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Soft bottom habitats include environments where the seabed consists of fine grain sediments, mud and sand. Their biodiversity and productivity vary depending upon depth, light exposure, temperature, sediment grain size and abundance of microalgae and bacteria.
Intertidal or shallow soft bottom habitats include mudflats and seagrass meadows, which are economically and ecologically important, but in geographic terms comprise only a small part of this extensive habitat type.
Overall, soft bottom is the ocean’s largest benthic habitat, forming the bottom of most of the continental shelves as well as vast expanses at depths of 3,000 to 6,000 m, which cover more than 60 percent of Earth’s surface (Snelgrove 2010; Ausubel et al. 2010).
Wherever they occur, soft bottom habitats are inhabited by burrowing animals such as worms, snails, clams, and some anemones, shrimps and crabs, among others. Sand dollars, brittle stars and sea cucumbers roam the muddy surface, feeding on the sediments and their inhabitants, including great numbers of copepods, nematodes and other animals that live in spaces between sediment particles. Fluke, founder, haddock, sculpin, skates, rays and other fish feed on all of these tasty populations.
The major cause of damage to soft bottom subtidal habitats is commercial fishing with heavy bottom trawls. The trawls. disturb the sediments and damage or kill many non-target animals as they are dragged along to catch fish that live on or near the bottom.
How Is It Measured
Subtidal soft bottom habitats are a Status component for the Biodiversity (Habitats) goal and a Pressure component for four goals (see below).
Hidden from view, the extent of soft bottom and hard bottom subtidal habitats must be estimated based on the composition of samples obtained from ships, as detailed in Halpern et al. (2008). The Institute of Arctic & Alpine Research (IAAR) at the University of Colorado, Boulder collected data on every bottom sample-- 544,543 samples as of June 2005--taken worldwide as part of the dbSEABED project. Information on the percentage of soft and hard sediment accompanied each sample. Those with more than 50% soft sediment were counted as soft bottom. Each 2 arc-minute grid cell (~3.7 km, or 13.69 km2 depending on latitude) was scored as either soft or hard bottom based on the samples it contained. Unsampled locations were assigned a score using a statistical technique (‘kriging’) based on the distribution of bottom types in the surrounding area. Maps of soft and hard bottom benthic habitats were created for shallow (0-60 m), shelf (60-200 m), slope (200 – 2000 m), and deep/abyssal water (>2000 m). About 80% of the samples in the IARR dataset were soft bottom: about 65 percent entirely soft sediment and about 15 percent mixed-type substrate.
The Status of soft bottom habitats was estimated by using the intensity of trawl fishing as a proxy. Trawl fishing normally occurs only on soft bottoms because hard bottoms damage the equipment. Trawl fishing data came from the Sea Around Us global catch database. Sea Around Us gathered catch data by gear type from the UN Food and Agriculture Organization (FAO) and other sources, then spatially disaggregated it into 259,200 grid cells, each 0.5° latitude by 0.5° longitude, based on species distribution maps for over 1,500 commercially exploited fish and invertebrate taxa and other information about fishing. Those grid cells were assembled into Exclusive Economic Zones to provide information on catch (metric tons) from trawling and dredging gears for each area.
Status was expressed as 1 minus the intensity of trawling in each area, with trawling intensity calculated as the annual catch made by trawling divided by the amount of trawlable soft bottom habitat available. ‘Trawlable habitat’ was defined as shallow subtidal (0-60m) and outer shelf (60-200m) soft bottom habitat. The reference point was the median value for trawling intensity across all years and any value greater than the median was set as 1.
Trend was calculated as the change in proportion of catch from trawl fishing per unit area of habitat within a region.
Damage or destruction to subtidal soft bottom habitat is used as a Pressure component for four goals. All pressures are ranked for their differing affects on different goals. For each goal, the effect of each pressure is weighted 'low' (1), 'medium' (2) or 'high' (3). The actual data-derived value of the pressure is then multiplied by the weight assigned to it for that goal. That process is repeated for each pressure-goal combination. The sum of those values divided by 3 (the maximum pressure-goal value) expresses the total affect of pressures on the goal.
Damage or destruction of subtidal soft bottom habitats has the following weights for goals where it is used: 'high' impact for Biodiversity (both subgoals) and Natural Products (sponges); 'medium' impact for Food Provision (Fisheries) and Natural Products (shells, fish oil) and Livelihoods & Economies (both subgoals) and 'low' impact for Opportunities for Artisanal Fishing.
The Pressure score for an area was 1 minus its Status score for subtidal soft bottom habitats.
Halpern et al. (2008). A global map of human impact on marine ecosystems.