When closely analyzing satellite images for his research on sea turtles, population ecologist Kyle Van Houtan, then at Duke University, kept noticing unusual tracks in coastal waters. Zooming in, he realized they were the ghostly sediment wakes of bottom trawlers, fishing boats that drag nets along the seafloor to capture shrimp, flounder, and other bottom-dwelling species. Van Houtan and fisheries researcher Daniel Pauly of the University of British Columbia realized that these tracks were visible on coastal satellite images worldwide. Some of these images are so high resolution that they show what appear to be seabirds flying overhead, ready to scavenge the dead fish that the trawlers leave behind.
"Bottom trawling is like clear-cutting the ocean floor," says Van Houtan. Besides the unwanted bycatch-four pounds for every pound of shrimp caught-bottom trawlers kick up sediment that adversely affects ocean environments in many ways. They destroy the structure of the sea bottom, reducing the complexity of the habitat. The disturbed mud can smother the remaining seafloor life and redistributes pollutants and nutrients that had settled out of the seawater, which can fuel harmful phytoplankton blooms.
Learn about bottom trawlers and see mudtrails along the Louisiana coast in this Bio Bulletin.