The most destructive of the industrial fishing methods involves a large net with heavy weights being dragged across the seabed, capturing all marine life on the seafloor, including the target species, and other species with no commercial value (bycatch). Trawling devastates entire marine ecosystems and habitats in its path (similar to the practice of clear-felling trees in a forest), including seaweed habitats, coral reefs and seagrass meadows that provide food and shelter for marine species, and are vital breeding grounds and nurseries for juvenile fish.
The bottom trawling method is the gear type that accounts for most of the discard (bycatch) from global marine capture fisheries (see Bycatch & Incidental Capture of Cetaceans under Impacts). Discard of non-target species in global marine capture fisheries between 2010 and 2014 was estimated to be 9.1 million tonnes, with 45.5% (or 4.2 million tonnes) of the total annual discard from bottom trawling, and an average discard rate of 21.8% (FAO 2019).
Dynamite (blast) fishing
The use of explosives to catch fish is illegal in many nations, although it is routinely practised in some countries. Small bombs are set off underwater, stunning and killing fish so they float to the surface to be collected. The rate of bycatch is high, as the explosion injures or kills many non-target species, so more fish are wasted than harvested (Ocean Health Index 2021).
The seabed is reduced to rubble, cratering coral reefs, destroying natural structures that take decades to rebuild. This method is used in some small-scale fisheries, but it is a major threat to local communities who rely on fish for their economic livelihood and as their primary food source (Katikiro & Mahenge 2016).
The use of poison to catch live fish for the food and aquarium industries is nominally illegal, yet widely practised in many countries. Sodium cyanide is squirted into seawater, stunning fish, making them easier to catch. Cyanide devastates coral reef ecosystems and kills marine organisms dependent on coral habitat. The rate of bycatch is low, as the poison injures or kills species it contacts, but there is no physical damage to habitat (Ocean Health Index 2021).
Cyanide poisoning damages coral zooxanthellae, slows photosynthesis, causing loss of coral pigment (bleaching). Cyanide poisoning is magnified under increased temperature and linked to higher mortality rates in simulated ocean warming and heatwave scenarios, compared to the control temperature (Madeira et al. 2020).
Most coral reefs are located between 30 degrees latitude north and south of the Equator where ocean temperatures are warmer, covering more than a quarter of a million square kilometres. Key findings of the most recent World Resources Institute (WRI) publication Reefs at Risk Revisited indicate most coral reefs globally are threatened by human activity.
More than 60% of reefs are immediately and directly threatened from at least one local source, including overfishing and destructive fishing (dynamite fishing and cyanide fishing). Overfishing and destructive fishing threatens 55% of coral reefs globally, and 95% of coral reefs in Southeast Asia. When local sources are combined with heat stress, about 75% of coral reefs are threatened, reflecting impact of rising ocean temperature and increased coral mortality from bleaching (WRI 2011).
Seagrass beds or meadows occupy a variety of marine habitats, but are typically found in shallow, protected, gently sloping estuaries and along coastlines with soft and sandy or muddy substrates. Seagrasses grow in salty and brackish waters in depths of between 1-3 metres, where light levels required for photosynthesis are high. Seagrass meadows may be mono-specific or consist of multi-species communities.
Seagrasses are home to a wide variety of mobile and sessile marine invertebrates. Mobile herbivorous species including gastropods, limpets and sea urchins graze on the epiphytes attached to their leaves, while sessile animals including oysters, barnacles and filter feeding sponges permanently attach themselves to the leaves.
Carnivorous invertebrates including crustaceans and molluscs frequent seagrass meadows. The cephalopod Sepia apama (Giant Australian Cuttlefish) form large aggregations in Spencer Gulf, South Australia for their annual mating ritual among the seagrass beds.
Vertebrates occupying seagrass beds include fish, rays, sharks, reptiles and mammals, including the Sirenian Dugong dugon (Dugong). Seagrass is a primary food source for these grazing animals that consume about 28-40 kilograms (wet weight) of seagrass every day.
Previous estimates of seagrass loss were 29% on a global scale, and a 7% rate of decline between 1990 and 2010 (Waycott et al. 2009), although current estimates are unknown.
Mangroves are the collective name for about 50 true mangrove species from 16 families, ranging from small shrubs to trees 60 metres tall. Mangroves thrive in muddy, water-logged soils, survive being flooded twice a day by ocean tides and can tolerate water many times saltier than other plants. Mangrove forests are found along riverbanks, shorelines and estuaries in the tropical and subtropical regions, usually within 25 degrees of the Equator.
Mangrove forests are important intertidal marine environments, act as natural buffer zones between land and sea, protect coastlines from hurricanes and storms, and provide various ecosystem services including food, shelter and economic livelihood for coastal communities. Mangrove forests are home to incredible biodiversity, act as refuges and nurseries for many species, including manatees, fish and marine turtles, and are dolphin hunting grounds.
Historical global estimates of mangrove deforestation between 1980 and 2000 were high, up to 35% of cover, with annual deforestation rates estimated between 1 and 8% (Friess et al. 2019). Between 1990 and 2020, global mangrove area decreased by 1.04 million hectares, although rate of loss more than halved (FAO 2020). Recent studies show mangrove forests may be at risk from sea level rise (Saintilan et al. 2020).
Mangrove forests are threatened by the environmental impact of human activity, including tourism, urbanization, aquaculture, agriculture, oil exploration, coastal development, and the charcoal and wood industries. Finfish and shellfish aquaculture were the leading cause of mangrove deforestation between 1950 and 2000 (Friess et al. 2019), devastating local economies and communities, creating environments unable to sustain fishing or farming.
© 2016 – 2021 Seafood Free September
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