Dredging fishing is a method of harvesting marine life that involves towing a heavy apparatus across the seabed to collect organisms living on or buried within the bottom substrate. This technique is specifically designed to target benthic species. Because of its physically invasive nature, dredging is a subject of significant debate within marine resource management and conservation. The practice extracts commercially valuable species but simultaneously generates widespread physical impacts on the marine environment.
Defining Fishing Dredges and Their Mechanics
A fishing dredge is a specialized piece of equipment consisting of a rigid metal frame with an attached collection bag, typically made of chain mesh or heavy netting. A vessel tows the frame across the seafloor, where it acts as a scoop or rake. The front edge usually includes a bar, often fitted with metal teeth, to rake through the sediment and dislodge target organisms.
The fundamental operation relies on physically disturbing the substrate to capture the shellfish. Tow speed, often around 2.5 knots, and duration are carefully managed to maximize the catch. Larger vessels may tow multiple dredges simultaneously, sometimes deploying up to twenty individual units.
Fishing dredges are broadly categorized into two main types based on their mechanism of substrate penetration. Mechanical dredges use sheer weight and the action of a rigid bar or teeth to scrape, plow, or rake the upper layer of the seabed. This physical force lifts the organisms and surrounding sediment into the collecting bag.
Hydraulic dredges employ a more complex system, utilizing powerful water jets to liquefy or fluidize the sediment before collection. The jets inject pressurized water into the substrate, chasing burrowing species out of the sand or mud and into the dredge’s mouth. This mechanism allows the gear to penetrate deeper into the seafloor, targeting organisms that live several inches below the surface.
Primary Target Species and Commercial Application
Dredging is primarily employed to harvest commercially valuable mollusks that are sessile or burrowing, making them inaccessible to most other types of fishing gear. The technique is essential for capturing bivalve species like scallops, oysters, and certain clams, such as surfclams and ocean quahogs. These organisms are concentrated on the seabed, requiring the direct physical action of the dredge for efficient harvest.
The economic importance of these dredged fisheries is substantial on a global scale, supplying a significant portion of the world’s shellfish market. Scallop fisheries, in particular, represent multi-million dollar industries in many coastal nations. Since these target species cannot move quickly or are permanently anchored, the dredge provides a highly effective means of mass extraction.
The commercial application of this method is driven by the high market demand for these shellfish species. The high yield potential of dense beds often justifies the investment in heavy dredging gear, making it the dominant harvesting method for these specific resources worldwide.
Ecological Consequences of Seabed Disturbance
The most significant consequence of dredging is the physical modification and destruction of the benthic habitat. As the heavy gear is dragged across the seafloor, it scrapes and plows the substrate, which can homogenize complex environments. This physical scraping is particularly damaging to structured habitats, such as seagrass meadows, maerl beds, and slow-growing cold-water corals.
These complex structures provide shelter and nursery grounds for a wide range of marine life, and their removal leads to a measurable loss of biodiversity. The disturbance reduces the overall complexity of the seafloor, favoring fast-recolonizing, opportunistic species over long-lived and immobile species.
Dredging also causes significant sediment resuspension, creating large plumes of fine particles that spread throughout the water column. This increase in turbidity reduces sunlight penetration, inhibiting photosynthesis in marine plants and algae. The plumes can also settle on nearby organisms, smothering sessile invertebrates and disrupting their feeding mechanisms.
The churning of the sediment can also release legacy contaminants, such as heavy metals or pollutants, that have been sequestered in the seafloor for decades. The resulting decrease in water quality and potential for oxygen depletion negatively affects fish and other mobile marine life.
Bycatch, the incidental capture of non-target species, is another major ecological concern associated with dredging. Organisms like starfish, whelks, and sea urchins are frequently scooped up along with the target shellfish. Larger, more vulnerable species, including sea turtles and small marine mammals, can also be caught or fatally injured by the heavy gear.
Many non-target organisms that pass through the dredge or are discarded are damaged or do not survive the experience. This high discard mortality rate further impacts the marine ecosystem by removing predators and prey that maintain the natural balance of the seafloor community.
Regulatory Frameworks and Management
To mitigate the environmental impact of dredging, governments and regional fishery management bodies implement various regulatory frameworks. A common strategy involves establishing spatial restrictions, designating areas where dredging is strictly prohibited. These include Marine Protected Areas (MPAs) or Habitat Management Areas, which protect sensitive benthic ecosystems from physical disturbance.
Fishery managers also employ temporal restrictions, such as seasonal closures, to protect target species during specific life stages. For example, areas may be closed during spawning periods to allow for stock replenishment before harvesting resumes. This ensures the sustainability of the population while also giving the seabed a period of respite.
Gear restrictions are another tool used to reduce the ecological footprint of the practice. Regulations may mandate the use of specific mesh sizes in the collection bag to allow undersized shellfish and smaller non-target organisms to escape. Some fisheries require the use of modified, lighter gear or the addition of devices, such as turtle deflectors, designed to guide vulnerable species over the dredge frame.
Scientific monitoring and mapping of the seabed are increasingly used to inform these management decisions. By understanding the location of sensitive habitats and the natural recovery rates of different sediment types, regulators can create rotational closures. This system allows fishing in certain areas for a period and then closes them for several years, giving the disturbed habitat time to recover before the next harvest cycle.