Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
APPENDIX F 332 APPENDIX F Ecological Effects of Marine Debris * GENERAL EFFECTS ON ECOSYSTEMS Judgments concerning the broad impact of plastic debris on marine ecosystems are speculative at present. The bioaccumulation of plastics through food chains, for example, may be a problem, based on observations of secondary and tertiary ingestion of plastics by certain species: bald eagles preying on parakeet auklets with plastics in their stomachs (Day et al., 1985); Antarctic skuas preying on broad-billed prions in the South Atlantic (Bourne and Imbet, 1982); and shorteared owls in the Galapagos Islands preying on blue- footed boobies that had ingested fish containing plastic pellets (Anonymous, 1981). There is little scientific information available on how debris may affect marine invertebrate species, plant life, or marine habitats in general, aside from observations that debris damages coral reefs, is ingested by squid (Araya, 1983; Machida, 1983), and may present a new habitat niche for encrusting marine species (Winston, 1982). ENTANGLEMENT OF MARINE SPECIES A major reason for the heightened concern over marine debris was the increasing number of reports that plastic was causing widespread mortality of marine species. Among the first species to be highlighted was the Northern fur seal. Studies indicated that each year as many as 50,000 of these seals were * Summary prepared by the Committee on Shipborne Wastes as a supplement to Chapter 2.
APPENDIX F 333 becoming entangled and dying in plastic debris, primarily fishing nets and strapping bands. Subsequent findings have shown that the increased use and subsequent disposal of plastics in the marine environment is causing widespread mortality among marine mammals, turtles, birds, and fish, either through entanglement or ingestion. However, most of this information is drawn from a few studies, and there has been no attempt to compile the data at one source, nor has there been any extensive effort to monitor trends. Entanglement in plastic debris poses a potentially serious threat to a number of marine species, at both the individual and population levels. (To date, the threat to populations has been documented only in the Northern fur seal.1) Marine mammals, sea turtles, seabirds, and fish have been found entangled in the loops and openings of fishing nets, strapping bands, and other plastic items. Once ensnared, an individual may be unable to swim or feed or may incur open wounds that become infected. There have been attempts to identify the circumstances that can lead to entanglement. In some cases, random encounters with debris are to blame. For example, an animal may not be able to see or otherwise detect plastic debris, especially fishing gear designed to be nearly transparent in water (Balazs, 1985). However, a number of biological factors appear to increase the risk of entanglement for certain species. Like natural ocean rubble such as sargassum weed and logs, floating plastics attract fish, crustaceans, and other species seeking shelter and concentrated food sources. Marine mammals, turtles, and birds also are attracted to floating debris, and they may become ensnared when attempting to feed. Predators, such as seals and sea birds, are at increased risk of becoming entangled in discarded fishing gear, which may have fish entrapped in netting. Finally, pinnipeds haul themselves out of the water to rest on natural debris, such as floating kelp mats and logs, while young seals are attracted to floating objects as playthings. If such debris includes plastics, entanglement can result. Due to their behavioral characteristics, seals and sea lions may be the most prone to entanglement. Individuals from at least 15 of the world's 32 species of seals have been observed ensnared in plastic debris; these include several species found in the United States, such as the northern fur seal (Fowler, 1985, 1988; Scordino, 1985), northern sea lion (Calkins, 1985), California sea lion, northern elephant seal, harbor seal (Stewart and Yochem, 1985, 1987), and the Hawaiian monk seal (Henderson, 1984, 1985), which is on the U.S. government's list of endangered species. 1 The Pribilof Islands of Alaska are home to a population of approximately 827,000 Northern fur seals, 71 percent of the estimated total world population of this species. Studies show that the Pribilof population is less than half that observed 40 years ago and is declining at an annual rate of 4 to 8 percent (Fowler and Merrell, 1986). Entanglement in plastic debris is thought to be contributing to the decline.
APPENDIX F 334 Most information available on pinniped interactions with debris has been compiled by the National Marine Fisheries Service (NMFS). The effects of entanglement on an individual animal may vary. Most entangled Northern fur seals have been observed with debris around their necks and shoulders. This kind of entanglement, if constricting, may directly impair swimming or feeding. Entangling debris also increases drag during swimming. Consequently, an entangled seal must use more energy to swim than it normally would and therefore must consume more food to compensate; unfortunately, drag inhibits the high-speed swimming required for pursuit of prey and therefore may lead to starvation of the animal. In other cases, abrasion from entangling debris may cause wounds that are susceptible to infection. Entanglement of breeding animals also can adversely affect their young. In field studies on St. Paul Island, Alaska, nine of 17 female northern fur seals observed entangled in debris never returned to their pups after foraging at sea (Fowler, 1988). The other entangled seals took twice as long to return as did unencumbered female seals (Fowler, 1988). Sea turtles also appear to be prone to entanglement in plastic debris. In the first comprehensive assessment of this problem, carried out by the NMFS, Balazs (1985) complied a list of 60 cases of sea turtle entanglements worldwide involving green, loggerhead, hawksbill, olive ridley, and leatherback turtles. The debris involved most often was monofilament fishing line. Other cases involved (in order of decreasing frequency) rope, trawl nets, gillnets, and plastic sheets or bags. As is the case for pinnipeds, entangled sea turtles are unable to carry out basic biological. functions such as feeding, swimming, or surfacing to breath; constricting debris also may cause lesions or even necrosis of flippers. According to Bourne (1990); however, there appears to be no evidence that the entanglement of turtles in debris is affecting their numbers, in contrast to the significant effects of other threats such as drowning in shrimp trawls, overfishing, direct harvesting for meat and eggs, disturbance of breeding habitat, and ingestion of debris. While pinniped and sea turtle entanglement in plastic debris has been documented, accounts of the impact of plastics on birds are entirely anecdotal. There has been no attempt by any agency to collect extensive data on bird mortality due to entanglement in debris. In the past, the entanglement problem has been overshadowed by the magnitude of seabird mortality related to active fishing operations, principally in the high seas drift-net fisheries in the Pacific. For example, the Japanese salmon gillnet fishery, in which more than 2,575 kilometers (km) (1,600 miles) of drift gill net were set each night, reportedly drowned over 250,000 seabirds in U.S. waters each year during a two-month fishing season (King, 1984). An international moratorium was enacted recently on high seas drift-gillnet fishing. Seabirds also are attracted to lost or discarded nets and have been found entangled in large pieces of lost gillnets that continue to ghost fish at sea (Jones and Ferrero, 1985).