More than nine out of ten sea birds have some plastic in their gut. They do not knowingly eat plastic, but there is so much plastic debris in the ocean that the birds have little choice. Sea birds eat fish amongst other things and fish also ingest plastic, again not for nutrition but by mistake or because they eat things that contain plastic.
The plastic enters the oceans as a result of humanity dumping what it no longer needs into a limitless (apparently bodies of water where a little bit more rubbish appears to make no difference. But it does make a difference and sea birds and the plastic enters the food chain.
Some fish eat seabirds including hake, cod, pollock and monkfish. When they do this they also eat plastic. Humans eat fish and no doubt studies will show the amount of plastic in a human gut will increase.
In 2014 the 5 Gyres Institute estimated that 269,000 tons of plastic are floating on the sea’s surface. More lurks in the depths.
It is believed, but not known, that plastic in the ocean contribute to persistent, bio-accumulative, and toxic substances in the human diet. It is believed but not known that these substances are being harmful to humans. Time will tell and until it does it seems merely common sense that we should stop discarding plastic in the oceans and do what we can to recover what we have already discarded there.