Bottom trawl

Bottom trawl

6-Bottom_trawl-(D)-Sketch_of_bottom_trawl--(copyright-)

Sketch of a bottom trawl

6-Bottom_trawl-(D)-Relative_size_of_bottom_trawl--(copyright-)

The picture shows the relative size of a bottom trawl in the groundfish fisheries. The trawl extends 200 m from the trawl doors to the cod end and is being trawled at 1,500 m (900 fathoms) by the stern trawler.

6-bottom-trawl-(g)-effort-distribution-(hafro).png

Location of effort with bottom trawl in 2011 (hours trawling), dark areas indicate highest effort.

Source: The Marine Research Insitute

6-bottom-trawl-(g)-bottom-trawl-total-catch-(statice).png

Bottom trawl catch (t) by species.

Source: Statistics Iceland, weight reports

6-bottom-trawls-(g)-bottom-trawl-catch-by-month-(statice).png

Bottom trawl catch (t) by months

Source: Statistics Iceland, weight reports

Trawls are funnel shaped bags of nets that are dragged (trawled) horizontally in the ocean. They are either bottom trawls or pelagic (or midwater) trawls depending on whether they are trawled along the ocean floor or in mid water. The trawls are classified further by the type of fisheries that they have been adapted to, such as bottom trawls for the groundfish, shrimp and lobster, pelagic trawls for the oceanic redfish, capelin, herring and blue whiting. The third main trawl type, beam trawl, as is commonly used in the North Sea is not used in Iceland.

The bottom trawl (also often called otter trawl) is the most important gear used in the Icelandic fisheries and has been adapted to suit various conditions of different fisheries. It is used at varying depths, ranging from 80 m to 1500 m. The fish species most often caught by bottom trawl are cod, demersal redfish, haddock, saithe and Greenland halibut but trawls also catch large amounts of plaice, Atlantic catfish, spotted catfish, ling, blue ling, tusk, great silver smelt and lemon sole. Trawls are used throughout the year, but the catch composition may vary depending on the season.

In the groundfish fisheries, the minimum mesh size is 135 mm and selectivity devices are also required in some fishing areas. The minimum allowed mesh size has been increased with time, from 110 mm in 1954, to 120 mm in 1963, and 135 mm in 1976, the largest minimum mesh size in the North Atlantic. Problems of a bycatch of small and immature fish may arise from time to time and from one area to another, in spite of the regulations for minimum mesh size. In order to overcome this, a range of selectivity devices has been developed that exclude the bycatch from the square part of the trawl. The devices are usually grids that will exclude the bycatch which may be either larger than the target species in case of immature small fish in the shrimp fisheries or it may be smaller than the target species such as small fry and immature shrimp in the shrimp fisheries.

Trawling is generally not allowed within 12 nm from the coast, except off the south coast during part of the year. Furthermore, some areas outside the 12 nm limit are permanently closed to trawlers due to abundance of juvenile cod. Temporary area closures for 1-2 weeks are also used if juvenile fish is abundant in the catch.

The first trawling reported in Icelandic waters was undertaken by an English trawler in 1890. There were more of them each year thereafter, and at the turn of the century the estimated numbers were around one hundred foreign trawlers operating in Icelandic waters, mostly English. In 1904, one year before statistics on the distant water fisheries in Icelandic waters became available, there were 180 trawlers reported to be fishing in Icelandic waters, 150 of them English. For comparison, 100 Norwegian boats fishing for herring, 150 French schooners, 100 Faroese and 130 Icelandic decked sailing vessels, and 2000 Icelandic rowing boats were also reported to operate in Icelandic waters in the same year. The trawlers were unpopular among the Icelanders, since they frequently destroyed the Icelandic longlines and were suspected of destroying the fishing grounds. The Icelanders, however, realized that their operation could not be prevented and soon began their own trawling experiments. The first Icelandic experiment with trawl was in 1901 with a sail trawler. The first operation of an Icelandic steam trawler took place in 1904 and trawls have been in continuous use by Icelanders since then.

Trawls and trawlers have been continuously evolving since their use began. The first trawlers were steam powered side trawlers. After the second world war they were replaced with oil powered, much larger side trawlers which were then in turn replaced by stern trawlers after 1978. The trawl itself has even seen larger changes. Originally hemp was used to make the trawls, later nylon took over and currently they are made of many types of synthetic fibers. Trawl doors were originally made of wood but later steel doors took over that are being constantly developed for better hydrodynamic shape and lighter construction. Bobbins were originally from wood also, but were replaced by steel and currently almost all boats use so called rock hoppers made from rubber. Various sensors are also attached to the trawl to measure how much fish is entering the trawl and how much is in the cod end (the end of the trawl).

References and further information

References: (Gunnarsson et al. 1998), (Şór 2003), (Şór 2005)

For full citation and further information on fishing gear see this page.

Hörğur Sævaldsson / Hreiğar Şór Valtısson University of Akureyri

 

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