Fishing Gear Beds, Coral Purses, and Bottle Cap Meals: Marine Debris Impacts in the Northwestern Hawaiian Islands
A monk seal lounges in marine debris on the beach.
A monk seal sleeps in marine debris on the beach. Credit: NOAA PIFSC

OCTOBER 16, 2014 -- Hawaiian monk seals love marine debris. It is not unusual for scientists visiting tiny islands in the Northwestern Hawaiian Islands to see one of these large grey marine mammals sleeping or lounging on a pile of derelict fishing gear on shore.

That moment of comfort for the seal could give way to a far more dangerous situation, either on the beach or in nearby waters: Hawaiian monk seals, in addition to their ill-advised attraction to debris, suffer the highest entanglement rate of any seal in the world.

They are also endangered. The International Union for Conservation of Nature lists the Hawaiian monk seal as critically endangered under its Red List of threatened species, and they are listed as endangered under the Endangered Species Act in the United States. The monk seal population is currently declining at about 4 percent annually, with an estimated 1,200 individuals remaining.

Marine debris poses a huge challenge for the Papahānaumokuākea Marine National Monument, where the majority of the Hawaiian monk seal population lives. Although the islands within the Monument are uninhabited by humans, they are not free from the impacts of everyday human life.

The tiny islands, atolls, and reef structures, with their turquoise waters, unique shells, vibrant corals, and powdery white sands, are perfectly situated in the Pacific Ocean for catching marine debris. They are an enormous comb for derelict fishing gear and consumer litter trapped in the North Pacific Subtropical Gyre, and one look at any of the remote beaches confirms it. Plastic and glass bottles, shipping crates, flip-flops, toys, buoys, floats, toothbrushes, cigarette lighters, nets, ropes, laundry baskets – just about any item imaginable is strewn on these otherwise untouched shorelines.

Debris also lurks in the water column. An estimated 52 tons of derelict fishing nets (Dameron et al. 2007) from commercial fisheries all over the Pacific Rim floats into the Monument every year, and many of these nets become trapped on the extensive reefs. Since 1996, NOAA and its various partners have removed 769 metric tons of debris from the Northwestern Hawaiian Islands.

The threats marine debris poses to the protected marine life within the Monument are many. Here is a look at a few:

Entanglement

For an animal entangled in derelict fishing gear, the outlook can be quite grim. Many fishing nets are made of durable plastic, and they continue to catch animals indiscriminately long after they are lost. Once an animal is trapped, the derelict fishing gear can drown it, prevent it from hunting, make it vulnerable to predators, or cause lacerations that expose the animal to infection.

Between 1982 and 2013, there were 339 observed monk seal entanglements in Hawaiʻi, with 96 percent occurring in the Northwestern Hawaiian Islands. According to NOAA scientists, 230 of those seals were disentangled, 92 escaped unaided, and nine died. What happened to the other eight seals remains unknown. In 2014, scientists from the Hawaiian Monk Seal Research Program rescued five seals from derelict fishing gear during their annual 3-month-long field season to study and support the species in the Monument.

Another vulnerable resident of the Monument’s waters is the threatened green sea turtle, which relies on the islands for survival. An estimated 90 percent of green sea turtles in this region breed and nest on the sandy islets of French Frigate Shoals, a large, 18-mile-wide crescent-shaped atoll roughly 550 miles from Honolulu.

Although few statistics exist on entanglement rates for turtles outside the main Hawaiian Islands, divers participating in this year’s marine debris removal mission have already rescued three of them from nets.

Coral Damage

The Monument encompasses 5,178 square miles of the healthiest and least disturbed coral reef habitat in U.S. waters. These corals play a critical role in maintaining the reef ecosystem by providing a framework for the ecological community – from algae to apex-predator sharks.

The nets that drift into the Monument are frequently large and heavy, and when tangled together, weigh hundreds of pounds. These net conglomerates are sometimes described as giant “purses” – they roll across the large reef structures, snagging on corals, breaking them, and collecting them within the tangle. The added coral heads can make the nets heavier than when they started. Once the nets settle, they smother and scour the substrate underneath, impeding growth.

Ingestion

At Midway Atoll National Wildlife Refuge, located on the far northern end of the Monument, birds loom large. Though plenty of monk seals, sharks, spinner dolphins, and sea turtles live in the atoll, its three small islands are home to nearly 3.5 million individual birds, which “nest on virtually every square foot of available habitat,” according to the U.S. Fish and Wildlife Service. Among them are several species of albatross, boobies, terns, and shearwaters.

Walking along some of the atoll’s sandy and rocky shores, it is nearly impossible not to step over decaying bird carcasses, which in some cases show stomach cavities full of bottle caps and cigarette lighters. At least one-third of all seabird species are known to eat plastic debris, perhaps mistaking brightly colored, slim plastic pieces for prey. Although it is not yet known whether plastics are directly causing bird mortalities, there is growing research into whether the chemicals in plastics are affecting them.

Unfortunately, there is plenty of this false food at Midway Atoll for the birds to eat; in 2013, the marine debris removal team collected 4,781 bottle caps from the shorelines during a 21-day mission there.

Invasive Species

One consequence of marine debris often overlooked by the public but deeply worrying to the Monument’s managers is alien species transport. A floating net, buoy, boat, or container can transport algae, mollusks, barnacles, crabs, or other species from other regions that are non-native to Hawaiʻi. The ocean has no borders, and currents could potentially bring debris containing alien species from every country on the Pacific Rim to Hawaii’s shores.

From studies conducted on derelict fishing gear in the Northwestern Hawaiian Islands, it has been shown that alien species can be transported on occasion. The greater threat comes from debris generated by extreme natural events such as tsunamis or hurricanes. These events introduce items into the open ocean from coastal zones such as small boats, floating docks and aquaculture gear (nets, cages, floats), that have been colonized by intertidal and shallow water organisms. For the past several years, NOAA has focused on the potential for marine debris generated by the 2011 tsunami in Japan to bring non-native species to the Monument. Several pieces of tsunami debris have turned up there, including a 20-foot fishing vessel, a blue container, and a sign. These items were collected and examined and no alien species were documented, though items of this type collected in the main Hawaiian Islands have been documented to have alien species. For that reason, one of the marine debris removal team’s objectives is to remove, document, and report anything that looks like it may have come from the disaster.

To learn more about the Monument, please visit www.papahanaumokuakea.gov.

In 2014, two Marine Debris Program staffers participated in NOAA's annual mission to remove derelict nets and other marine debris from sensitive coral reefs and shorelines in Papahānaumokuākea Marine National Monument, one of the largest marine conservation areas in the world. An estimated 52 tons of derelict fishing gear washes up in the Monument each year, threatening the pristine ecosystem. Follow their journey.

Read more about 2014's mission and its history.