Deep-sea trawling harms biodiversity and carbon storage

Source: October 2017. Bottom Trawling.

Preface. Overfishing has eliminated 90% of the world’s large predatory fishes and is devastating marine ecosystems.

Bottom trawling is one of the most devastating ways our oceans are being overfished, degraded and biodiversity destroyed .  This industry tossed 437 million tonnes of unwanted fish in just the past 65 years, a huge waste  (Cashion 2018). About 20 pounds of bykill are caught for every pound of desired species.

There are tens of thousands of trawlers dragging an area equivalent to twice the lower 48 states every year to catch shrimp and fin fishes.

Trawling is done by large industrial fishing vessels dragging large nets along the sea floor, often pulling up unwanted fish, and generating the most waste of any fishing method because the unwanted catch is dumped back into the ocean.  This has been going on since the Middle Ages, but the damage is orders of magnitude greater now with motorized fishing fleets, powered by government subsidies, using heavier nets to get at fish in much deeper water that are further offshore.

Satellite images show that spreading clouds of mud remain suspended in the sea long after the trawler has passed. But what satellites can see is only the “tip of the iceberg,” because most trawling happens in waters too deep to detect sediment plumes at the surface.

In addition to bottom trawling that’s every more widespread and goes deeper, oceans are also threatened by sea-mining and oil drilling.

Cashion, T., et al. 2018. Reconstructing global marine fishing gear use: Catches and landed values by gear type and sector. Fisheries Research 206: 57

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Pusceddu, A., et al. June 17, 2014. Chronic and intensive bottom trawling impairs deep-sea biodiversity and ecosystem functioning. Proceedings of the National Academy of Sciences 24: 8861-8866

Abstract. Bottom trawling has many impacts on marine ecosystems, including seafood stock impoverishment, benthos mortality, and sediment resuspension. Historical records of this fishing practice date back to the mid-1300s. Trawling became a widespread practice in the late 19th century, and it is now progressively expanding to greater depths, with the concerns about its sustainability that emerged during the first half of the 20th century now increasing. We show here that compared with untrawled areas, chronically trawled sediments along the continental slope of the north-western Mediterranean Sea are characterized by significant decreases in organic matter content (up to 52%), slower organic carbon turnover (ca. 37%), and reduced meiofauna abundance (80%), biodiversity (50%), and nematode species richness (25%). We estimate that the organic carbon removed daily by trawling in the region under scrutiny represents as much as 60–100% of the input flux. We anticipate that such an impact is causing the degradation of deep-sea sedimentary habitats and an infaunal depauperation. With deep-sea trawling currently conducted along most continental margins, we conclude that trawling represents a major threat to the deep seafloor ecosystem at the global scale.

Trawling represents one of the most common fishing practices along the coastal oceans of the world. However, it can have a plethora of impacts on the sea bottom, including stock impoverishment, alterations to the sea-bottom morphology, sediment resuspension, and increased bottom-water turbidity, epibenthos mortality, altered nutrient cycles, and alteration of the benthic biodiversity (1).

Historical records of this fishing practice date back to the mid-1300s, and it became widely practiced with the industrialization of fisheries in the late 19th century (24). Because shallow coastal water resources have steeply declined in the last 50 years (5, 6), fisheries are expanding offshore and trawling is being carried out at progressively increasing depths (7, 8).

In contrast to what was believed up to a few decades ago, deep-sea habitats (>200 m in depth) are rich in biodiversity, and they host many endemic and commercially important species (9, 10). Compared with shallow-water areas, the impact of trawling on deep-sea benthic ecosystems is deemed more severe and long-lasting, because of their lower resilience and higher vulnerability (10). However, our knowledge of the impact of trawling on deep-sea ecosystems has remained limited and has mainly focused on hard-bottom systems, such as seamounts and cold-water coral reefs (11, 12).

Sedimentary environments (i.e., the soft sea bottom) represent the greatest area of the deep-sea floor and host a vast fauna biodiversity (10). In these environments, the metazoan fauna (i.e., multicellular organisms) include almost all of the 35 modern animal Phyla. The smaller components of this fauna, the meiofauna, are characterized by relatively short life cycles, high turnover rates, and a lack of larval dispersion. For all oceanic seafloors, nematodes account for >90% of meiofauna abundance in the deep sea (13) and are characterized by very high species richness and recognizable feeding types and life strategies (14, 15). In this sense, nematodes have been recently used as a model to demonstrate that any loss in deep-sea fauna biodiversity is associated with an exponential decrease in ecosystem functioning (16).

Recent investigations carried out in the north-western Mediterranean Sea have revealed that the continuous stirring, mixing, and resuspension of surface sediments by intensive and chronic trawling activities has caused changes to the present-day sediment dynamics and has permanently smoothed the seafloor morphology of the continental slope over large spatial scales (1719). In this region, deep-sea trawled grounds are subjected to levels of sediment disturbance whose effects are larger than the changes in sediment properties associated with seasonal variability (20). Smoothed trawling grounds are also exposed to a reduced habitat heterogeneity. Because high habitat heterogeneity is crucial to preserve high biodiversity levels (21, 22), trawling activities might represent a major threat to the integrity of deep-sea ecosystems (12, 18).


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