Entangled in Debris: Marine Animals Threatened by Human Trash

By Emma Moore
Published: March 2007

Sea lion entangled in debris. Credit: Emma Moore

Biodiversity in the marine world is under threat by five factors: over-exploitation, physical alteration, pollution, invasive species and climate change.  One very visible impact of pollution is entanglements observed in marine species, from large whales to tiny seabirds.  A beached animal caught in plastic or fishing gear can be an appalling sight.

Entanglement is defined as “an interaction between marine life and discarded material whereby the loops and openings of various types of debris entangle animal appendages or entrap animals.”  Animals can be injured or killed by the human debris.  If not killed directly, an animal can die indirectly if the material causes the animal to incur wounds and infections, weakens its ability to catch food, or creates aberrant behavior patterns that place it at a competitive disadvantage. 

The materials that cause entanglements can be categorized into two main groups:

• fishing gear, either active or discarded
• marine debris, for example, six-pack holders and balloons

Observations of entanglements at sea are often chance encounters, hence entanglement studies tend to be made from land-based observations, where live or dead animals strand on beaches, or where animals are seen as entangled during population surveys.

In central California there are a number of research studies that document information on entanglements encountered during data collection.  Beach Watch, a long-term beach-monitoring program, is one of the groups whose citizen scientists record entanglements encountered during their surveys.  From 1995 to 2005, approximately 1.2 carcasses were encountered per km surveyed, and 1 entangled carcass was found per 100 km surveyed.

A recent graduate project used the data collected by Beach Watch volunteers over the past decade in carrying out a study into entanglements observed on California’s coast.  The other data sets in the research project included:

• Beach monitoring surveys
• Pinniped population surveys
• Rescue and rehabilitation centers

Observations of entanglements occurred every year.  The data indicated that entanglement is not a major cause of mortality; however these land-based observations represent an unknown fraction of entanglements occurring at sea.  To address the sources of entanglement, it was recommended that the programs should adopt new survey categories providing details of entanglement material type.  Continued monitoring by all the groups will be valuable in providing an overview of the impacts for each species, identifying trends and highlighting any particular areas of concern.

Emma Moore will be presenting an overview of the research project at a potluck at the Sanctuary headquarters on Wednesday, March 21, 2007.  Please contact slyday@farallones.org for further information.

 

References

Laist, D.W. (1987) Overview of the Biological Effects of Lost and Discarded Plastic Debris in the Marine Environment.  Marine Pollution Bulletin, 18(6B), 319-326.

Laist, D.W., Coe,J.M and O’Hara, K.J. (1999) Marine Debris Pollution.  In: Twiss Jr. and R.R.Reeves (eds), Conservation and Management of Marine Mammals.  Smithsonian Institution Press, Washington and London, pp 342-366.

Norse, E.A and Crowder, L.B. (2005) Marine Conservation Biology: The Science of Maintaining the Sea’s Biodiversity.  Island Press.