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.
