Since the early 1980s, field biologists working in the remote Northwestern Hawaiian Islands have found Hawaiian monk seals entangled in derelict fishing gear and other plastics. These entanglements can result in lethal injuries and drowning—they’re a serious threat to these endangered seals. NOAA scientists began documenting and removing debris from entangled seals they encountered while patrolling island shorelines. Biologists began cleaning up dangerous plastic garbage on Northwestern Hawaiian Island beaches in the 1980s with the hope of reducing seal entanglement. A new study published in Science shows that these efforts have been a success, with declines in entanglement rates at many sites.
Seals continued to suffer entanglement in marine debris, mostly consisting of abandoned, lost, and otherwise discarded fishing gear. This lethal ocean plastic pollution comes from all over the North Pacific Ocean and is concentrated and deposited in Hawai‘i by ocean currents. Cleaning up dangerous marine debris that had washed up on land was not enough to reduce the threat to seals.
NOAA scientists hatched a bold proposal in the late 1990s: What if we didn’t stop at the water’s edge? What if we cleaned up ocean plastics in the reef and lagoon waters, too? Would that be possible, and could it be done at a scale that would make a difference to monk seals and other marine life?
Cleaning up Debris Saves Seals
A large-scale, multi-agency program was established in fall 1999 to clean up marine debris in the Northwestern Hawaiian Islands. NOAA led this successful debris removal effort until the early 2020s, when it was taken on by a non-profit organization, the Papahānaumokuākea Marine Debris Project. These efforts have removed hundreds of metric tons of destructive ocean plastics from the Northwestern Hawaiian Islands over the past 25 years. But the question remained: Have monk seals benefitted from the reduction of plastics in their habitats? A new study published in Science provides the answer—a definitive yes!
Scientists drew upon more than four decades of data on Hawaiian monk seal entanglement at six different islands and atolls. They devised a new and statistically robust way to calculate seal entanglement rates. The research team found that seal entanglement rates were lower at five of six locations in the years after large-scale debris removal began. Notably, the largest change in entanglement rate, a 71 percent decline, occurred at Pearl and Hermes Reef (Manawai). More debris was removed from that location than from the other five sites combined.
Many other species that share the monk seal’s marine habitat—from corals, to fish, to sea turtles—are injured and killed by the same ocean plastics. While these other species have not been so closely studied as the seals, there is little doubt they have also benefited from the cleanup.
The study was a collaboration among NOAA’s Pacific Islands Fisheries Science Center, the Papahānaumokuākea Marine Debris Project, and Hawai‘i Sea Grant. All these institutions have been deeply involved in the cleanup efforts.
Cleanup Efforts Must Continue
Now we know that all the hard work and dedication did reduce monk seal entanglement, saving seal lives and improving nearshore habitats. We know this only because of the decades of detailed monk seal studies and conservation work carefully conducted by NOAA and its partners.
Now it is clear that cleaning up plastic garbage in the monk seal’s habitat has reduced the threat of entanglement. However, marine debris continues to accrue. A conservative estimate is that more than 50 metric tons of plastic derelict fishing gear is deposited in the Northwestern Hawaiian Islands every year. Until the flow of these harmful plastics into the ocean is greatly reduced, the cleanup program and disentanglement efforts must continue.
