The problem of plastic pollution of the marine environment has been known since the 1970s, but it has only recently emerged as a serious threat. More than 300 million t of plastic were produced in 2015, an amount that is expected to double by 2025. As understanding of the issue has improved over the last decade, increased attention has been focused on microplastics. Microplastics particles are less than 5 mm and include primary plastic in the form of microbeads in cosmetics that are now banned in the US and UK, as well as pellets that are the raw material for plastic extrusion. So-called secondary microplastics, in the form of fibers, fragments, and films, are produced by fragmentation of larger plastic debris.

Like macroplastics, microplastics are ubiquitous in the marine environment — at the water surface, in the water column, in deep sea sediments and from the tropics to the poles. Microplastics are concentrated at the center of the five large oceanic gyres in the world’s major oceans. Microplastics also occur in major freshwater rivers and lakes.

The source of microplastics is highly variable. In areas with intensive fishing, more than 70 percent of microplastics are derived from fishing gear, much of it so-called ghost gear that continues to fish after it is lost. In one case study in South Korea, most of the plastic pollution in a particular area was associated with lost floats used for mussel culture longlines. In most developed countries, plastic waste in landfills are fairly well controlled and regulated. Some developing countries, however, do not have good management control over disposal areas, especially those in the coastal zone.

Microplastics are ingested by a wide array of marine life, including commercially important bivalves and crustaceans. Extractive forms of aquaculture, such as bivalve shellfish farming, have been promoted as environmentally friendly because they filter water and contribute to eutrophication control. Unfortunately, filter feeding is not highly selective and shellfish can also remove microplastics from water. Laboratory studies have demonstrated that oysters can efficiently remove plastic microbeads as small as 6 μm in diameter. Ingestion of microplastic particles can cause various physical impacts, as well as inflammation.

Many microplastics are of a similar size to large plankton and fish eggs. Microplastics can be ingested by forage fish that are used to produce fishmeal and the likelihood that microplastics are transferred from whole fish to fishmeal cannot be discounted. The risk of microplastics in fishmeal to fed aquaculture is currently unknown.

The public health risk of consuming seafood containing microplastics appears to be negligible, although uncertainties are substantial. Microplastics are typically restricted to the guts of organisms that consume them. For most fish used as food, gut contents are removed prior to consumption. However, microplastics can be transferred to consumers of fish and shellfish that are eaten whole. The EU has concluded that levels of food contaminants are generally well below thresholds of concern for human safety. As with other risks of seafood consumption, the potentially negative health impacts of consuming microplastics should be evaluated against the beneficial effects of seafood consumption.

Of potentially greater concern are a category of microplastics known as nanoplastics (1-100 nm), some of which can be absorbed across cell membranes, including gut epithelia. Nanoplastic particles can cross cell membranes and bioaccumulate following transfer across trophic levels. Furthermore, plastics often contain potentially toxic additives that impart certain desirable qualities to plastic polymers. Microplastics are also hydrophobic and will adsorb persistant bioaccumulative toxins, among other compounds, from water. There are large knowledge gaps and uncertainties about the human health risks of nanoplastics.

Aquaculture makes wide use of durable plastics, which are used for tanks, fish cage collars, pond liners, netting, rope, and floats, among other items. In the context of global plastic pollution of the oceans, aquaculture is a tiny contributor, although estimation of the size of the contribution remains a knowledge gap and lost or derelict gear from aquaculture can be a locally important contributor.

In places where coastal aquaculture has considerable plastic infrastructure on the water, maintaining gear in good working condition and retrieving lost or derelict gear is responsible aquaculture. There is scope for localized improvement here. A disproportionate amount of plastic waste comes from fisheries and aquaculture in intensively managed coastal zones. All the prominent aquaculture certification systems have some variation on the 3 R’s of reduce, reuse and recycle.

Aquaculture is both affected by and contributes to the presence of microplastics. As such, aquaculture is a major stakeholder in discussion about how to develop adaptive management plans to address an issue that is global in scope but with substantial geographic variation in magnitude. It can use its role as a major stakeholder to influence outcomes that reduces the risk of microplastics to aquaculture. Aquaculture needs to keep its own house in order but also needs to advocate for good management control of solid waste disposal in coastal areas and for improved wastewater treatment. Aquaculture can also play a role in raising awareness of the microplastics issue and promoting anti-litter campaigns.

The biggest problem is that production and use of plastic results in waste accumulation. In particular, single-serve packaging has been responsible for the biggest growth in plastics production. While such packaging is hygienic and contributes to food safety and product shelf-life, it has greatly increased the flow of plastic solid waste to disposal sites. A major shift in consumption patterns is needed, one that treats the economy of plastics production, consumption, use as more circular, with waste designed out of the cycle.

Finally, changes in cultural values are needed to embrace an ethic of environmental stewardship, including changes in attitudes towards littering. It seems highly likely that plastic pollution of the oceans will increase unless values shift. Aquaculture can make a positive contribution to the discussion by emphasizing the moral imperative and the utilitarian necessity of maintaining healthy oceans. —
John A. Hargreaves, Editor-in-Chief