Plastics – een bedreiging voor zeevogels

Large accumulations of plastic along beaches show that these long-lived materials are widely distributed in the marine environment. Relatively little attention has been given to plastic as an environmental pollutant: the UNEP-report “Health of the Oceans” (GESAMP 1982) does not discuss plastics. However, the gigantic production of plastics, combined with their effect on marine organisms, justifies concern. This paper discusses the different types of plastic pollution, and the effects on seabirds in particular. Referring to their source, marine plastics can be distinguished into ‘users-plastic’ and ‘industrial-plastic’. The users-plastics are mainly a result of waste-dumping from ships. Basic types are sheets (bags, wrappings), foamed plastics (packages, cups), threads (nets, ropes, clothes) and a wide variety of moulded plastics (bottle (bottles, cases, shoes, 6-tin-plastics, toys,...). Major concentrations occur near shipping routes, but anywhere on the oceans floating plastics are commonly seen. Industrial plastics consist of small inconspicuous particles: small spheres (spherules), generally smaller than 1 mm, but occasionally considerably larger; disc-shaped particles (pellets) of several mm; and other shapes like rectangular pieces. All these types were represented in a sample taken from the Dutch beach, shown in figure 1.: pellets (A) were the most abundant type, generally transparent or opaque, but sometimes light-brown (B) or brightly coloured (C); a smaller number of particles was rectangular (D) and spherules were scarce (E); the large spherule was sampled selectively. The large majority of the pellets is probably made of polyethylene. The industrial plastics are the feedstock of plastic-industry and enter the sea due to spillage during transport and the lack of proper filtering at factories. They are widely distributed in the marine environment, even in the central parts of the major oceans (table 1.). Marine organisms become entangled in plastic materials or consider it to be food. Large numbers of birds, but also turtles and sea-mammals become entangled in fishing-nets and drown, both during the use of the nets and later when they are dumped or lost. Animals become entangled in other materials as well, like in fishing-line, ropes, bags and 6-tin-plastics. Many marine organisms eat plastics. It is shown in fishes and turtles. Especially a large number of seabirds seems te consider plastics as proper food (table 2.) and even feed such materials to their nestlings. Procellariiform birds frequently eat large quantities of plastic. In general, surface-feeding birds with a diet of mainly crustaceans, squid and carrion have more plastics in their stomach than birds diving for fish. Most gulls contain few plastics in spite of their scavenging feeding-method; probably this is a consequence of their ability to regurgitate indigestable food-items. In many other birds, plastics are probably retained for long periods of time, at least in the gizzard. Eventually they disappear through wear (Day 1980). The Fulmar Fulmarus glacialis consumes considerable quantities of plastic. About 80% of the Fulmars found in the Netherlands contain plastic (table 3.). On average, each Fulmar has 5 fragments of plastic in its stomach. Three of these are industrial pellets, the remaining ones consist of all types of users-plastic. Sometimes extreme quantities of plastic are accumulated (figures 2. and 3.). The phenomenon certainly is not restricted to birds that are weak or diseased; other investigators present similar data on ‘healthy birds’. When discussing the consequences of the eating of plastics, generally attention is focussed on complete gastro-intestinal blockage and subsequent starvation. It is emphasized however that sublethal, long-term effects may be much more important. Long-term effects of plastic eating are a decrease in the uptake of food and an increased assimilation of toxic chemicals. Both factors result in a deterioration of the health of the animal, which has negative effects on its resistance to stress and on its reproduction-capacity. A decrease in the uptake of food may result from: partial gastrointestinal blockage; lowered feeding-stimulus caused by a “full” stomach; a decreased maximum volume for food in the stomach; and a lower efficiency of the proventriculus in pre-digesting the food, caused by ulcerations or plastics ‘embedded’ in the tissue. An increased assimilation of toxic chemicals results from the considerable quantities of additives like PCB’s and colorants that are incorporated in users-plastics. Most industrial plastics contain fewer additives. When plastics wear off in the stomach, the substances enter the intestines and are assimilated. Adverse effects of PCB’s and many colorants are well known; not surprisingly, because hardly investigated, no effects of plastics themselves are known. The sublethal effects of plastic-eating become truly dangerous when combined with the effects of other marine pollutants. An example is given of periods of strong winds during winter. Food is relatively scarce in winter and bad weather makes the food harder to obtain; the birds need a perfect health to keep their energy-balance. However, during the same periods maximum oil-pollution occurs, resulting in increased numbers of oil-contaminated birds: minor contaminations can be survived but demand much energy. Many birds will have to use their stored fats in order to resist the scarcity of food or oil-contamination. This results in high levels of those toxic chemicals that are normally stored in the fat, but now enter the total body. Such chemicals, like PCB and DDT adversely affect the nervous system and thus reduce the capability of obtaining food. Sufficient uptake of food (energy) is the essential factor during windy winters, to resist scarcity of food, small oil-contaminations and the quick absorption of stored fats. In such a situation even a minor decrease in the efficiency of food-uptake, caused by plasticeating, may result in the death of large numbers of birds, many of them being already at the upper limits of their capability of resistance to environmental stress. In addition, the danger of fat-absorption is increased by plastic-eating because the plastics are a source of toxic chemicals. The effects of different pollutants may not be viewed separately, and in that way plastics are truly dangerous to seabirdpopulations. Most of the discussion above is valid for other groups of animals as well. Special concern is expressed on baleen-whales. An other aspect of concern is the fact, that plastic objects in sea eventually embrittle into very small fragments: nothing is known about their effects on the large number of small filter-feeding organisms that are the basis of marine food-chains. Both the known and the unknown adverse effects of plastic in the marine ecosystem justify strong opposition to further plastic-dumping in the oceans. Ships should have proper incinerators or storagesystems to prevent dumping of waste; improved transport-systems and filter-techniques should put an end to industrial plastic pollution.