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Fisheries   By-catch   

By-catch

By-catch is a big problem: for every kilogram of flatfish another 0.28 to 0.55 kilogram of other animals are caught. By-catch are thrown overboard but most do not survive. Regulations such as largers mesh widths should limit the amount of b-catch. Many marine mammals are also victims of fisheries. Worldwide, almost 1000 whales and dolphins are found dead in nets and fishing gear.

  • By-catch from beam trawl fisheries

    Beam-trawler fishermen catch quite a lot of extra organisms, such as fish (cod, dab, tub and grey gurnard, undersized sole and plaice, and rare species such as sharks and rays), starfish and shellfish. A distinction is made between worthwhile by-catch (edible fish and crustaceans which are not the aim of the fishery) and by-catch which are thrown overboard after sorting, the so-called discards.
    There are large differences in estimates as to how much by-catch is caught: from two to six kilograms undesired benthic animals and fish per kilogram flatfish (estimate Greenpeace) to an average of 0.45 kilogram per kilogram flatfish (estimate IMARES). According to IMARES, the by-catches from flatfish fisheries have greatly decreased in past years due to various causes: establishment of a required mesh width above latitudes of 56°N, establishment of a plaice box and a market for dab and flounder (former discards). The market for dab and flounder has certainly decreased the number of 'discards', but the catch of these species naturally remains the same.

  • By-catch of porpoises

    Studies of porpoises by Naturalis in the period 1990-2000 showed that half of the stranded porpoises died due to by-catch. The standing rigging fisheries has caused a lot of damage in Denmark, Germany and England. The mesh size of the nets in the Netherlands is smaller than in these countries, so that the Dutch standing rigging fisheries were probably not a problem. However, there is a good chance that the large towed nets which fish for the same prey as porpoises and beam trawlers do cause many deaths.

  • Pingers to warn marine mammals

    Pingers are devices that send off a signal which are meant to scare away dolphins, porpoises and seals. The EC has declared that pingers are required on fishing nets as of 2007. TNO-researcher Wim Verboom believes it is a rash action since the difference in range of sensitivity for sound by the various marine mammals is so great that certain species will not hear anything and will swim into the nets while others will incur hearing damage. Because the fish to be caught must not be scared off by the pingers, a study is being done in Zeeland as to the behaviour of fish with the various types of pingers. A number of fish also react to sound. Therefore, the government should determine the specifications for pingers so that fish will not be negatively affected and yet dolphins will be scared off. According to American researchers, an extra complication is that pingers might actually attract curious marine mammals