Making up an estimated 60 - 80% of marine debris, plastics in the oceans is more than just unsightly. It's a threat to many of the critters living there. Marine species throughout the food web - from the very small to the not so small accidentally or deliberately ingest plastics, often mistaking it for food. Aside from filling the animals stomach with garbage that provides no nutritional value what-so-ever - (just like with the Green turtle in the image below), plastics can either be toxic in themselves, or become saturated with toxins. Plastics and other marine debris often finds itself converging around gyres - strong circular currents in the ocean, just like the North Pacific Subtropical Gyre which circles around the Hawaiian archipelago.
Anela Choy and Jeffrey Drazen of the University of Hawai'i analysed the stomach contents of some 595 individuals representing 10 species of pelagic fish that were captured in the North Pacific Subtropical Gyre. They found plastics in 7 out of the 10 species, though the amount varied quite considerably. Small-eye Lampris' seemed to be most prone to ingesting plastics, with 58% of examined stomachs containing plastics. The stomach contents of Smith’s escolar, yellowfin tuna, and skipjack tunas were much more promising - these guys had no detectable levels of plastics in them. It's not entirely clear why there should be such a disparity in the frequency of plastics found in the stomach contents of different species but interestingly the top 3 plastic ingestors are thought to be mesopelagic predators, often living at deeper depths during the day and moving up to shallower waters at night. The authors put forward a few suggestions, such as secondary ingestion (their prey consumed plastic, and thus ended up in their stomachs) but this is certainly something to be explored.
It's also not clear just how well these critters can cope with plastic ingestion. It may be that they can quite happily pass them with minimal impact, or it may be that they suffer ill-health from the accumulation of toxins or, like what may have happened to the Green turtle in the image below, become unable to feed efficiently and starve to death
According to one UNEP report, 80% of marine debris comes from land based sources...just something to consider before tossing that empty water bottle out the car window...
If you fancy reading the original paper yourself, there is an open access copy of the paper available.