Beam-trawl fisheries

A beam trawl is a fishing net held open with the help of a steel beam. Two shoes are attached at the ends of the beam. A trawler tows two of these nets over the sea floor. The beam trawler is used on the tidal flats and in coastal waters for catching shrimp, using a groundrope with bobbins which roll over the bottom to flush up the shrimp. When fishing flatfish, the nets are provided with heavy chains (tickler chains) which scrape the sea floor. An energy-saving form of fishing is the electro-fisheries. A new method being developed is fishing with a net which floats just above the bottom.

The flatfish fisheries with beam trawlers occurs primarily in the southern and central part of the North Sea, in coastal areas and in sea channels. Beam trawl fisheries is the most important form of benthic fishing in the Netherlands. This country possesses the largest and most modern fleet of beam trawlers. Approximately 80% of all the fish landed by Dutchmen are caught with a beam trawl. For Belgium, this percentage lies around 65 %.

Larger vessels have gear extending twelve meters across and a weighing approximately 9000 kilograms. Beam trawlers are equipped on the underside with ten to twenty chains which are meant to stir up the fish on the seabed, in particular plaice and sole.

At places where the ground is too stony, the tickler chains are sometimes replaced with a mat of chains. The risk that the fishing gear is ruined is then much smaller.

When a ship sails at a fast speed with a beam trawl which is too light, the chance exists that the fishing gear will rise up from the floor, and therefore not catch any fish. Since more fish are caught with heavier gear, the Dutch captains have been changing over to more powerful vessels and likewise heavier beam trawl gear. While in 1972 the motor capacity never rose above 1100 horsepower, a capacity greater than 2000 HP is no exception nowadays. This form of fisheries is very uneconomic, energy-wise. After fishing for four days, a 2000 HP-beam trawler will have used an average of four liters of fuel per kilogram flatfish. Because oil prices are so high, the development of the more economic electro-fishing and the suspended beam trawl (sumwing) is very interesting for the fishermen.

The nurseries for North Sea fish (the 12-mile zone and the plaice box) are forbidden territory for beam trawl cutters with more than 300 HP motor capacity.

From otter fishing to beam trawling

Up till 1960, Dutch fishermen fished mostly using otter trawlers, first for catching flatfish but later on for catching pelagic fish as well. German shrimp fishermen have always fished with beam trawlers. The nets became more manageable when a boom was attached, making it possible to fish with two nets simultaneously. This system was taken over by Dutch shrimp fishermen, firstly only in the Wadden Sea but since 1950 outside the sea channels as well. The results were so good that eventually all shrimp fishermen switched over to the beam trawl. The positive results led to an attempt to catch flatfish in this manner. The steel beam at the front of the net made it possible to place more chains to scrape the bottom (in comparison to a net kept open with boards or 'trawl doors'). The increased amount of chains led to extra yields, particularly with fish species such as sole. Because the results were so good, a number of fishermen switched to beam trawl fishing prior to 1960 and the traditional otter fishing ended soon after 1960.

Distribution of the beam-trawl fisheries in the Netherlands

The fishermen register where and when they fish. These data are collected in the VIRIS databank, in which the fishing frequency is stored for areas of 30 by 30 sea miles. Basing their opinions in the early 1990s on this VIRIS databank, scientists at the NIOZ concluded that the larger Dutch cutters fish every square kilometer of the Dutch section of the North Sea outside of the 12-mile zone on the average of one to two times a year. According to this calculation, some areas in the southern North Sea are fished 3 to 10 times a year.

Recent studies from the Netherlands Institute for Fisheries Research has shown that the beam trawl fisheries concentrate on a smaller section of the North Sea than one has assumed up till now. The report 'The Micro Spreading of the Dutch Beam Trawl Fisheries' appeared in the Spring of 1997.

Twenty-four Dutch beam trawlers have been sailing for three years long with an apparatus on board which recorded the position of the ship every 6 minutes. In addition, the fishermen kept a log book for how much fish they caught. In this way, one was able to determine how often a 3 by 3 sea-mile area was fished.

The 24 cutters form around one tenth of the Dutch 'bokker' (a Dutch boat) fleet. One therefore had to calculate the data based on the whole fleet, whereby some uncertainty enters into the result. Not all fishermen will fit into the analysed pattern.

According to this study, of the 125,000 square sea miles that the North Sea measures, the Dutch fleet fishes a total of 50,000 square sea miles, 70 % of the surface is fished less than once a year, 14,000 square sea miles are fished more than two times per year.

The Dutch beam trawl fisheries appear to concentrate on a wide stroke along the coast, directly outside the 12-mile zone and at a few fishing grounds further north in the vicinity of Doggersbank, the German Bight and to the north of the Frisian Front. The deeper parts of the North Sea and the areas, which are relatively quiet due to a lack of current and weather influences, are rarely if ever fished.

