http://www.marlisco.eu/summary-of-current-methods-of-monitoring-and-assessment-for-marine-litter.en.html

Methods to Monitor and Assess Marine Litter

Gathering data on marine litter is a relatively new concept and this
report provides an overview and summary of possible approaches,
internationally agreed monitoring methods and protocols. The guidance is
also intended to support litter monitoring by the general public or
other non-research trained persons and includes links to relevant
sources of information.

http://www.marlisco.eu/summary-of-current-methods-of-monitoring-and-assessment-for-marine-litter.en.html?file=tl_files/marlisco/Downloadables/WP%201/MARLISCO_D1.2_Methods%20to%20Monitor%20and%20Assess%20Marine%20Litter.pdf