Bottom trawling is an industrial fishing method where a large net with heavy weights is dragged across the seafloor. When the weighted nets and trawl doors are dragged along the seafloor, everything in their path is disturbed or destroyed, including seagrasses, coral reefs or rock gardens where fish hide from predators.
Bottom trawls are used in catching marine life that live on the seafloor, such as shrimp, cod, sole and flounder.
Impact of Bottom Trawling
- The damage from bottom trawling is not limited to habitat destruction. As the net drags along the seafloor, all creatures in its path—fish, animals, marine mammals, plants, and turtles—are scooped up along the way.
- The fishing vessel keeps the targeted commercial species and discards the remaining, unwanted fish and animals—virtually all of it dead or dying.
- Once coral and sponge communities are destroyed, commercial fish and other species dependent on them for spawning, shelter, nurseries, protection, and food, may also disappear. In addition, overfished species such as rockfish and crab may need corals and other seafloor structures to provide appropriate habitat for recovery.
- By re-suspending bottom sediment, nutrient levels in the ambient water, and the entire chemistry of the water is changed.
- Re-suspended sediment can lower light levels in the water, and reduce photosynthesis in ocean-dwelling plants, the bottom of the food web.
- The re-suspended sediment is carried elsewhere by currents, and often lost from the local ecosystem. It may be deposited elsewhere along the continental shelf, or in many cases, permanently lost from the shelf to deeper waters. Changing parts of the seafloor from soft mud to bare rock can eliminate those creatures that live in the sediment. Species diversity and habitat complexity are directly affected by changing the physical environment of sand, mud or rock that result from trawling.