Dutch beam trawlers concentrate along a wide strip along the coast, directly outside of the twelve-mile zone as well as on a few fishing grounds spread out further seaward in the vicinity of Doggersbank, the German Bight and north of the Frisian Front. The deeper parts of the North Sea and the grounds that are relatively quiet due to a lack of current and weather influences are rarely if ever fished.

Dutch fishermen register where and when they fish and with what sort of equipment. Since 1990, the catch data have been collected in a Fisheries Registration and Information System (VIRIS). All data is registered per 30 x 30 mile plot (around 0.5 latitude x 1 longitude), which gives an impression of the fishery movements.

The F-project

Limiting regulations such as quota and minimum mesh widths are based upon studies made by fishery biologists. Fishermen did not often agree with the results of the studies because they had very different impressions of the fish population, based upon their own yields. The F-project was set up in order to bridge the gap: a joint project between North Sea fishermen and biologists. The F stands for Fishing mortality, the parameter used by scientists to describe the cause of death due to fishing.

The project was set up to get a better insight into the manner in which the fishermen's catch can say something about the size of the flatfish stock (plaice and sole) and its management. Around forty fishing vessels participated in 2002. By participating in the research, the fishermen have a better understanding and confidence in the TACs and the assigned quotas. In May 2008, the method for determining the fish population was ready. Fishermen and biologists from IMARES were in agreement. The only barrier for using the method lay by the ICES.

Effects on sea life

In early 1998, the NIOZ and fisheries department of IMARES brought out the report Impact II, in which the studied effects of the various types of fishing on the ecosystems of the North Sea and Irish Sea were described. Thirteen institutes of five EC member states worked together on this study.

The results in short showed that per kilogram by-catch an additional 2.5 to 3.5 kilograms shellfish, crabs and lobsters are left damaged on the sea floor after a beam trawl trek. Further conclusions include:

- the trawls leave behind a visible track on the sea floor. The tickler chains plowed four to eight centimeers deep. Depending upon the local circumstances, it takes between 37 hours (with a beam trawl in a region with fine sediment and exposed to tidal currents) to 18 months (otter trawls on a muddy sediment in a sheltered region) before the tracks disappear.
- the species diversity is higher in unfished regions than in fished and disturbed regions.
- despite the high mortality of the small fish as by-catch and the most invertebrate animal species, this mortality is extremely low (a few percent) when seen as a percentage of the density of these animals on the sea bottom before the fishing boat passes over.
- the otter trawls dig obviously less deep into the sea floor than the beam trawlers; the otter trawlers cause less direct death to invertebrate species.
- ships with small beam trawls produce just as much damage per square meter than vessels with large beam trawls.
- the conclusion is that observed changes in the bottom habitats in the long term are certainly caused by direct and indirect effects of the fisheries, and not just by eutrophication, meteorologic changes and/or pollution.

Improved techniques

Various institutes, including IMARES-IJmuiden, perform studies for improving the technique for flatfish fisheries. Can the fishing gear of beam-trawlers be adapted in such a way that the negative effects on marine life are decreased and what are the effects on the catches? Which technique conserves the most on fuel?

An alternative is to replace the tickler chains of the beam-trawler with a device that uses electric shocks to startle the fish: pulse fishing. There are also experiments with beam trawls using jets of water instead of chains.

Otter trawling, which was the method used for fishing flatfish prior to the beam-trawler, solves the problem with bottom disturbance. However, the classical otter trawler with one net is less economical and costs much more energy per kilogram caught fish. The modern variation (twin-trawling) seems to be the most attractive environmentally friendly alternative.

Texel fishermen are experimenting with the sumwing, a beam-trawler with a horizontal wing which significantly reduces the resistance of the net.

In 2006, the UK95 experimented with a combination of fishing with two nets attached together, held open with a newly developed beam. This beam is called the twin-fly beam.

Finally, the anchor seining method is getting popular once again.

Renovation costs

The changeover from a beam trawler to a more environmentally friendly method requires a tremendous investment. The table below shows what is necessary to convert a beam trawler into a twin rigger:

vessel	euro cutter	large beam trawler
rebuild ship for twin rigging	90,000*	200,000*
acquisition twin rigger	20,000*	36,000*
savings in fuel per week trip	8-12 ton	3-5 ton
necessary yield per week to keep the ship operating at a profit	13,600*	25,000*
*(in euros) source: RIKZ

To convert a beam trawler to anchor seining, the costs are much higher. For an euro cutter, the costs are around 225,000 euros.

Weblinks

http://en.wikipedia.org/wiki/Bottom_trawling

Source: de Vleet, Ecomare