Scientists have known for years that when fishing trawlers drag nets and gear across the ocean bottom they trap or kill almost all the fish, mollusks and other creatures they encounter. And the dragging destroys underwater features like reefs, turning the bottom to mud.

Now, scientists have used satellite images to show fleets of trawlers leaving plumes of mud behind them like contrails. They hope the images will focus wider attention on trawling damage, and on the possible uses of satellites to monitor fishing.

One of the researchers, Kyle Van Houtan, who earned his doctorate in environmental science in December at Duke, began the work when he was studying the nesting success of sea turtles and wanted to check the influence of shrimpers, who trawl the bottom for their catch. He turned for guidance to Daniel Pauly, director of the fisheries center at the University of British Columbia, which maintains an elaborate global database on fishing.