A study has measured the amount of plastic debris found in a region of the Atlantic Ocean over a 22-year period.
US researchers, writing in Science, suggest the volume of plastic appeared to have peaked in recent years.
One reason could be tighter marine pollution rules that prevent vessels dumping their waste at sea.
The team said monitoring the free-floating plastic also provided an insight into the behaviour of ocean surface currents.
They found plastic, most pieces measuring no more than a few millimetres, in more than 60% of 6,136 samples collected by dragging fine-meshed nets along the ocean’s surface.
“Plastic marine pollution is a significant environmental concern, yet a quantitative description of the scope of this problem is the open ocean is lacking,” they wrote.