Marine Pollution

Five groups of wildlife are most susceptible to injury from discarded plastic: sea turtles; marine and terrestrial mammals; birds; fish; and crustaceans. Marine debris affects wildlife in one of two ways — they can become entangled in it or ingest it…. When an animal becomes entangled or ensnared in plastic debris (strapping bands, beverage container rings, or nets, ropes, and lines), it can strangle, suffocate or exhaust itself. Injury or death may be immediate or prolonged. When an animal ingests small pieces of plastic, it is likely to pass (regurgitate or defecate) it. Large sheets of plastic and other items have blocked intestinal passages of turtles and whales.

Currently available reports on wildlife entanglement in marine debris indicate that many species of marine and terrestrial birds and mammals, as well as marine and freshwater fish, become entangled in debris. Anecdotal reports and local systematic surveys are increasing each year. Although individual animals of many species are harmed, to date, scientists can identify adverse impacts to only a few wildlife populations, e.g., northern fur seals and the endangered Hawaiian monk seals.

Without supportive data on population impacts, determining “significance” of marine debris impacts on wildlife depends to a large extent on one’s interpretation of “significant.” Is the loss of a single or a few individuals “significant?” What if the species is threatened or endangered? Is the potential, but presently undocumented loss of larger numbers of individuals, considered significant? We need considerably more data relating marine debris relationships to wildlife population levels before we can conclude that marine debris has significant adverse impacts on such populations. It is known, however, that marine debris affects individuals of many endangered, threatened, and commercially valuable species. Data are not sufficiently reliable to conclude that its effects are not a significant factor in the health of at least some of these species’ populations.

Scientists regularly report “scars” and bruises on marine mammals as evidence of entanglement. They point out that it is difficult, if not impossible, to know if the scar is from active or discarded fishing gear. Scientists doing this research collect most of their data on land, using stranded sea turtles and cetaceans as specimens. There are no reliable estimates of the fate of marine animals which entangle in debris while at sea or ingest plastic products, because these animals either sink or are eaten, or go unnoticed by human observers due to the vastness of the oceans.

Marine debris affects commercial and recreational fishermen in several ways. First, target species continue to be captured in lost traps, nets, or lines. The extent of losses is unknown, but in some cases, such as lost crab traps, losses may be significant. Second, marine debris may foul or damage active fishing gear as well as damage fishing boats, foul ship propellers, and clog vessel water intake systems. The latter two impacts also affect pleasure, merchant, and military vessels, although such occurrences do not appear to be common.

A. Fish and Wildlife.

1. Marine Mammals.

Reports of marine mammals entangled in marine debris have increased. This may be due to an increase in the actual number of such incidents or an increase in concern and recognition of the problem. Most of the reports come from areas where fishing or marine transportation is common. The most commonly reported entangling debris items are fishing nets, scraps of fishing nets, plastic strapping bands, ropes, and plastic sheeting….

a. Seals and Sea Lions.

Perhaps the best documented research on entanglement in marine debris has been done on northern fur seal populations on the Pribilof Islands…. From the mid-1970s to the mid-1980s, the Pribilof fur seal population has been declining at about 4 to 8 percent per year. Scientists monitoring annual seal harvest between 1981 and 1984 examined harvested seals for evidence of entanglement such as scars around the seals necks, or materials attached. The most common debris entangling seals was net fragments with mesh size of 20 cm or greater. Of the animals harvested, 0.42 percent had indications (scars or bruises) that they had encountered debris.… The observed entanglement has remained relatively constant at about .04 percent since it peaked in 1976 at .076 percent. Fowler (1985, 1987) concludes that this observed rate of entanglement contributes significantly to the decline of the population. He bases this mortality theory on correlations between observed entanglement, pup production, and female harvest. Scientists count only those animals that become entangled in small enough net fragments that they are able to swim back to land. However, they believe that most entangled animals never return to land. Coe (1986) concludes that existing data are insufficient to conclude reliably that the decline of fur seals population is due solely to entanglement in lost fishing gear.

Scientists have also studied entangled seals and sea lions at other breeding and haul-out beaches. Stewart and Yochem (1985) examined sea lions, elephant seals, and harbor seals at San Nicholas and San Miguel Islands in California and found the numbers of seals and sea lions plastic encounters shown in Table II.

Henderson (1985) summarizes reports of 27 to 35 Hawaiian monk seals, a highly endangered species, entangled between 1974 and 1984. Since these seals inhabit remote, seldom visited, islands in the Hawaiian archipelago, this is likely to be a minimum estimate. Hawaiian monk seals are highly endangered and number only 1,000 to 1,800 animals. They are particularly susceptible to buoy lines marking spiny lobster traps, and net debris from high seas gill net and ground-fish trawl fisheries in the north Pacific, much of which washes ashore on the leeward side of the archipelago. In general, it appears that pups and young animals are more likely to become entangled and killed, than older, more experienced individuals.

b. Cetaceans.

