Hawai'i's Environment
Debris continues to threaten marine life
By Jan TenBruggencate
Advertiser Columnist
The role of marine debris in the deaths of Hawaiian monk seals has grown dramatically, researchers say.
The National Marine Fisheries Service has been able to document 173 cases of seals entangled in nets, lines and other manmade materials from 1987 to 1996, according to a paper by service researcher John Henderson, printed in the Marine Pollution Bulletin this year.
The youngest seals are the most likely victims, he said.
The entanglements — most of them in the Northwestern Hawaiian Islands, where most seals are found — are occurring despite major efforts to clear the islands of debris. Henderson said more than 19,000 individual pieces of marine debris on Northwestern Hawaiian Island beaches were destroyed in the same 10-year period.
The most common constituents of such marine debris are rope, cable, cord and other lines.
Next are sections of trawl net. Chunks of lighter driftnet are third in frequency.
The threats caused by these products are comparatively new to the animals, since the long-lived plastics have been common in the oceans for less than four decades.
The very first sighting of a tangled Hawaiian monk seal occurred 32 years ago, in 1969.
"Polypropylene and nylon nets have replaced antiquated and once-prevalent tarred cotton and linen webbing, and various plastic lines are now used in place of manila and other natural fibers such as hemp," Henderson said.
The natural fibers posed less of a threat because many of them would sink instead of floating to island beaches, and many would rot comparatively quickly if not carefully maintained.
The threat from plastics crosses all oceans and all lines of species. One study estimated that 267 marine species are subject to either eating or being snagged in marine debris, Henderson reported.
For Hawaiian monk seals, pups are the most susceptible to being trapped in nets, while older seals are more likely to be caught by rope or fishing line, Henderson found.
There is some evidence that the amount of trawl webbing that is entering the ocean is being reduced as a result of marine pollution treaties. However, researchers have not seen a reduction in the amount coming ashore in Hawai'i. Driftnet fragments continue to wash ashore, even though that fishery has been banned on the open ocean.
Henderson concluded that some fisheries and other marine operations may still be dumping or losing gear at sea, but also that long-lived plastics dating to before the treaty's 1989 passage are still common and floating on the Pacific's currents.
Jan TenBruggencate is The Advertiser's Kaua'i bureau chief and its science and environment writer. Call him at (808) 245-3074 or e-mail jant@honoluluadvertiser.com.
