Trawling is a fishing method that involves pulling a large net through the water, either along the seafloor or in the midwater column. This technique is widely used in commercial fisheries globally to catch various marine species. Trawlers, the vessels used for this practice, range in size from smaller boats to large factory ships capable of processing fish onboard. Trawl nets are typically cone or funnel-shaped, designed with a wide opening to capture marine life and a narrow, closed end where the catch accumulates.
Physical Damage to Ocean Habitats
Bottom trawling, a specific form of trawling, alters the physical structure of ocean habitats. The heavy fishing gear, including weighted nets, metal doors, and chains, is dragged directly across the seafloor, scraping and ploughing the substrate. This action impacts benthic organisms and disrupts geological features. Fragile, slow-growing habitats such as deep-sea coral reefs, sponge gardens, and seagrass meadows are particularly vulnerable to this disturbance.
These habitats are important for marine ecosystems, serving as nurseries, feeding grounds, and shelters for a diverse range of marine species. The physical disruption from repeated trawling can change the composition of the seabed, impairing its ability to recover and support marine life. In deep-water environments, where organisms are less accustomed to physical disturbances, recovery times from trawl impacts can extend from several years to many decades.
Unintended Catches and Waste
Trawling, particularly bottom trawling, is indiscriminate, leading to bycatch—the capture of non-target marine animals. This incidental capture affects a wide array of marine life, including juvenile fish, marine mammals like dolphins, whales, and porpoises, sea turtles, seabirds, non-commercial fish species, sharks, and rays.
Most bycatch animals suffer high mortality, often discarded back into the ocean dead or dying. Bycatch can constitute a significant portion of a trawl’s total catch, sometimes reaching 60% or more, and exceeding 80% in certain shrimp trawling operations. This represents a loss of marine resources and threatens incidentally caught species.
Threat to Fish Populations
Trawling contributes to the depletion of marine fish stocks, impacting both targeted species and those caught unintentionally. The efficiency and large scale of trawl operations enable the rapid removal of large quantities of fish from specific areas, often leading to overfishing. This constant pressure can affect the reproductive capacity of fish populations by catching mature, breeding individuals before they can reproduce, or by removing juveniles before they reach reproductive age.
The continuous removal of fish through trawling can result in declines in overall fish abundance and lead to shifts in population structures, such as a dominance of smaller individuals within a species. This practice also reduces genetic diversity within fish populations, making them less resilient to environmental changes. Deep-sea fish species are susceptible to these impacts because they often exhibit slow growth rates and low reproductive output, making their populations slow to recover from intense fishing pressure.
Wider Ecosystem Disruptions
The direct impacts of trawling, such as habitat destruction and species depletion, trigger broader ecological consequences throughout the marine environment. The damage to seafloor habitats and the reduction of specific species can lead to imbalances within the marine food web. This disruption affects predator-prey relationships, as the removal of certain species can cascade through different trophic levels, impacting the availability of food sources for other marine animals.
Disturbing the seabed through trawling also resuspends sediments into the water column. This sediment resuspension reduces water clarity, which can decrease light penetration and negatively affect photosynthesis in marine plants. This stirring of sediments can also release stored organic carbon and other nutrients, altering natural nutrient cycling and potentially releasing carbon dioxide into the water column and atmosphere. Heavily trawled areas may experience a shift towards less diverse ecosystems dominated by more opportunistic species.