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Animals of the floor of the North Sea, and beam trawling

For some animals of the sea floor, beam trawling is an important cause of death.

Maximum lifespan of bottom-dwelling animals
species Maximum lifespan Mortality from trawling
     
  Years % per year
Velvety seamouse 10 21 1)
Faroe sunset shell 14 35 2)
Pelonaia corrugata 10 14
Rayed trough-shell 12 15
Sand star 10 14
Transparent razor 10 17
Razor sp. 12 11
Iceland cyprina 220 11 2)
Auger 10 13
Thick trough-shell 10 24 1)
Striped venus 12 24
White furrow-shell 10 25 1)
1) 90% of individuals killed by trawling before achieving maximum lifespan
2) 99% of individuals killed by trawling before achieving maximum lifespan
Source: NIOZ   Alterra/NC/Oct02

Impacts


Beam trawling is causing high mortality among many invertebrates that live on the sea floor. Most of the population of species that have a long lifespan are dying prematurely, i.e. before they can die from natural causes. In some species, as many as 90% and more of the population are killled prematurely by trawling. As a result, the age composition has shifted in favour of younger animals.

Research on the long-term impacts has indicated that bivalves and certain crustaceans are likely to decline, but small, short-lived worms are likely to increase.

Technical note


The table shows bottom-dwelling invertebrates whose lifespan exceeds 10 years. Per species, the annual mortality resulting from beam trawling is given, as a percentage of the total number of individuals. A high annual mortality indicates that beam trawling is killing 90 or 99% of individuals before they achieve their maximal lifespan.

If the annual mortality (of the velvety seamouse, for example) is 21%, then within 10 years, trawling will have killed over 90% of the number of individuals. The significance of such a high annual mortality can be seen from the fact that the velvety seamouse has a maximum lifespan of 10 years. So if trawling continues, only 10% of the population will be able to reach this age.

References


  • Lavalije M.S.S., H.J. Lindeboom en M.J.N. Bergman (2000). Macrobenthos van het Nederlands Continentaal Plat. Rapport ecosysteemdoelen Noordzee. NIOZ-rapport 2000-4. Texel.

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This page was last changed on 13 May 2004  (version 01).