Some human impacts go rather unnoticed in comparison with hunting and by-catch. They may not kill directly, and their impact is more difficult to assess, but their long-term effects might be detrimental at the population level. Pollution and ocean acidification are such “soft” or “silent” killers.
GESAMP (Joint Group of Experts on the Scientific Aspects of Marine Pollution) defines marine pollution as “the introduction by man, directly or indirectly, of substances or energy into the marine environment (including estuaries) resulting in such deleterious effects as harm to living resources, hazards to human health, hindrance to marine activities including fishing, impairment of quality for use of sea water and reduction of amenities” (Sciortino and Ravikumar 1999).
Marine pollution poses a chronic, vast, dynamic and growing threat to the marine environment and marine mammals. The continuous addition of large quantities of all kinds of pollutants induces a gradual build-up in the marine environment. It can take the form of chemical substances (such as contaminants and oil spills), energy (such as noise) and debris (such as plastic, nets, rubber and other persistent trash) that degrade slowly, if at all. They transfer from their sources, in industrialised regions, to remote, usually non-industrialised, regions and have an impact at the global level.
Contamination is “the presence of elevated concentrations of substances in the environment above the natural background level for the area and for the organism” (Sciortino and Ravikumar 1999). Contaminants (or chemical pollutants) represent a significant threat to the environment and marine mammal populations, as most of them are not biodegraded and accumulate through the food chain. They may affect the reproductive, immune and nervous systems of organisms.
You can read more on contaminants here.
Marine debris in the ocean, especially plastic debris, is an increasing environmental issue at the global level. Between 5 and 13 million metric tons of plastic are dumped in the ocean every year from coastal countries (Jambeck et al. 2015). Plastic pollution is a conservation and welfare concern threatening the marine environment and marine mammals, notably through entanglement, ingestion and as a vector of toxins, pathogens and alien species (NPI 2018). Over 50% of all marine mammal species have suffered from ingestion of marine debris or entanglement.
You can read more on marine debris here.
Micro- and nanoplastics
Microplastics are tiny plastic particles ranging from 5mm to 1µm in diameter, while nanoplastics are smaller than 1 µm (Framsenteret 2018). These tiny particles can act like a sponge as they accumulate contaminants and become a vector of toxins and pathogens. They are likely the most numerically abundant items of marine plastic debris and can easily be ingested by marine species, both directly and indirectly, including those that end on our plates, such as shells, fish and marine mammals.
You can read more on micro- and nanoplastics here.