Tuesday, May 25, 2010

Difference Between Pelagic & Demersal Fishing

Written by Choleena DiTullio of Demand Studios for publication on eHow.com. The published article at eHow.com.

Pelagic fishing is the collection of fish or seafood at or near the surface of the ocean. Demersal fishing is the collection of fish or seafood on or near the seabed in open ocean. While the biggest difference is the location within the ocean, the fishing practices and seafood types vary as well.

Oceanological Zones
The pelagic zone of the ocean measures from the surface to 36,000 feet (11,000 meters) in depth. The demersal zone is simply on or near the seabed of the ocean. The depth differs depending on the depth of the ocean.

Some of the most common pelagic fish catches are mackerel, herring, sprats. Some of the most common demersal fish catches are cod, haddock, whiting and flatfish. Shellfish species include, shrimp, scallop, lobster and crab.

Demersal Trawling
Bottom trawling is the practice of dragging a large funnel-shaped net across the seabed. Weights in the net ensure contact with the sea bottom. As the net is all-encompassing, it captures all aquatic life, not just food species. A single trawler may rake more than 6.25 square miles (10 square kilometers) of seabed daily.

Pelagic Trawling
Large-scale commercial and industrial pelagic trawling, such as purse seining, captures schooling fish. The dense concentration of fish is surrounded by a deep curtain of netting supported by floats at the surface. The curtain is then pursed or gathered together at the bottom to fully enclose the school. The seins measure 100 to 200 feet (30 to 60 meters) in length.

Large-scale demersal fishing, such as bottom trawling, has been responsible for the destruction of some seabeds. The habitat is overturned and sediment billows, causing water quality to significantly decrease for species that rely on that habitat for living or as a food source.

Large-scale pelagic trawling has been implicated in the overfishing and significant depletion of some stocks, such as codfish. For example, in 1992 the Canadian government declared a moratorium on cod fishing as a result of overfishing. Reduced stock not only means fewer fish for human consumption but also fewer fish to feed the food chain, thereby possibly affecting other stock.

Encyclopedia Brittanica: Pelagic Zone
ProFortis International Limited: Sea Bed Fishing (Demersal)
Fisheries Research Services: Description of Commercial Gears
Canadian Geographic: Atlantic Cod

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