Many observers throughout the world have reported incidences of whales and dolphins entangled in fishing gear. The most common types of gear entangling cetaceans are gill-nets (and fragments of them) and buoy lines used to mark traps. Weinrich (1987) reports that 56 percent of endangered right whales photographed by the New England Aquarium have scars from probable gill-net or lobster gear entanglement. Center for Coastal Studies photographs of endangered humpback whales in New England waters show nearly 40 percent of those animals bear scars indicating they have encountered nets or lines…. It is not clear whether these scars are due to entanglement in lost or active fishing gear. As with seals, juvenile whales appear to be more susceptible than adults. Weinrich estimates that net mortality of whales in New England waters caused by entanglement is a “moderate, but not severe, problem.”

In the Pacific offshore fisheries, there are few reports of cetacean entanglement in gear known to be lost or abandoned. Mate (1985) reports several incidences of gray whales off the Oregon coast with active crab-pot buoy lines between their baleen plates, and each year there are a few reports of gray whales becoming entangled in gill nets.

There is some evidence that whales ingest plastic materials. Of 38 sperm whale stomachs examined, one contained “about 1 liter of tightly packed trawl net.”…. Walker (1988) reports that pelagic cetaceans rarely have plastic in their digestive tracts. He collected data from approximately 1,500 free roaming cetaceans (6 different species including Dall’s porpoise, bottlenose dolphins, sperm whales, killer 20 whales, Stenella species, and Baird’s beaked whales) and found no evidence of plastic in their stomachs. Whales and dolphins, which feed on the bottom, rarely had ingested any type of man- made objects. However, he identified a single incident of a captive killer whale dying after it had eaten a plastic bag.

c. Seabirds and Shorebirds.

Plastics in the marine environment cause similar problems for all types of birds. Seabirds and shorebirds, particularly, become entangled in actively fishing nets, discarded nets, and other marine debris such as beverage container rings and monofilament line. These can lead to drowning, choking, or lacerations.

There have been some reports of pelicans with beverage container rings and monofilament around their beaks which prevented them from catching prey. A greater source of concern, however, appears to be entanglement of monofilament fishing line around the wings and legs of pelicans and egrets which then become tangled in trees where the birds roost. Birds become caught in trees then die of exhaustion or starvation….

Day, Wehle, and Coleman (1985) compiled data on ingestion of plastic debris by seabirds. Researchers have found plastic particles in the stomachs of approximately 25 percent of the world’s 250 seabird species. Of the species identified as having ingested plastic, only 12 species have consumed an average of one or more plastic particles per individual….

Shearwaters and parakeet auklets show the highest incidences of ingesting plastic, as high as 21.7 particles of plastic per individual (short-tailed shearwater) in a California study….  Resin pellets are the most frequently identified plastic type consumed by birds…. Several shearwater and alcid species, e.g., puffins, auklets, and murres, ingest floating resin pellets, probably because they resemble fish eggs or invertebrate prey. They are the most common form of plastic ingested by seabirds. These pellets and other plastics may be especially harmful to young birds which rely on regurgitated food from their parents during the first weeks of life. When an adult feeds resin pellets to young birds, the young may not able to pass them. Therefore, their stomachs can fill with pellets, preventing them from obtaining adequate nutrition.

No scientists have yet attributed a decline of a seabird population to persistent marine debris.

3. Sea Turtles.

Five species of sea turtles inhabit U.S. waters — Kemp’s ridley, hawksbill, leatherback, green, and loggerhead — each of which is listed as either endangered or threatened under the Endangered Species Act. Their preferred habitats and foraging patterns differ significantly. However, like other wildlife, they can ingest and become entangled in plastic debris. Entanglement can cause a turtle to drown, reduce its swimming efficiency, and lacerate exposed appendages.

Balasz (1985) reviewed published literature documenting sea turtle encounters with plastic debris worldwide. He identified 79 reports on ingestion of plastic and 60 reports of entangled sea turtles from all over the world. He further reports the findings of several researchers who have examined sea turtle digestive tracts and found very little or no plastic items: green turtles off Nicarauga…; loggerhead turtles from Cumberland Island, Georgia…; and loggerhead, Kemp’s ridley and leatherback sea turtles from Virginia, Maryland, and Delaware….

Sea turtles spend up to their first five years of life drifting with oceanic currents. As with other floating material, they appear to be concentrated along lines of current convergences or at the centers of major current gyres, such as the Sargasso Sea in the Atlantic. Carr (1987) reports that these zones have high concentrations of plastic marine debris, particularly pieces of styrofoam, resin pellets, and floating nets which intermingle with sargassum weed and other flotsam. Young sea turtles are especially vulnerable to plastic debris because these zones are their essential habitat and the plastic beads are suggestively similar to their standard food — sargassum floats — and resembles natural prey items such as invertebrates. Also, as the turtles move about in the sargasso weeds, they may become entangled in nets and line.

