The primary pressures on open ocean biodiversity are overexploitation, pollution from land-based activities and climate change; coastal ecosystems have additional pressures associated with habitat destruction, aquaculture and invasive species (see Section 7.2). Although data are limited, these pressures affect the state of marine biodiversity from populations to ecosystems.
Coastal systems are particularly vulnerable; for example, between 20 and 35 per cent of mangrove area has been lost since 1980 (Innis and Simcock eds. 2016) and the current annual rate of seagrass habitat destruction is about 8 per cent (Innis and Simcock eds. 2016). Coral reefs are among the most biodiverse marine ecosystems, yet they are also among the most fragile (see Section 7.3.1).
The decline in the health of marine ecosystems and biodiversity is increasingly affecting people (WWF 2015). Marine capture fisheries provide healthy food and support livelihoods (see Section 7.3.2). However, overexploitation is leading to population declines in marine fisheries with the percentage of global stocks fished at biologically unsustainable levels increasing from 10 per cent in 1975 to 33 per cent in 2015, with the largest increases in the late 1970s and 1980s (FAO 2018b; Figure 6.17). In 2015, over 50 per cent of the stocks in the Mediterranean, Black Sea, the Pacific Southwest and the Atlantic Southwest were fished at biologically unsustainable levels (FAO 2018b).
Exploitation of target species is coupled with additional negative biodiversity impacts from by-catch and damage to benthic environments from trawling, although some seabird populations have increased through feeding on discards (Foster, Swann and Furness 2017). The rise of aquaculture can reduce pressures of exploitation for some wild species, but can also lead to invasive species, inter-species breeding, eutrophication and disease spread (Ottinger, Clauss and Kuenzer 2016) (see Section 7.4.3).
Pollution, including marine plastic litter and microplastics (see Box 6.2), and loss and degradation of habitat leads to further reduced contributions from natural systems, such as declining fish nursery grounds or mangrove wood supply (Nordlund et al. 2016; Quinn et al. 2017), as well as increases in vulnerability to extreme events (see Box 6.3) through reduced coastal protection.