For nearly three decades, the National Oceanic and Atmospheric Administration has conducted or supported regular hauls of marine debris from the Northwestern Hawaiian Islands. Most of it is made up of derelict fishing gear, and most of that is non-biodegradable plastic.
But what effect has the removal had on the protected species that call the Papahanaumokuakea Marine National Monument home? There is plenty of photographic evidence of the harm that entanglement poses to the corals, green turtles, monk seals and seabirds. Until recently, though, no quantitative study of the impact of debris removal had been done.
But in September, the journal Science published a report that details the positive benefit on monk seal pups that debris removal has had, especially at Pearl and Hermes Reef, the source of more than half of the 945 metric tons – more than two million pounds – of marine debris collected between 1996 and 2022 by NOAA and its partners. The study, “Four decades of Hawaiian monk seal entanglement data reveal the benefits of plastic debris removal,” was authored by researchers at the Pacific Islands Fisheries Science Center, an agency of the National Oceanic and Atmospheric Administration, the University of Hawaiʻi School of Ocean and Earth Science and Technology, and personnel affiliated with the Papahanau Marine Debris Project, a non-profit group founded by former NOAA researchers in 2020 that is dedicated to debris removal from the area.
A monitoring program for monk seal entanglements launched in the 1980s, but even before that, entanglements were recorded “opportunistically,” the authors write. Altogether, from 1974 to 2022, 474 monk seals, representing both sexes and a wide range of ages, were seen entangled in marine debris. “Plastic derelict fishing gear (net, line, and trap components) accounted for 76 percent of the items observed entangling seals. Only one non-plastic entangling item (a loop of copper wire) was observed,” the report states.
The number of seals observed to be entangled does not reflect the total number of seals caught up in debris: “Documented incidents represent only a subset of the actual number of seal entanglements as entangled seals are not always observable… Seals alternate time at sea with periods ashore but are practically only available for detection of entanglement when on land. For example, seals entangled in debris anchored to offshore substrate or in debris that limits their ability to swim to shore are essentially unavailable for detection and almost certainly perish unseen and undocumented. Further, field staff are only present for part of the year at most sites where monk seals reside.”
In an effort to quantify the effect of debris removal, the team reviewed data from 1996 to 2022. NOAA began a large-scale debris removal program in 1999 but had begun to haul off debris and weigh it three years earlier. The authors used statistical modeling to determine the relationship between removal of debris and survival of monk seal pups. They found that after considering the effect of debris removal for each subpopulation of seals, “estimated entanglement rates were lower at five of the six seal subpopulation sites after large-scale debris removal began.”
At Pearl and Hermes Reef (also called Manawai), where a total of 505 metric tons of debris was removed over the study period, the entanglement rate declined by 71 percent. This site was also where debris removal was highest, amounting to between more than 13 times the amount collected at Kure Atoll, where the least debris was removed, to more than twice the debris removal at Midway, where around 200 metric tons were collected.
Pearl and Hermes Reef was the only subpopulation site where the rate of entanglement of weaned pups was found to be statistically significant following debris removal. This, the report says, is linked to three factors: “First, this subpopulation had a relatively high ‘baseline’ entanglement rate prior to debris extraction efforts. Second, the amount of debris removed from Pearl and Hermes Reef was higher than that of any other location. Lastly, the rate of debris removal was very high in the early years of the debris cleanup program, resulting in more numerous subsequent years when entanglement hazards were reduced as compared to other sites where debris removals accrued more gradually.” Still, at five of the six subpopulation sites, post-removal entanglement rates decreased.
Laysan Island was the only site where entanglement rates increased, a fact the authors explain by noting the rates at Laysan were relatively low to begin with, “leaving little room for detection of statistically significant improvement.” At French Frigate Shoals, where the pre-entanglement rate was even lower than that at Laysan, the entanglement rate decreased, but not in a statically significant quantity.
At other sites – Lisianski Island, Midway Atoll, and Kure Atoll – reduced entanglement rates were seen and seals may have benefited from the debris removals, but “the intensity of removal efforts may have been insufficient to result in statistically significant findings at each location,” the report states.
The authors see the benefits of debris removal on monk seals as a “proxy for many undocumented benefits to other Northwestern Hawaiian Island species as well as ecosystem function, health, and resilience. That we were able to detect a signal in seals suggests that the impact of plastic debris on many other reef-associated species may have also eased.” They note that fish, turtles, seabirds, and corals are all harmed by plastic debris.
“Our findings suggest that if the level of debris removal achieved at Pearl and Hermes Reef were attained at subpopulations where monk seal entanglement rates remain relatively high, benefits to monk seals and the ecosystem at large could be maximized,” they write.
In addition to debris removal, the authors conclude by noting the problem still needs to be addressed at its source: “[R]educing environmental plastic inputs is preferable to only mitigating their damage after the fact. The entangling plastic debris on Hawaiian reefs and beaches largely originates from multinational regulated fisheries throughout the Pacific, as well as illegal, unreported, and unregulated fishing activity…. Efforts to reduce inputs from fisheries coupled with ambitious removal programs … will likely be required to minimize the damage wrought by abandoned, lost, or otherwise discarded fishing gear.”
Just this year, field camp teams “disentangled two turtles (one fully submerged) from a net mass anchored on a reef at Manawai,” according to a recent Pacific Islands Fisheries Science Center report.
Meanwhile in Hawaiʻi
The state Department of Land and Natural Resources’ Division of Aquatic Resources has supported marine debris removal efforts through its Marine Debris Rapid Response program. A hotline – 833-4-DA NETS (833 432-6387) – allows members of the public to report sightings of marine debris too large to collect by hand. The reports are transmitted to organizations that partner with the DLNR on all islands except Lanaʻi and Niʻihau.
Through this program, more than 100,000 pounds of marine debris have been collected since its start in 2020.
One of the partners, the Hawaiʻi Wildlife Fund, has been undertaking beach clean-ups on Maui and the Big Island since the early 2000s. According to its president, Megan Lamson, it has collected roughly 350 tons of debris – almost all plastic – from the Big Island, with almost all that gathered from the district of Kaʻu.
— Patricia Tummons
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