GEO-6 Chapter 7: Oceans and Coasts

7.4.4 Marine litter

7.4.3 Mariculture

7

Although the greatest accumulation of marine litter is in coastal environments (Derraik 2002), plastic (including microplastic) is distributed worldwide in the ocean, with increased accumulation in the convergence zones of each of the five subtropical gyres (Cozar et al. 2014; Van Sebille et al . 2015; Yang et al. 2015; see Figure 7.7 ). Plastic pollution has been recognized for decades as a threat to marine biodiversity (Gray 1997). One of the most visible impacts is death or injury of marine life from entanglement with derelict fishing gear and plastic packaging. Many animals also ingest litter, either accidently or intentionally when it is mistaken for food. This can cause starvation due to intestinal blockage or lack of nutritian (UNEP and GRID-Arendal 2016). Recent reviews have found that a growing number of turtles, marine mammals and seabirds are endangered or killed by floating litter (Thiel et al. 2018; O’Hanlon et al. 2017).

Mariculture has a substantial impact on the marine ecosystem, and documentation of these effects is growing. Conversion of mangroves for mariculture has resulted in widespread habitat loss with far-reaching implications for dependent species. In open, dense culture facilities, antibiotics and other medications used to prevent disease are carried by currents and tides well outside the waters in the culture area. Excessive feed sinking through the cages can accumulate on the sea floor, decompose and reduce oxygen levels. These and other effects, such as being vectors or resources for parasites and diseases, or increasing risks of non-adaptive gene-flow and invasive species, can be managed through careful, albeit sometimes costly operations (Bernal and Oliva 2016). However, the ecosystem approach is also being applied in aquaculture, with comparable objectives and rapid uptake by industry (FAO 2010).

Figure 7.7: Plastic litter in the open ocean

Plastic currents A giant distribution system for marine plastics

North Atlantic gyre

North Paci c gyre

South Atlantic gyre

South Paci c gyre

Indian Ocean gyre

Sample points used in the model

Microplastic concentration* Kilograms per square kilometre

0

10

Surface current

Source: GRID-Arendal (2016b), based on data from Van Sebille et al . (2015)

188

State of the Global Environment