the fishermen can tell whether the grid is working correctly. Studies have suggested that shrimp trawling is responsible for the highest rate of by-catch. Environmental damage Trawling is controversial because of its environmental impacts. Because bottom trawling involves towing heavy fishing gear over the seabed, it can cause large-scale destruction on the ocean bottom, including coral shattering, damage to habitats and removal of seaweed. The primary sources of impact are the doors, which can weigh several tonnes and create furrows if dragged along the bottom, and the footrope configuration, which usually remains in contact with the bottom across the entire lower edge of the net. Depending on the configuration, the footrope may turn over large rocks or boulders, possibly dragging them along with the net, disturb or damage sessile organisms or rework and re-suspend bottom sediments. These impacts result in decreases in species diversity and ecological changes towards more opportunistic organisms. The destruction has been likened to clear-cutting in forests. The primary dispute over trawling concerns the magnitude and duration of these impacts. Opponents argue that they are widespread, intense and long-lasting. Defenders maintain that impact is mostly limited and of low intensity compared to natural events. However, most areas with significant natural sea bottom disturbance events are in relatively shallow water. In mid to deep waters, bottoms trawlers are the only significant area-wide events. Bottom trawling on soft bottoms also stirs up bottom sediments and loading suspended solids into the water column. It is estimated that 21.87 gigatons of sediment from the sea floor is resuspended annually due solely to the activity of trawlers. For scale, the amount of sediment deposited into the ocean by all rivers in the world is estimated to be 17.8 to 20 gigatons annually. These turbidity plumes can be seen on Google Earth in areas where they have high resolution offshore photos (see bottom trawling). When the turbidity plumes from bottom trawlers are below a thermocline, the surface may not be impacted, but less visible impacts can still occur, such as persistent organic pollutant transfer into the pelagic food chain. Additionally, rototilling the sea floor and resuspending bottom sediment affects the nutrient levels and changes the entire chemistry of the ambient water, greatly reducing the photosynthesizing ability of plants and kelps while also impacting any animal living on the ocean floor. For example, in an article published in the New Zealand Journal of Marine