It has been known for decades that marine animals, especially seabirds, sea turtles, seals, and sea lions, are frequent victims of ghost fishing (Kühn et al. 2015).
There is an another issue that poses serious health hazards to them, namely when marine animals eat plastic garbage, and this is of great concern. Recently, scientists have discovered that when plastic waste becomes microplastic, it acts as a sponge and delivers chemical contaminants that can be passed up the food web to humans.
Microplastic has found its way into every area of the world’s oceans. It is too late to completely clean up the plastic pollution from the oceans, and the microplastic has begun to negatively affect the marine ecosystem. There are numerous marine animals, such as fur seals and others, that unknowingly ingest microplastics.
Conservation groups often bring up the issue of seals becoming entangled in ghost nets (Laist 1997). A further study has revealed that 12 species from the seal superfamily (Otaridae and Phocidae) inadvertently eat plastics (Kühn et al. 2015).
In a research conducted at Macquarie Island, south of New Zealand, plastic particles were found in seals’ feces (Eriksson & Burton 2003).
The majority of plastic fragments found in these feces were very small, with most of them less than 1 cm, so scientists assume that these fragments were probably not directly ingested by the fur seals, but rather came from their staple food: the lantern fish (Eriksson & Burton 2003). This way of eating plastics without knowing it is called secondary ingestion of plastic.
In a study conducted in the Netherlands, scientists found plastic fragments in harbor seals’ stomachs. The harbor seals that had plastic in their stomachs represented about 10% of the total population, and the cause of the plastic in their stomachs was probably through secondary ingestion; it was derived from the fish they ate (Bravo Rebolledo et al. 2013).
Small microplastics accumulate in the stomachs of lantern fish, and lantern fish are a staple food for many larger marine species (Boerger et al. 2010, Davison ＆ Asch 2011). With this equation it makes it easy for us to show that secondary ingestion occurs more often than we estimate, especially for fur seals and seals (Kühn et al. 2015).
In general, if wild animals eat plastic debris by mistake and cannot regurgitate it, the plastic debris accumulates in their gastrointestinal tract, causing a nutritional disorder, reducing their reproductive power, and decreasing their survival rate (Bjorndal et al. 1994, McCauley & Bjorndal 1999).
Because microplastic is a relatively lighter material in the oceans, it has a larger surface area than other materials with the same volume, so more harmful chemical substances such as PCBs and DDT can adhere to its surface (Rochman 2015). With this evidence, it can be said that microplastics are among the worst carriers of pollutants in the marine environment.
It is possible that dangerous chemical substances are becoming concentrated throughout the food chain. This issue has become a growing concern. However, we are not sure about the possible deleterious effects of their presence in the marine food web.