blue Petrel Broad-billed Prion Antarctic Prion Salvin's Prion Thin-billed Prion Fairy Prion Great Shearwater Sooty Shearwater Little Shearwater Grey-backed Storm Petrel White-faced Storm Petrel White-bellied Storm Petrel Gannet (?) Southern Skua Black-backed Gull
Eudyptula minor Eudyptes chrysocome Pygoscelis adelíae Diomedea exulans Diomedea epomophora Diomedea cauta salvini Diomedea melanophris Diomedea chrysostoma Diomedea chlororhyrtchus Phoebetria fusca Macronectes halli Macronectes giganteas Daption capense Pterodroma brevirostris Pterodroma mollis Pterodroma incerta Halobaena caerulea Pachyptila vittata Pachyptila desolata Pachyptila salvini Pachyptila belcheri Pachyptila turtur Puffinus gravis Puffinus griseus Puffinus assimñis Garrodia nereis Pelagodroma marina Fregetta grallaria Sula serrator Catharacta antarctica Larus dominicanus ingested form of plastic pollutants in this region. Following the wreck of several hundred sea birds on beaches of the west coast of the North Island, New Zealand, in September 1981, Reed (1981) noted that all 27 Blue Petrels dissected contained plastic pellets. In contrast, only one of 26 Kerguelen Petrels contained a plastic pellet (Reed, 1981). Similarly, in Victoria, Australia, Brown et al. (1986) found plastic pellets in 60% of beach-washed Blue Petrels but only in one Kerguelen Petrel. Furthermore, Harper and Fowler (1987) have shown that between 1958
and 1977 over 15% of storm-killed prions recovered from beaches near Wellington contained plastic pellets, with incidence being greatest in local residents (Broad-billed Prion, Fairy Prion) and least in Indian Ocean migrants (Thin-billed Prion).
Larger fragments of plastic and items such as bottle tops and small fishing floats are also ingested (Day et al., 1985). Gregory (1987) reported a Salvin's Mollymawk chick on Proclamation Island (Bounty Islands) regurgitating a small handline type, orange fishing float at the foot of an observer, and various plastic objects (Fig. 11.9) are scattered around Southern Royal Albatross nesting sites on Campbell Island. It has also been noted that pieces of cordage and strips of blue coloured plastic, derived from packing straps, were incorporated into nesting mounds of Salvin's Mollymawk on both the Bounty and Snares Islands (Gregory, 1987). Regurgitations are not necessary evidence for extensive contamination of local waters, for albatrosses from Auckland Island are known to feed off Sydney, Australia (C.J.R. Robertson, pers. comm.). Although Day et al. (1985) commented that the phenomenon is not known in penguins, several pieces of shredded plastic and a small ball of thread partly buried amongst pebbles of Adelie Penguin nesting sites at Cape Adare, North Victoria Land, Antarctica, suggest that this species may also in rare instances ingest plastics. Careful inspection of several other large Adelie Penguin colonies around the Ross Sea revealed no further examples.
Whilst some species of seabird apparently ingest plastic randomly, the feeding strategies of many suggest selective preference for certain types of plastic (Day et al., 1985; Harper and Fowler, 1987). Plastic objects of all consumable sizes are probably mistaken for normally-sought prey. The encrusting biota is seldom present in other than minor amounts, suggesting it is not an important food objective. Prince (1980) has commented that red coloured toothpaste-tube tops and melamine laminate, regurgitated by Grey-headed Albatross chicks on Bird Island, South Georgia, could easily have been mistaken for reddish crustacean prey. Similarly many of the plastic items observed in Southern Royal Albatross regurgitations from Campbell Island sites are red or pinkish (Fig. 11.9). It has also been suggested that small colourless virgin plastic pellets could be taken on board as an unwitting substitute for the pumice granules that many seabirds use as cropstones. Granule- and larger-sized pieces of pumice are broadcast across surface waters of the Southern Ocean (Gregory et al., 1984b; Gregory, 1987) and frequently ingested by seabirds of the region.
Intuitively, many would argue that ingestion of plastics should be harmful to life, if not a significant cause of death. The direct effects, it has been suggested (e.g., Bourne and Imber, 1982; Day et al., 1985), could include intestinal blockage leading to starvation, or local ulceration of delicate internal tissue following damage by sharp and irregularly jagged objects. The "quality" of life and reproductive performance could also be detrimentally affected. On the other hand, it must also be acknowledged that plastic granules could behave as crop stones improving digestive efficiency through grinding action (Day, 1980).
