Plastic Debris And Pelagic Fish: A First Look
The world’s oceans are being increasingly polluted by plastic debris as a result of irresponsible waste disposal practices. Such debris is often ingested by marine organisms, including fish, sea turtles, seabirds, and other wildlife. This paper, published in Marine Pollution Bulletin, reports on the first investigation of the occurrence of plastic debris in the stomach content of large pelagic fish (fish that live in the open ocean) in the Mediterranean Sea.
Specifically, the authors investigated the presence of plastic in three pelagic fish species: albacore (Thunnus alunga), bluefin tuna (Thunnus thynnus), and swordfish (Xiphias gladius). They captured a total of 121 fish in the Central Mediterranean Sea, measured them, and examined their stomach content after the fish were killed. They also categorized plastic particles according to number, color, and weight, as well as size using the categories of microplastics (<5mm), mesoplastics (5-25mm), and macroplastics (>25mm).
Overall, the authors found 29 plastic fragments from the stomach content of 22 fish, representing 18.2% of the total specimens. They found particles in 11 bluefin tuna (32.4% of bluefin tuna specimens), 7 swordfish (12.5% of total swordfish specimens), and 4 albacore (12.9% of albacore specimens). Additionally, they found that the plastic debris differed in size in each species, as mesoplastics were more abundant in swordfish stomachs, albacore ingested more microplastics, and bluefin tuna contained similar amounts of macroplastics and mesoplastics.
In their discussion, the authors suggest that the bluefin tuna specimens contained the highest frequency of plastic particles due to their feeding behavior, as they generally hunt schools of small fish close to shore and prey on mesopelagic fish—fish that live in the middle zone of the ocean and have been previously found to ingest plastic particles. This suggestion indicates that some large fish may experience secondary ingestion of plastic by eating prey who initially ingested the plastic.
The authors state that their findings “underline the ubiquitous presence of plastic in the Mediterranean marine biota, including the water column where large pelagic fish live and feed.” They note that plastic litter in the marine environment represents a significant toxicological threat in two ways: through accumulation of persistent, bioaccumulative and toxic (PBT) substances; and through the spread of toxic chemicals that are added to enhance plastic during production. Previous research has shown that such substances may alter the reproductive systems of fish, a particular concern for bluefin tuna, a species listed is endangered by in the IUCN Red List. Additionally, the authors caution that these findings may have implications for people who consume large amounts of pelagic fish, and they call for further research on both seafood safety and impacts of plastic on the marine environment.
For advocates, the paper offers clear evidence of the significant presence of plastic debris in the Mediterranean Sea. The fact alone that nearly one-third of bluefin tuna in the study had plastic in their stomachs should surely be cause for concern. While the paper does not investigate the impact on fish of ingesting plastic, it highlights the need for more research on this subject, in particular on bluefin tuna and other generalist feeders. It should also be noted that advocates will likely not approve of some of the methods used in the study, in particular killing fish in order to identify their stomach contents.