http://marinedebrisblog.wordpress.com/2014/01/09/monitoring-and-assessment-using-standard-techniques-to-measure-our-progress/

Monitoring and Assessment: using standard techniques to measure our progress
January 9, 2014 by NOAA Marine Debris Program

By: Sherry Lippiatt

How do scientists and policy makers figure out if efforts to prevent 
trash from becoming marine debris are working? Ultimately, to evaluate 
any campaign, or initiative, or policy, we need to see a change in the 
amount of debris washing ashore or a decrease in the number of 
particular items, such as plastic bags, in the marine environment or 
Great Lakes.

This change can be tricky to detect, given that we need a baseline 
understanding of how much and what types of debris are out there in the 
first place. Determining a baseline is not as simple as going to the 
beach once or twice and looking for trash – it requires going back to 
the same spot over time and recording data in the exact same way. We 
also need to know how environmental factors, such as winds and coastal 
currents, impact the way debris moves and builds up on certain shorelines.

To gather this kind of information, the NOAA Marine Debris Program (MDP) 
developed standard techniques for assessing debris on shorelines, on 
water surfaces, at-sea, and in benthic environments, and the guidelines 
are now available for the public’s use. They were developed over several 
years, based on literature reviews, discussions with experts, and field 
testing by the MDP and contractors.

http://marinedebris.noaa.gov/sites/default/files/Lippiatt%20et%20al%202013.pdf