September 5, 2025 — The open ocean may seem infinite… but it is not immune to human impact. Every year, millions of tons of plastic and synthetic fibers enter the seas, breaking down into tiny particles that move with currents, accumulate in sediments, and enter marine food webs. Recent research shows that one of the ocean’s most iconic predators, the blue shark (Prionace glauca), may play an unexpected role in this cycle.
Found throughout temperate and tropical waters, blue sharks are one of the most abundant shark species caught as bycatch in tuna longline fisheries. Their feeding habits are broad — they eat fish, cephalopods, and crustaceans — covering hundreds or even thousands of miles in a single year. With such a wide-ranging diet (both in what they eat but also where they eat it), they’re exposed to particles in the water column as well as through the actual prey they consume. A recent study carried out by a team of researchers led by Chenxuan Du of Shanghai Ocean University, China, analyzed blue shark intestines and revealed both plastic and non-plastic particles in every section of the scroll-shaped organ, with the highest concentration in the posterior region. On average, a single intestine contained roughly 11 plastic fibers and 48 non-plastic fibers. Fibers dominated the found mixture, making up more than 95 percent of the particles identified. Plastics were primarily polyester and polyethylene terephthalate, while non-plastics included rayon and cotton, materials known to commonly shed during laundry wash cycles and be carried out to our big, blue ocean through the numerous river systems covering our planet. The size of these fibers varied, ranging from about 0.004 inches to over 8 inches (102 micrometers to 8.1 millimeters), highlighting that while they are small enough to be ingested by other organisms, they are also large enough to remain intact during digestion. Sharks can then excrete these particles back into the ocean miles from where they swallowed them, turning them into unexpected couriers of pollution.