The frequency with which seabirds swallow, and regurgitate, naturally occurring, hard and resistant objects such as pebbles, bones, nuts and squid beaks with little apparent ill-effect (Kenyon and Kridler, 1969; Rothstein, 1973; Hays and Cormons, 1974) suggests that plastic items are similarly unlikely to be of any great biological consequence (Gregory, 1978,1983; Bourne and Imber, 1982; Day et al., 1985). Whilst the presence of ingested plastics is, without question, of concern to all environmentalists, recent studies suggest that it seldom impairs digestive efficiency (Harper and Fowler, 1987; Ryan and Jackson, 1987) and the direct risk to adult birds is considered minimal (Fry et al., 1987). Larger plastic objects may well cause death of seabirds through intestinal blockage (e.g., Dickerman and Goelet, 1987), but, with pellets and granules, it is difficult to decide whether they are a cause or effect of starvation (Bourne and Imber, 1982; Harper and Fowler, 1987).
Although ingestion of discarded plastic items by larger marine animals appears to be less of a problem than entanglement (Laist, 1987), intestinal blockage leads to some deaths (cf. Wehle and Coleman, 1983). Cawthorn (1985) has cited two New Zealand examples. Leatherback turtles, although uncommon, are not rare visitors to these waters. Necroscopy of a specimen that died shortly after beaching near Whakatane, Bay of Plenty, in the summer of 1979-80 revealed an oesophagus packed by polythene bread bags. It is assumed these were mistaken for its normal prey of salps and medusoids, for marine turtles are known to have an appetite for synthetic drift items (Wehle and Coleman, 1983; Balazs, 1985). Similarly, a juvenile minke whale (Balaenoptera acutorostrata), stranded at
Palliser Bay east of Wellington in 1976, had a polythene bag stuck deep in its oesophagus.
Those very properties which mankind finds so highly desirable in plastics, lightness, strength, manufacturing adaptability and flexibility, together with relative inertness and resistance to degradational processes, are also the reasons why today they are a marine pollutant of global proportions. At the present time, it is difficult to estimate disappearance rates for plastics and other synthetic debris from the marine milieu (Gerrodette, 1985).
Gregory (1978,1983) cited evidence to suggest that, despite their unquestioned durability, raw plastic granules both afloat and left stranded and exposed on sandy shores at low to middle latitudes may have a life expectancy of five years or less. Ingested pellets have an expected half life of at least one year (Ryan and Jackson, 1987). Progressive embrittlement through oxidative ageing may lead to initial fragmentation of fabricated high density polyethylene objects, like plastic bottles and containers, within three years of disposal at sea (Dixon and Dixon, 1981). On the other hand, antioxidants or other additives may ensure survival of 50 or more years for some monofilament netting and polypropylene cordage (Gregory, 1978; Wehle and Coleman, 1983). At higher latitudes where insolation is much less and temperatures lower, degradation rates will be reduced (Merrell, 1980; Gregory, 1987). If they survive physical battering, some objects may persist almost indefinitely.
Many plastic items cast up on the Subantarctic island shores are much abraded or exhibit other signs of physical battering and mechanical battering (Figs 11.2 and 11.3). Although impossible to quantify at present, evidence for disintegration through oxidative ageing appears to be of less significance than for comparable materials found on mainland New Zealand shores to the north. Similarly, most pellets taken from these southern waters have a fresh, non-crazed surface.
Gill net floats and sandals stranded on Amchitka island, Alaska, are often gnawed by rats (Merrell, 1980). There is no evidence of this taking place on Campbell and Enderby Islands. Lichen encrustations could also lead to the deterioration of some plastics in a manner analogous to their pedogenic weathering action on rocks (Jones et al., 1980).
Longevity of plastics and continued input of fresh material outpaces ultimate loss of visibility and/or adsorption into the environment through processes such as:
1. physical and mechanical disintegration;
2. degradation and deterioration in response to embrittlement following photo-oxidation, reaction with seawater, microbial and other biological activity;
3. burial after stranding at the shore;
4. sinking as buoyancy is lost either by fouling of marine growth or becoming water-logged.
As a consequence, unlike pelagic tar balls where numbers have reached an equilibrium state (Butler, 1975a), seaborne plastics are likely to continue increasing in quantity for the foreseeable future (Gregory, 1978).
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