Plastic Bags and Packaging Are Among Most Lethal for Marine Animals

In one of the saddest news stories of recent weeks, scientists from Hobart, Tasmania, took up the heartbreaking task of figuring out what kinds of plastic pollution are worst for killing large marine animals and seabirds. The study, which was published in the journal Conservation Letters, analyzes the results of 655 studies on marine debris, 79 of which described associated deaths of cetaceans (whales and dolphins), pinnipeds (sea lions and seals), sea turtles, and seabirds. 

What the researchers found is that film-like plastics, such as bags and packaging, and fishing nets or rope are "disproportionately lethal" for larger animals, while items like balloons, rope, and rubber are more dangerous for smaller animals. Film-like plastics caused the most deaths in cetaceans and sea turtles; fishing debris caused the most deaths in pinnipeds; and hard plastic pieces caused the most deaths in seabirds.

When it comes to cetaceans, the films they ingest cause fatal gastric obstructions, typically in the stomach. Often these obstructions prevent them from swimming and diving properly and so they remain at the surface for days, increasing their risk of being hit by ships and boats. The study says that half of ship-struck cetaceans have ingested plastic, which suggests that "mortality resulting from plastic may be more common than direct mortalities from confirmed gastric obstructions or perforations would suggest."

Sea turtles suffer greatly as well. The plastic they ingest is a mixture of films and hard pieces, and it tends to form a bolus, or small rounded mass, that blocks the stomach or intestines. Similar to cetaceans, this affects buoyancy and forces the turtle to remain at the surface, where it's likely to be struck and killed by a ship or boat. 

Seabirds mainly ingest fragments of hard plastic, typically "buoyant hard plastic polymers such as polyethylene and polypropylene [that] float at the oceans' surface where foraging seabirds mistake them for food." Although hard pieces pose less risk than soft plastic films, the hard pieces cause more deaths because they're more frequently ingested and can get stuck internally.

Armed with this dismal information, the researchers make a few key suggestions. First, they want scientists to start recording more detailed information about the plastic found during necropsies. Up until now it's been frustratingly vague, making projects like this difficult to execute. Take rubber, for example, which is described as "the most disproportionately lethal debris items highlighted by this review" – except that the source of the rubber is seldom described in studies, thus limiting the policy recommendations that can be made.

Next, the authors call for policy changes that limit the disposal of plastic into marine environments. From the study:

"We propose that the most cost‐efficient way to prevent megafauna mortality would be by prioritizing the prevention of large and more lethal items. We have already seen a global response in the form of plastic bag bans and fees for bags, which are reducing or eliminating single‐use thin film bags in cities and countries around the world."

These are steps in the right direction, but must be expanded more broadly and as rapidly as possible. 

Fishing-related debris is another significant threat to marine life, and this could be reduced with tighter oversight, improved fisheries management practices, and engineering solutions to reduce fishing gear losses. The study authors write,

"[Commercial] fisheries have high gear loss rates; 5.7% of all nets and 29% of all lines are lost annually ... Solutions to reduce loss of fishing gear include repair or port disposal rather than at‐sea disposal of damaged nets, enforcing penalties associated with dumping, failure to retrieve lost items, and restricting fishing activity in conditions/locations where loss is likely."

Microplastics, which have received a lot of attention in recent years, do not pose as immediate a threat to marine megafauna as larger pieces do. These were "seldom implicated in mortality," though their presence is "likely underestimated in our summary, as many studies of larger taxa did not count small items." Microplastics are known to be harmful to small seabirds and turtles, contributing to blockages. 

By identifying specific kinds of plastic as key threats, policy-makers are then able to create legislation to reduce use and improve disposal methods.