Large leatherback sea turtles are pelagic and highly migratory. They subsist primarily on jellyfish. Researchers have found dead leatherback sea turtles stranded on beaches with plastic bags and sheets in their digestive tracts. They attribute leatherback consumption of plastic sheeting to its resemblance in the water to jellyfish….

The National Marine Fisheries Service within the National Oceanic and Atmospheric Administration, Department of Commerce, has coordinated a nationwide sea turtle stranding reporting network since 1978. NMFS scientists and others have necropsied stranded sea turtles, some of which included analysis of stomach contents for some of the turtles. These indicate that one-third to one-half of endangered and threatened sea turtles are ingesting plastic products or byproducts…. These estimates are slightly higher than information presented by Balazs that 13 percent of 140 leatherback sea turtles and 23 percent of 39 green sea turtles examined had consumed plastic bags.

Only Plotkin (1987) attributes the death of any animals directly to ingestion of plastic. She attributes the death of one loggerhead turtle to a plastic bag which was completely blocking the animal’s digestive tract.

4. Fish and Shellfish.

&Fishing pots (traps), gill nets, and other fishing gear may continue to capture fish and shellfish after they are discarded or lost. Commercial and recreational fishermen primarily use traps or pots to capture crustaceans such as crabs and lobsters although they are sometimes used to catch demersal fish (e.g., snappers). High (1985) estimates that king crab and Dungeness crab fishermen in Alaska and along the Pacific coast lose over 10 percent (over 1,200) of their pots annually. In baited pots, predators consume bait within several days. Lost pots continue to trap fish and shellfish which in turn attract new predators, such as crabs. Scientists found that escape panels prevent about 20 percent of legal size crabs and 8 percent of sub-legal crabs from escape. The cycle continues until the trap disintegrates. This may take up to several years.…

The advent of monofilament gill nets enabled fishermen to string long curtains of nets in ocean waters. Fishermen use different types and sizes of gill nets throughout the world, depending on target species. In the Bering Sea and North Pacific Ocean, Japanese salmon and squid fishermen use nets up to 25 miles long. The mother-ship salmon fleet within the U.S. EEZ sets approximately 1,100 miles of gill nets each night during the 4 to 6 week salmon season…. U.S. observers aboard these fishing vessels estimate that this fishery incidentally captures thousands of pelagic seabirds, approximately 2,000 Dall’s porpoises, and other species of marine mammals each year. These are data from actively fishing nets, not debris. However, one anticipates continued capture of target as well as non-target species in lost gill nets as long as they fish. Even a small rate of net loss from such an extensive fishery could produce substantial quantities of net debris.

One study shows the changes in shape of derelict gill net fragment of varying sizes over time and determines the fishing ability of these nets… Gerrodette deployed and tracked gill net sections of 50, 100, 350, and 1,000 meters. He observed them for between 57 and 309 days. The 50 meter net had collapsed, or folded in accordion fashion within 30 minutes after deployment, although it was still hanging freely in the water. The 350 meter net collapsed to 40 percent of its original length in a few hours, and had collapsed completely by the 10th day after deployment. The 1,000 meter net followed a similar pattern of slower contraction. Very little marine life was caught in any of the gill nets during the first 3 days of observation, only a small marlin and a large flying fish. After 10 days, a rotting shark and several unidentifiable bony fish were observed. No large animals were entangled when the nets were recovered. The study showed that drifting gill nets will eventually collapse and ball up, thus reducing their fishing ability. Nets less than 100 meters long collapse in less than a day, while longer ones may take several days to several weeks.

The collapse rate may be shortened if a large animal is caught and struggles. It may be lengthened if any buoys are attached. A collapsed gill net can still catch fish, but its effectiveness is diminished. Collapsed gill nets still pose a hazard because non-target species attract fish which may attract predators which may be more effectively ensnared in the many folds of the collapsed net.

No one knows for sure how long lost gill nets continue to capture animals after they are lost. High (1985) reports that synthetic gill net materials in the marine environment remain strong enough to capture fish and wildlife for at least six years. Carr, et al (1985) surveyed from a submersible over 100 acres (40.5 ha) of actively fished gill net areas near Gloucester, Massachusetts. They found nine lost gill nets, six of which were balled up and rising from the bottom up to 10 feet (3.6 m). The other three stretched horizontally with reduced float line heights. These researchers estimated the time that nets had been submerged by algal and invertebrate animal growth on them. Eight of nine nets were thought to be over three years old. The nets had caught over ten species of marine life. Like lost crab traps, lost gill nets continue a cycle of ensnaring marine life which dies, thereby attracting more predators and scavengers which themselves become entangled and die.

No data are available on frequencies of fish ingesting plastic debris, although this is known to occur….