What we know
A blue whale can swallow up to 43.6 kg of microplastic in a single day. Over a full feeding season, the count passes one billion particles. And the surprising part is where it comes from: not — or not mainly — from the water the animal filters. Over 98% of the plastic ingested passes through the prey — krill and small fish — that whales eat by the ton at exactly the depth where microplastics concentrate most, between 50 and 250 metres. Where they feed, in other words, is where the plastic is: the krill they live on is already loaded with it. The study measured all this by combining microplastic concentrations in the water column with data from 191 tagged whales — blue whales, fin whales and humpbacks — in the California Current, off the US Pacific coast.
Why it matters
What makes the difference isn't the animal's size, but its diet. A blue whale ingests on average about 10.9 million particles a day, a fin whale 5.7 million. But the sharpest contrast is among humpbacks: those that feed on krill reach 3.9 million particles a day, while those that eat fish stop at around 200,000 — an order of magnitude less. It's the diet that counts, not the size. And this shifts how we see the problem: microplastic isn't just a contaminant animals ingest by chance from the environment, but something that concentrates and moves up the food chain.
What's still uncertain
The limits, up front. The study is an estimate built on a model, not direct observation of stomach contents. It covers only the California Current: elsewhere, the Mediterranean included, the numbers may change. And it measures how much plastic whales ingest, not what harm it does to their health — that remains to be understood. Nanoplastics too, the smallest and potentially more harmful fragments, aren't measured directly: the study only assumes their presence.