Plastic fibres and particles in West Coast waters are being consumed and passed up the food chain by tiny marine creatures that apparently mistake them for food, according to a new study from the Vancouver Aquarium Marine Science Centre.

Researcher Peter Ross and his colleagues found plastic litter in the digestive systems of two key species of plankton that are eaten in large numbers by salmon and baleen whales.

Adult salmon returning to the Strait of Georgia may be consuming up to 91 plastic particles a day by eating plankton, and juveniles leaving fresh water up to seven particles a day, while a humpback whale could ingest more than 300,000 particles a day, according to the researchers' estimates.

Several recent studies have documented ingestion of plastics in the wild by fish, bivalves and crustaceans. Plastic particles have also been detected in the scat of marine mammals.

"Most salmon species feed heavily on (plankton) during their juvenile and adult life stages," said Ross. "These particles could pose a serious risk of physical harm to the marine animals that consume them, potentially blocking their gut or leaching chemicals into their bodies."

Images of dead seabirds with their guts full of plastic particles brought global attention to the problem of microplastics several years ago, Ross said.

Whether microplastics in the marine food web pose an immediate threat to human health is unknown, but the risk to humans is certainly lower than for marine mammals, he said. The plastics do not pass into the flesh of fin fish, which is the part of the animal we usually consume.

However, shellfish consumed in their entirety may pose a risk. A 2014 study by Ghent University found that consumers of mussels could be ingesting 11,000 plastic particles a year.

Barely visible to the eye, the microplastic particles found in plankton result from the breakdown and decay of plastics. Microplastic fibres — commonly found in sewage effluent — likely result from washing clothes made of synthetic fabrics and the decay of fishing and fish-farming gear.

Plastic pollution is typically higher in ocean waters near population centres, or they may be highly concentrated in mid-ocean waters by circling currents, called gyres.

The study, which focused on the Strait of Georgia, West Coast Vancouver Island, Haida Gwaii and mid-Pacific, found that the concentration of ingested plastics was highest for both species of plankton in the Strait of Georgia, a crucial feeding ground for young fish before they migrate to open water.

In a previous study, the concentration of microplastics in sea water was as much as 20 times higher in the Strait of Georgia than offshore.

The study was published this month by the Archive of Environmental Contamination and Toxicology.