Indicator Code: EEZTD
Objective: Ecosystem Vitality
Policy Category: Productive Natural Resources
Subcategory: Fisheries
Indicator Short Name: Trawling Intensity
Indicator Full Name: Percentage of Exclusive Economic Zone Area Trawled
Indicator Description: Benthic trawling is a fishing method that targets fish and invertebrates that inhabit ocean floor (or benthic) ecosystems. These include cod, scallops, shrimp, and flounder. Such trawling comes at a heavy environmental cost. Bottom trawling and dredging equipment has been described as the most destructive fishing gear in use today (Watson, 2004 and 2006). Benthic trawls are boats equipped with large heavy nets that are dragged across the living seafloor. The nets are held open at the front by a metal beam or by large “doors”, which can weigh several tons, and which are designed to scour the bottom as the trawl is dragged along, forcing the fish and invertebrates up into the net. This process exerts a heavy toll on the natural habitats of the sea floor, breaking off brittle bottom flora and fauna such as sponges and corals. Marine species such as turtles that try to escape the gear suffer stress, injury, and quite frequently, death (FAO, 2005).
The damage can last many years and continuous trawling and dredging does not allow the time needed for habitat recovery. Deep-sea coral communities can be wiped out by a single trawl sweep and repeated trawling can change the species composition of the ecosystem toward small opportunistic species, such as sea stars and small short-lived clams, and diminishes the abundance of commercially valuable species.
In addition to disrupting the living seafloor, trawling kills a large number of animals as “by catch,” the accidental harvest of untargeted species, such as other fish and invertebrate species, marine mammals, seabirds, and turtles. Some of this by catch is retained for sale, but a portion of it is returned to the sea, usually dead or dying. These animals returned to sea are known as discards. Bottom trawled fisheries have the highest discard rates of all fisheries. By catch is a contributor to the depletion of fish stocks, and can have a significant impact on endangered species of fish, mammals, turtles and seabirds.
The habitat destruction caused by trawling and dredging directly affects the human communities that depend on marine resources for food and income. Key nursery habitats such as seagrass are essential for sustaining a range of commercially important species. When these nursery habitats are destroyed, the entire local environment is impacted and the productivity of local fisheries, including those employing sustainable fishing methods, decreases.
The 2008 EPI uses a simple calculation of the percentage of the shelf area in each country’s EEZ that is fished by trawlers. There are no direct data available for the area trawled on a country-by-country basis. However, there are good data available describing fish landings and the gear used to catch these fish, and acceptable data on the composition of each country’s fishing fleet.
Units: Percentage Area
Country Coverage: 175
Reference Year: 2004
Target: 0%
Target Source: Expert Judgment
Short Source: Watson et al. 2004; 2006
Source: Watson, R., Hoshino, E., Beblow, J., Revenga, C., Kura, Y., & Kitchingman, A. (2004). Fishing gear associated with global marine catches. Fisheries Centre Research Reports 12(6), 32p.
Watson, R., Revenga, C., & Kura, Y. (2006). Fishing gear associated with global marine catches: II Trends in trawling and dredging. Fisheries Research 79, 103-111.
Source URL: http://www.seaaroundus.org/
Methodology: This indicator is calculated based on the amount of catch that is trawled per one-half degree (30 arc-minute) grid cells. This results in a metric of the area (sq km) associated with combined bottom trawl or dredge catch (supergears 8 or 9) rates >0.05 tonnes/sq km/year within declared EEZ areas. The marine area of the cells are added up to find the total area trawled and then divided by total EEZ. Cells that have a minimal catch are not included in the analysis.
Additional Citations: FAO. (2005). Mortality of fish escaping trawl gears (No. 478). Rome:Food and Agriculture Organization of the United Nations.