A dead gray whale washed ashore on West Seattle's Arroyo Beach last week. In itself, that is not an  inherently unusual occurrence: five to ten gray whales die annually in Puget Sound waters, unable to complete the lengthy migration from the lagoons of Baja California, where they breed, to their summer feeding waters north of Alaska. Indeed, that whale was the fourth discovered in the region in a little over a week.

What was especially interesting about this particular whale, however, was its stomach contents. According to a postmortem examination by Cascadia Research:

The animal had more than 50 gallons of largely undigested stomach contents consisting mostly of algae but also a surprising amount of human debris including more than 20 plastic bags, small towels, surgical gloves, sweat pants, plastic pieces, duct tape, and a golf ball.

(If you're looking for a good gross-out, this slideshow contains images of those stomach contents).

However, the trash was likely not to blame for the whale's death, despite its extent:

The debris, while numerous, made up only 1-2% of the stomach contents and there was no clear indication it had caused the death of the animal. It did clearly indicate that the whale had been attempting to feed in industrial waters and therefore exposed to debris and contaminants present on the bottom in these areas. Gray whales are filter feeders that typically feed on the bottom and suck in sediment in shallow waters and filter the contents to strain out the small organisms that live there. They have been known to accumulate material including rocks and other debris from the bottom ingested in this process. While debris has been found in the stomachs of some previous gray whales found dead in Puget Sound, this appeared to be a larger quantity than had ever been found previously.

Unfortunately, there is little reason to believe that gray whales will be any less likely to ingest such pollutants in future.

A recent study in the journal Marine Pollution Bulletin found the average density of marine debris along seafloor habitats off the California coast to be "significantly higher" than the 1990s. The list of debris detected by the study's authors is not dissimilar to that found in the dead whale's stomach: monofilament fishing lines, longlines, fishing nets and traps, cables, beverage cans and bottles, tires, hub caps, and even outboard motors and 55 gallon drums.

The study's authors note that plastic "was the most abundant material and will likely persist for centuries."

Nor is debris along the seabed the only plastic peril facing gray whales – and other marine wildlife – along their migration route. The authors of another study in the same journal retrieved and examined 870 lost or abandoned gillnets in Puget Sound and environs, and found the nets had ensnared almost 32,000 marine invertebrates, over 1,000 fish, more than 500 seabirds and 23 marine mammals. They wrote:

The legacy of such gear can be devastating to marine populations in Puget Sound; divers reported a single derelict gillnet suspended between rocks off the southwest corner of Lopez Island in the San Juan Islands that had thousands of bones piled 1–3 feet deep and running the length of the 30 m span of the net.