The oceans have become more polluted by large plastic objects such as bags over the past 60 years.Credit: Angel Fitor/Science Photo Library

Scientists have uncovered the first strong evidence that the amount of plastic polluting the oceans has risen vastly in recent decades — by analysing 60 years of log books for plankton-tracking vessels.

Data recorded by instruments known as continuous plankton recorders (CPRs) — which ships have collectively towed millions of kilometres across the Atlantic Ocean — show that the trackers have become entangled in large plastic objects, such as bags and fishing lines, roughly three times more often since 2000 than in preceding decades.

This is the first time that researchers have demonstrated the rise in ocean plastics using a single, long-term data set, says Erik van Sebille, an oceanographer at Utrecht University in the Netherlands. “I’m excited that this has been finally done,” he says. The analysis was published1 on 16 April in Nature Communications.

Although the findings are unsurprising, long-term data on ocean plastics had been scant: previous studies looked mainly at the ingestion of plastic by sea creatures over shorter timescales, the researchers say.

Fishing for data

CPRs are torpedo-like devices that have been used since 1931 to survey plankton populations, by filtering the organisms from the water using bands of silk. Today, volunteer ships such as ferries and container ships tow a fleet of CPRs around the world’s oceans.

But in recent years, the devices have more often returned from sea tangled up in plastic, says Clare Ostle, a marine biogeochemist at the Marine Biological Association in Plymouth, UK, which oversees the fleet of devices.

Each time a ship tows a CPR, the crew fills in a log book and notes any problems with the device. So Ostle and her colleagues looked through all tow logs from the North Atlantic between 1957 and 2016, to determine whether plastic entanglements have become more common.

Over the 60-year period, 208 such entanglements were recorded, including a few in the 1950s and 1960s. But since then, encounters with large plastic items, or macroplastics, have become much more common, rising through the 1980s and 1990s and peaking in the 2000s when the CPRs were becoming entangled in roughly 3% of tows (see ‘Plastic entanglements’). Fishing gear was the biggest culprit — involved in 55% of all entanglements.

Source: Ref. 1

Evidence analysis

It’s unlikely that the increase results from better recording of entanglements: when the team analysed the number of CPR encounters with natural items, such as seaweed and fish, there was no significant change over the same time period.

Van Sebille says that because the study focused on large plastic items, it doesn’t reveal much about the quantity of microplastics — fragments fewer than 5 millimetres long — in the oceans. These tiny contaminants come from sources such as disposable plastic packaging, rather than from fishing gear.

Nevertheless, he adds, the study demonstrates that fisheries play a major part in plastic pollution, and will provide useful baseline data for tracking whether policy changes affect the levels of plastic in the oceans. “As fisheries become more professional, especially in the North Sea, hopefully we might see a decrease,” he says.