Marine debris has been found in the digestive tracts of 47 species of whale, that is 59% of the 80 species of cetaceans still extant (Kühn et al. 2015).
In cases where whale deaths were caused by eating marine debris, 60% of them were due to swallowing fishing gear while the remaining 40% were due to swallowing plastic waste (Baulch & Perry 2014).
It is very common for plastic waste such as plastic bags to be found in the stomachs of whales stranded a beach (de Pierrepont et al. 2005).
Recently, however, it has been found that whales eat not only large pieces of plastic waste but also microscopic microplastic.
From the stomach of a humpback whale that was stranded in the Netherlands, pieces of microplastic smaller than 5mm were retrieved (Besseling et al. 2015).
Baleen whales swallow plankton with their big mouths; at the same time, they swallow microplastics as well (Fossi et al. 2016).
For example, when sei whales eat their food – krill – they swallow 71 ㎥ of seawater at a time (Goldbogen et al. 2007). If we suppose that a bathtub at home can contain 200 ℓ of water, this would be equal to about 350 bathtubs, and they end up swallowing microplastic with their food.
The size of the microplastics floating in the sea is the same as the size of the zooplanktons that the whales eat.
Scientists examined the size of microplastics in the Mediterranean Sea, where sei whales live. Of these, 50% were tiny, between 1mm and 2.5mm, and 40% were between 2.5mm and 5mm (Fossi et al. 2016). This is exactly the same size as zooplankton.
In the Mediterranean Sea, microplastics tend to accumulate intensely in the places where sei whales come to find food (Fossi et al. 2016). Therefore, when sei whales swallow zooplankton, they inevitably swallow microplastic as well.
A sei whale filters about 5,800 ㎥ of seawater per day. Therefore, it is estimated that the sei whales living in Mediterranean sea are swallowing thousands of pieces of microplastic a day with krill, their normal food (Fossi et al. 2014).
An Italian research team studied the density of chemical substances contained in the fat of sei whales living in the severely contaminated Mediterranean Sea as well as from the much less contaminated Gulf of California (Fossi et al. 2016).
They found higher densities of persistent organic pollutants (POPs) among whales in the Mediterranean than those in the Gulf of California; these POPs include phthalic acid-based plasticizer*, which is often used to soften plastics, as well as DDTs, polychlorinated biphenyls (PCBs) and hexachlorobenzene (HCB), which may be absorbed by plastics (Fossi et al. 2016).
*That found in whales was MEHP (mono(2-ethylhexyl)phthalate), which is metabolized from bis(2-ethylhexyl)phthalate), DEHP, which is often used as a plasticizer for PVC (Fossi et al. 2016).
Furthermore, MEHP was also detected in krill in the Mediterranean, meaning that the food of the whales has also been shown to have been contaminated by chemical substances from plastic (Fossi et al. 2014).
In this way, sei whales in the Mediterranean Sea accumulate toxic substances within their body, both directly and indirectly, by swallowing microplastic that contains toxic chemical substances in addition to plankton that is also contaminated by chemical substances.
A sei whale eats 900 g of plankton a day (Fossi et al. 2016). Huge amounts of microplastic must be included in that figure.
Sei whales are believed to be removing huge quantities of microplastic from seawater by swallowing it (Fossi et al. 2016). They transfer them to the other areas of the ocean or the deep sea by excreting them. But the chemical substances that were adhered to the plastic probably still remain within their bodies (Fossi et al. 2016).
In spite of the fact that the sei whales have a wide-spread, they are listed as endangered species and as a vulnerable species in the Mediterranean Sea (IUCN Red List). Scientists have pointed out that their lives are being increasingly threatened by plastic contamination, unless we take the necessary steps (Fossi et al. 2016).